Pathology Flashcards
What is the definition of hyperplasia, hypertrophy, atrophy and metaplasia?
Hyperplasia - increase in the number of cells
Hypertrophy - increase in the size of cells
Atrophy - decrease in the size of a cell that has at one time been of normal size
Metaplasia - change of epithelial cell type from one to another at a specific site or location (the epithelium is normal in appearance in an abnormal location)
What are the two types of hyperplasia? Give an example for each
Physiological hyperplasia - occurs due to a normal stressor (breasts in pregnancy, liver growth after resection
Pathological hyperplasia - occurs due to an abnormal stressor (adrenal gland growth due to production of ACTH by a pituitary adenoma)
Hyperplasia only happens in cells that divide (not the heart or neuron)
What are the two types of hypertrophy? Give an example for each
Physiological hypertrophy - occurs due to a normal stressor (enlargement of skeletal muscle after exercise)
Pathological hypertrophy - occurs due to an abnormal stressor (increase in heart size due to aortic stenosis
What are the two types of atrophy? Give an example for each
Physiological atrophy - occurs due to a natural stressor (decrease in size of uterus post-pregnancy)
Pathological atrophy - occurs due to an abnormal stressor (usually due to a loss of stilumus to that organ - blood supply, innervation, decreased workload, aging)
What is the mechanism of metaplasia? Give an example
The epithelium normally present at a site cannot handle the new environment so it converts to a type of epithelium that can adapt (Barrett’s oesophagus - squamous to glandular)
What are some causes of cell injury?
- Trauma
- Thermal injury, hot or cold
- Poisons
- Infectious organisms
- Ionising radiation
- Drugs
What are the six mechanisms of cellular injury? Give examples for each
Mechanical disruption - trauma, osmotic pressure
Failure of membrane function integrity - damage to ion pumps, complement-mediated cytolysis, specific blockage of ion channels, alteration of membrane lipids
Membrane damage - free radicals
Deficiency of metabolites - oxygen, glucose, hormones
Blockage of metabolic pathways - interruption of protein synthesis, respiratory poisins
DNA damage or loss - ioinising radiation, chemotherapy, free radicals
What are the two ways in which cell damage can occur?
Reversibly and irreversibly
What are the four things that the effect of cellular injury on a tissue depend on?
- the duration of injury
- the nature of the injurious agent
- the proportion and type of cells that are affected
- the ability of tissues to regenerate
What is the definition of ischaemia and infarction?
Ischaemia is the pathological process resulting from a lack of oxygen due to impaired blood supply
Infarction is the specific process of necrosis resulting from lack of blood supply
What is reperfusion injury?
Under some conditions it is possible for the onset of cell death to be delayed until blood flow has been restored.
It may be due to the generation of reactive oxygen species, free radicals and damage to calcium pups.
In reperfusion injury, are cells damaged via apoptosis or necrosis
Cells damaged in this way probably go through apoptosis rather than necrosis
What is a free radical?
Atoms or groups of atoms with an unpaired electron, as such they may enter into chemical bond formation
What are some of the clinicopathological events involving free radicals?
- toxicity of some poisons
- oxygen toxicity
- tissue damage in inflammation
- intracellular killing of bacteria
What is necrosis and what causes it?
Death of tissues, causes include ichaemia, metabolic and truma. It is a pathological process
What are the six different types of necrosis? Give examples of each
- Coagulative necrosis in most tissues, firm pale area with ghost outlines on microscopy (most common)
- Colliquative necrosis is seen in the brain, the dead area is liquefied
- Causeous necrosis is seen in TB, there is a pale yellow, semi-solid material
- Gangrene is necrosis with putrefaction: it follows vascular occlusion or certain infections and is black
- Fibrinoid necrosis is a microscopic feature in arterioles in malignant hypertension
- Fat necrosis may follow trauma and cause a mass (often in the breast), or may follow pancreatitis visible as multiple white spots
What are the different types of gangrene?
- Wet - in the bowel (incarcarated hernia) or appendicitis
- Dry - usually seen in the toes (gradual arterial or small vessel obstruction in diabetes)
- Gas - result of infection by Clostridium perfringens
- Synergistic - infection by a combination of organisms
What are the two types of cell appearances following injury? Describe them
- Hydropic change - cells when the cytoplasm becomes pale and swollen due to the accumulation of fluid (hypoxia or chemical poisoning) - often reversible
- Fatty change - vacuolation of cells often due to the accumulation of lipid droplets as a result of a disturbance to ribosomal function and uncoupling of lipid from protein metabolism. (often in the liver) - moderate changes are reversible but severe fatty change is irreversible
What is the process of coagulative necrosis?
Following devitalisation, the cells retain their outline as their proteins coagulate and metabolic activity ceases.
Later, it may become soft as a result of digestion by the macrophages
What is the morphology of caseous necrosis?
A pattern of necrosis in which the dead tissue lacks any structure
What is the process of fibrinoid necrosis?
Arterioles are under such pressure that there is necrosis of the smooth muscle wall. This allows seepage of plasma into the media with consequent deposition of fibrin.
How does trauma cause fat necrosis?
The release of intra-cellular fat elicits a brisk inflammatory response, the polymorphs and macrophages phagocytosing the fat, proceeding eventually to fibrosis
What is the process of fat necrosis in pancreatitis?
There is release of pancreatic lipase. As a result, fat cells have their stored fat split into fatty acids, which then combine with calcium to precipitate out as white soaps
What is apoptosis?
Individual cell deletion in physiological growth control and in disease
What does reduced apoptosis cause?
Cell accumulation (neoplasia)
What does increased apoptosis cause?
Extensive cell loss (atrophy)
What are some of the roles of apoptosis?
- Continuining control of organ size
- Unwanted or defective cells undergo apoptosis
What factors control apoptosis?
Inhibitors include growth factors, cell matrix, sex steroids and some viral proteins
Inducers include growth factor withdrawal, loss of matrix attachment, glucocorticoids, some viruses, free radicals, ioinising radiation, DNA damage
How do apoptosis inducers and inhibitors work?
They act via the bcl-2 protein family, inhibiting or activating the death pathway, resulting in activation of caspases
OR
Activation of plasma membrane receptor Fas (CD95) by its ligand bypasses bcl-2 to activate other caspases
What is the process of apoptosis?
Degradation of the cytoskeletal framework
Fragmentation of DNA and loss of mitochondrial function
Nucleus shrinks (pykonosis) and fragments (karyorrhexis)
Cell shrinks, retaining an intact plasma membrane
Alteration of this membrane rapidly induces phagocytosis
What happens to dead cells not phagocytosed?
They break into smaller membrane-bound fragments called apoptotic bodies.
What gene checks the integrity of the genome before mitosis? What happens next?
p-53 gene
Defective cells are swtiched to apoptosis instead
What diseases are associated with increased apoptosis?
AIDs (activate CD4, inducing apoptosis
Neurodegenerative disorders
What is the difference in induction of apoptosis vs necrosis?
Apoptosis - may be induced by physiological or pathological stimuli
Necrosis - invariably due to pathological injury
What is the difference in biochemical events of apoptosis vs necrosis?
Apoptosis - energy-dependent fragmentation of DNA by endogenous endonucleases - lysosomes intact
Necrosis - impairment or cessation of ion homeostasis - lysosomes leak lytic enzymes
What is the difference in extent of apoptosis vs necrosis?
Apoptosis - single cell
Necrosis - cell groups
What is the difference in cell membrane integrity of apoptosis vs necrosis?
Apoptosis - maintained
Necrosis - lost
What is the difference in morphology of apoptosis vs necrosis?
Apoptosis - cell shrinkage and fragmentation to form apoptotic bodies with dense chromatin
Necrosis - cell swelling and lysis
What is the difference in inflammatory response of apoptosis vs necrosis?
Apoptosis - none
Necrosis - usual
What is the difference in fate of dead cells of apoptosis vs necrosis?
Apoptosis - ingested (phagocytosed) by neighbouring cells
Necrosis - ingested (phagocytosed) by neutrophil polymorphs and macrophages
What is the definition of inflammation?
A response of vascularised tissues to infection and damaged tissues that brings cells and molecules of host defence from the circulation to the sites where they are needed, in order to eliminate the offending agents
In a broad sense
What are the key mediators of inflammation?
Phagocytic leukocytes, antibodies and complement proteins
What are some examples of key components of the innate immune system?
Natural killer cells, dendritic cells, and epithelial cells, as well as soluble factors such as the proteins of the complement system.
What are the steps of the typical inflammatory reaction?
- The offending agent, often in extrasvascular tissues, is recognised by host cells and molecules
- Leukocytes and plasma proteins are recruited from the circulation to the site where the offending agents are located
- The leukocytes and proteins are activated and work together to destroy and eliminate the offending substance
- The reaction is controlled and terminated
- The damaged tissue is repaired
How do blood vessels help in inflammation?
- The blood vessels dilate to slow down blood flow, and by increasing their permeability, they enable certain selected proteins to enter the site of infection of tissue damage
- The blood vessel epithelium characteristics change, such that circulating leukocytes first stop then enter the site of infection
What do leukocytes do once they are recruited?
They are activated and acquire the ability to ingest and destroy microbes and dead cells, as well as foreign bodies
What are some of the harmful effects of inflammation?
- Acute temporary things such as pain and functional impairment
- There are many diseases in which the inflammatory reaction is misdirected (against self tissues in autoimmune diseases), occurs against normally harmless substances (allergies) or is inadequately controlled
What cells are involved in these acute diseases:
1. ARDS
2. Asthma
3. Glomerulonephritis
4. Septic shock
- ARDS - neutrophils
- Asthma - IgE antibodies; eosinophils
- Glomerulonephritis - antibodies and complement, neutrophils, monocytes
- Septic shock - cytokines
What is the role of inflammatory mediators such as plasma proteins?
They initiate and amplify the inflammatory response and determine it’s pattern, severity, and clinical and pathological manifestations
Tell me about acute inflammation e.g. duration, mechanisms, and resolution
- Typically develops within minutes or hours and is of short duration, lasting for several hours or a few days
- Main characteristics are the exudation of fluid and plasma proteins (oedema), and the emmigration of leukocytes, particularly neutrophils,
- When it reaches its desired goal, the reaction subsides, but if the response fails to clear the stimulus, the reaction can develop into chronic inflammation
Tell me about chronic inflammation
- It is of longer duration and is associated with more tissue destruction
- Associated with the presence of lymphocytes and macrophages, the proliferation of blood vessels, and the deposition of connective tissue
What is the onset of acute vs chronic inflammation?
Acute - minutes to hours
Chronic - days
What are the cellular infiltrates in acute vs chronic inflammation?
Acute - neutrophils
Chronic - lymphocytes and macrophages
How are local and systemic signs present in acute vs chronic inflammation?
Acute - prominent
Chronic - less
How does the termination of inflammation occur?
It is terminated when the offending agent is eliminated
The reaction resolved because the mediators are broken down and dissipated, the leukocytes have a short life span in tissues
In addition, anti-inflammatory mechanisms are activated, serving to control the host and prevent it from causing excessive damage
What are some examples of things that trigger inflammation?
- Infections - bacterial, viral, fungal, parasitic
- Tissue necrosis elicits inflammation regardless of the cause of cell death, which may include ischaema, trauma, physical and chemical injury
- Foreign bodies - splinters, dirt, sutures - or endogenous molecules depositing in tissues e.g. urate in gout, cholesterol in atherosclerosis
- Immune reactions - autoimmune diseases, allergies
What are some cells and receptors that recognise microbes and damaged cells?
- Cellular receptors for microbes in the plasma membrane (extracellular), the endosomes (ingested), and the cytosol (intracellular) - e.g. Toll-like receptors TLRs
- Sensors of cell damage - all cells have cytosolic receptors that recognise a diverse set of molecules that are liberted of altered as a consequence of cell damage
- Other cellular receptors involved in inflammation - many leukocytes express receptors for the Fc tails of antibodies and for complement proteins
- Circulating proteins - the complement system reacts against microbes and produces inflammatory mediators, mannose-binding lectin recognises microbial sugars and promotes ingestion of the microbes and activation of the complement system
Where are TLRs expressed?
What do they do?
They are expressed on many cell types, including epithelial cells, dendritic cells, macrophages and other leukocytes
Engagement of these receptors triggers production of molecules involved in inflammation including adhesion molecules on endothelial cells, cytokines and other mediators
What are some examples of cytosolic sensors of cell damage?
- Uric acid - a product of DNA breakdown
- ATP - released from damaged mitochondria
- Reduced intracellular K+ concentrations - reflecting loss of ions because of membrane injury
- DNA in the cytoplasm because it should normally be in the nucleus
What do cytosolic sensors of cell damage do?
- They activate a multiprotein cytosolic complex called inflammasome, which induces the production of IL-1
- IL-1 recruits leukocytes and thus induces inflammation
What do the leukocyte receptors for the Fc tails do?
They recognise microbes coated with antibodies and complement (opsonisation) and promote ingestion and destruction of the microbes as well as triggering inflammation
What are the three major components of acute inflammation?
- Dilation of small vessels, leading to an increased blood flow
- Increased permability of the microvasculature enabling plasma proteins and leukocytes to leave the circulation
- Emigration of the leukocytes from the microcirculation, their accumulation in the focus on injury, and their activation to eliminate the offending agent
What is an exudate vs a transudate?
Exudate is an extravascular fluid that has a high protein concentration and contains cellular debris. Its presence implies an ongoing inflammatory response
Transudate is a fluid with low protein content, little or no cellular material, and low specific gravity. It is essentially an ultrafiltrate of blood plasma that is produced as a result of imbalance between hydrostatic and osmotic pressure without an increase in vascular permability
What is edema?
An excess of fluid in the interstital tissue or serous cavities, it can be either exudate or transudate
What is pus?
A purulent inflammatory exudate rich in leukocytes (mostly neutrophils), the debris of dead cells, and in many cases, microbes
What are the changes in vascular flow and caliber after injury?
- Vasodilation is induced by the action of many mediators, notably histamine, on vascular smooth muscle. First the arterioles then capillaries, causing increased blood flow and erythema
- Increased permeability of the microvasculature, with the outpouring of protein-rich fluid into extravascular tissues
- These changes lead to engorgement of small vessels, which slowly moving red cells, a condition termed stasis (again causing erythema)
- As stasis develops, blood leukocytes, principally neutrophils, accumulate along the vascular endothelium, simultaneously, endothelial cells are activated by inflammatory mediators and express increased levels of adhesion molecules, adhering to leukocytes, which then migrate through the vascular wall
What are the different mechanisms of increased vascular permability in short? (we’ll go into more detail after this)
- Contraction of endothelial cells resulting in increased endothelial spaces is the most common mechanism of vascular leakage
- Endothelial injury, resulting in endothelial cell necrosis and detachment
- Increased transport of fluid and proteins called transcytosis, through the endothelial cell
What chemical mediators trigger endothelial cell contraction?
Histamine, bradykinin, leukotrienes, and other chemical mediators
When does endothelial cell contraction happen and how long does it last for?
It is called the immediate transient response because it happens occurs rapidly after exposure to the mediator and is often short-lived (15 to 30 minutes)
What is delayed prolonged leakage?
- In some forms of mild injury (e.g. sunburn), vascular leakage begins after a delay of 2-12 hours and lasts for several hours or even days
- It may be caused by contraction of endothelial cells or mild endothelial damage
What is the mechanism behind endothelial injury causing increased vascular permeability?
Direct damage to the endothelium is encountered in severe injuries and is induced by the action of microbes and microbial toxins that target endothelial cells
Neutrophils that adhere to the endothelium during inflammation may also injure the endothelial cells and thus amplify the reaction
What does leakage occur due to endothelial injury, necrosis and detatchment after injury?
In most instances, leakage starts immediately after injury and is sustained for several hours until the damaged vessels are thrombosed or repaired
How do lymphatic vessels respond to inflammation?
In inflammation, lymphatic flow increases and helps drain oedema fluid. In addition to fluid, leukocytes and cell debris, as well as microbes, may find their way into the lymph.
Lymphatic vessels proliferate during inflammatory reactions to handle the increased load.
What are the most important leukocytes in inflammation?
Neutrophils and macrophages
What is the function of leukoytes in the inflammatory response?
- Ingest and destroy bacteria and other microbes, as well as necrotic tissue and foreign substances
- Produce growth factors that aid in repair
- When strongly activated, they may induce tissue damage and prolong inflammation
The journey of leukocytes from the vessel lumen to the tissue is a multi-step process that is mediated and controlled by what?
Adhesion molecules and cytokines called chemokines
What are the three phases of leukocyte migration?
- In the lumen: margination, rolling, and adhesion to the endothelium
- Migration across the endothelium and vessel wall
- Migration in the tissue toward a chemotactic stimulus
How does margination occur?
In normal flowing venules, red cells are confined to a central axial column, displacing the leukocytes towards the wall of the vessel
Because of stasis (blood flow slowing) in early inflammation, haemodynamic conditions change, and more white cells assume a peripheral position along the enothelial surface
At what stage of the inflammatory process do neutrophils predominate?
6 - 24 hours
At what stage of the inflammatory process do monocytes predominate?
24 - 48 hours
What are the mediators that trigger the following:
1. Leukocyte rolling on endothelium
2. Firm attachment to the endothelium
- Selectins (P-selectin (platelets), L-selectin (leukocytes), E-selectin (endothelium)) trigger leukocyte rolling
- Integrins trigger firm attachment to the endothelium
Which cytokines promote expression of selectins and integrins on endothelium?
TNF
IL-1
Which cytokines increases the avidity of integrins for their ligands and promote directional migration of the leukocytes?
Chemokines
What is transmigration or diapedesis?
It is migration of the leukocytes through the endothelium.
It often happens mainly in the postcapillary venules
After exiting the circulation, the leukocytes move in the tissues toward the site of injury. What is this process called?
Chemotaxis
What are some endogenous chemoattractants? (things released in an injury that attract leukocytes in inflammation)
- Cytokines, particularly those of the chemokine family e.g. IL-8
- Components of the complement system (mainly C5a)
- Arachidonic acid (AA) metabolites, mainly leukotriene B4
What are the three steps of phagocytosis?
- Recognition and attachment of the particle to be ingested by the leukocyte
- Engulfment, with subsequent formation of a phagocytic vacuole
- Killing or degradation of the ingested molecule
What phagocytic receptors bind and ingest molecules?
- Mannose receptors - a lectin that binds to terminal mannose and fucose (which are only on microbes) therefore recognises microbes and not host cells
- Scavenger receptors - bind to LDLs
- Receptors for various opsonins - main ones are IgG antibodies, C3b breakdown product and certain lectins (mannose-binding lectins)
How does engulfment take place in phagocytosis?
- After a particle is bound to the phagocyte receptors, extensions of the cytoplasm (psuedopods) flow around it
- The plasma membrane pinches off to form a vesicle (phagosome) that engulfs the particle
- The phagosome fuses with a lysosome granule, resulting in release of the granules contents into the phagosome
What molecules are used to perform intracellular destruction of microbes and debris once in the phagosome and how?
- Reactive oxygen series are produced by the oxidisation of NADPH by NADPH oxidase - it accompanies phagocytosis - produced by lysosome - may be released extracellularly, causing tissue damage
- Reactive nitrogen series
- Lysosomal enzymes
What are neutrophil extracellular traps?
Extracellular fibrillar networks that provide a high concentration of antimicrobial substances at sites of infection and prevent the spread of the microbes by trapping them in the fibrils
What are mediators of inflammation?
Substances that initiate and regulate inflammatory reactions
What are the most important mediators of acute inflammation?
Vasoactive amines
Lipid products (prostaglandins and leukotrienes)
Cytokines (including chemokines)
Products of complement activation
How are inflammatory mediators usually created?
They are either secreted by cells or generated by plasma proteins.
Cell-derived mediators are usually sequestered by intracellular granules and can be rapidly secreted by granule exocytosis (e.g. histamine in mast cell granules) or are synthesised de novo (e.g. prostagladins, leukotrienes, cytokines) in response to a stimulus
What are the main cells that create inflammatory mediators?
The major cell types that produce mediators of inflammation are the sentinels that detect invaders and damage in tissues, such as macrophages, dendritic cells, and mast cells.
How long do inflammatory mediators last for?
They are usually short-lived.
They quickly decay or are inactivated by enzymes, or they are otherwise scavenged or inhibited.
Can one inflammatory mediator stimulate another one?
Yes!
For instance, products of complement activation can stimulate the release of histamine. the cytokine TNF acts on endothelial cells to stimulate the production of another cytokine IL-1.
Where is histamine created and what does it do?
It is created in mast cells, basophils and platelets.
It causes vasodilation, increased vascular permeability and endothelial activation
Where are prostaglandins created and what do they do?
It is created in mast cells, macrophages, endothelial cells and leukocytes
It causes vasodilation, pain and fever
Where are leukotrienes created and what do they do?
They are created in mast cells and leukocytes.
They cause increased vascular permability, chemotaxis, leukocyte adhesion, and activation
Where are cytokines (TNF, IL-1 and IL-6) created and what do they do?
They are created in macrophages, endothelial cells and mast cells.
Locally, they cause endothelial activation (expression of adhesion molecules).
Systemically, they cause fever, metabolic abnormalities, hypotension (shock)
Where is complement created and what does it do?
It is in the plasma, created by the liver.
It causes leukocyte chemotaxis and activation, direct target killing (membrane attack complex), vasodilation (mast cell stimulation)
Where are kinins created and what do they do?
They are found in the plasma, created in the liver.
They cause increase vascular permeability, smooth muscle contraction, vasodilation and pain
How does histamine increase vascular permeability?
It binds to H1 receptors on microvascular endothelial cells, creating interendothelial gaps in venules and causing endothelial contraction
What things stimulate the release of histamine?
- physical injury, such as trauma, cold or heat
- binding of antibodies to mast cells (immediate hypersensitivity reactions)
- products of complement called anaphylatoxins (C3a and C5a)
- Neuropeptides (substance P) and cytokines (IL-1 and IL-8) may also trigger histamine release
What are some examples of arachidonic acid metabolites?
The lipid mediators prostaglandins and leukotrienes are produced from arachidonic acid (AA) present in membrane phospholipids, and stimulate vascular and cellular reactions in acute inflammation.
Arachidonic acid (AA) is usually found esterified in the membrane phospholipids. How is it released?
Through the action of cellular phospholipases (mainly phospholipase A2).
An increase in cytoplasmic Ca2+ and activation of various kinases in response to external stimuli, activates phospholipase A2
What is created when AA is released from the membrane phospholipids?
From the AA, two major enzymes: cyclooxygenases and lipoxygenases synthesise many AA-derived inflammatory mediators - called eicosanoids
Cyclooxygenases create prostaglandins
Lipooxygenases create leukotrienes and lipoxins
How are prostaglandins generated?
By the actions of cyclooxygenase 1 and 2.
What is the difference between COX-1 and COX-2?
COX-1 is produced in response to an inflammatory stimuli and is also constitutively expressed in most tissues, where it may serve a homeostatic function
COX-2 is induced by inflammatory stimuli and thus generates prostaglandins, but it is low or absent in normal tissues
What is the function of prostacyclin?
It is a vasodilator and a potent inhibitor of platelet aggregation, and also markedly potentiates the permeability-increasing and chemotactic effects of other mediators
What are the most important cytokines in inflammation? What do they do?
TNF & IL-1 serve critical roles in leukocyte recruitment by promoting adhesion of leukocytes to endothelium and their migration through vessels.
* They increase expression of adhesion molecules (E- and P- selectin)
* They increase the production of other cytokines, growth factors and eicosanoids
* They produce fever and can cause sepsis
What are chemokines and what is their role?
A family of small proteins that act as chemoattractants for specific types of leukocytes
What are the main morphological features of acute inflammation?
Dilation of small bloods vessels, accumulation of leukocytes and fluid in the extravascular tissue
What are the different morphological types of acute inflammation?
- Serous inflammation is marked by exudation of cell-poor fluid into spaces created by cell injury or into body cavities lined by the peritoneum, pleura or pericardium
- Fibrinous inflammation is where a fibrinous exudate develops when the vascular leaks are large or there is a local procoagulant stimulus
- Purulent (suppurative) inflammation is characterised by the production of pus, an exudate containing neutrophils, the liquefied debris of necrotic cells and edema fluid
- Ulcers are a local defect, or excavation, of the surface of an organ or tissue that is produced by the sloughing of inflamed necrotic tissue
What is (or isn’t) in the fluid of serous inflammation?
Typically, the fluid is not infected by destructive organisms and does not contain large numbers of leukocytes.
The fluid may be derived from plasma or from the secretions of mesothial cells
What is the mechanism of fibrinous inflammation?
With greater increase in vascular permeability, large molecules such as fibrinogen pass out of the blood, and fibrin is formed and deposited in the extracellular space.
A fibrous exudate is characteristic of inflammation of the body cavities, such as the meninges, pleura and pericardium.
How does fibrinous inflammation present histologically?
Fibrin appears as an eosinophilic meshwork of threads, or sometimes as an amorphuos coagulum
How does fibrinous inflammation resolve?
Fibrinous exudates may be dissolved by fibrinolysis and cleared by macrophages.
If the fibrin is not removed, over time it may stimulate the growth of fibroblasts and blood vessels and thus lead to scarring.
What are abscesses?
They are localised collections of purulent inflammatory tissue caused by a suppuration buried in a tissue, an organ or a confined space
What components make up an abscess?
Abscesses have a central region that appears as a mass of nectrotic leukocytes and tissue cells. There is usually a zone of preserved neutrophils around this necrotic focus, and outside this region, there may be vascular dilation and parenchymal and fibroblastic proliferation, indicating chronic inflammation and repair.
How does an ulcer develop?
During the acute stage, there is intense polymorphonuclear infiltration and vascular dilation in the margins of the defect.
With chronicity, the margins and the base of the ucler develop fibroblastic proliferation, scarring and the accumulation of lymphocytes, macrophages and plasma cells
What are the outcomes of acute inflammation?
- Complete resolution - the injury is short-lived, there is little tissue destruction and the damaged parenchymal cells can regenerate
- Healing by connective tissue replacement (scarring or fibrosis) - there is substantial tissue destruction, abundant fibrin exudation that can’t be cleared, connective tissue grows into the area of damage, converting it into a mass of fibrous tissue, a process called organisation
- Chronic inflammation occurs when the acute inflammatory response cannot be resolved, either due to the microbe or an inadequate immune response
What is chronic inflammation?
A response of prolonged duration (weeks to months) in which inflammation, tissue injury and attempts of repair co-exist, in varying combinations
What are the causes of chronic inflammation?
- Persistent infections by infections that are difficult to eradicate (e.g.mycobacteria, funghi, parasites) - these may be a delayed hypersensitivity reaction or granulomatous reactions
- Hypersensitivity diseases such as autoimmune diseases or allergic diseases
- Prolonged exposure to potentially toxic agents, either endogenous or exogenous for example silicosis (exogenous) or atherosclerosis (endogenous)
What are the morphological characteristics of chronic inflammation?
- Infiltration with mononuclear cells, which include macrophages, lymphocytes and plasma cells
- Tissue destruction, induced by the persistent offending agent, or by the inflammatory cells
- Attempts at healing by connective tissue replacement of damaged tissue, accomplished by angiogenesis and fibrosis
What is the most dominant cell in chronic inflammation and what does it do?
Macrophages contribute to the reaction by secreting cytokines and growth factors that act on various cells, by destryoing foreign invaders and tissues, and by activating other cells, notably T lymphocytes
What are the stages in the life cycle of a macrophage?
- They are tissue cells derived from haematopoietic stem cells in the bone marrow and from progenitors in the embryonic yolk sac and fetal liver during early development. They circulate as monocytes
- Monocytes enter the blood, migrate into various tissues and differentiate into macrophages
- The life span of a monocyte in 1 day, whereas a tissue macrophage can last for several months or years
What are the names of macrophages in the following organs:
1. Liver
2. Spleen and lymph nodes
3. Central nervous system
4. Lungs
What do they form together?
- Liver - Kuppfer cells
- Spleen and lymph nodes - Sinus histiocytes
- CNS - microglial cells
- Lungs - alveolar macrophages
Together they form the mononuclear phagocyte system
There are two major pathways of macrophage activation. What are they?
- Classical macrophage activation may be induced by microbial products such as endotoxins, which engage TLRs and other cells by T cell-drived signals (cytokine IFN-gamma). Classically activate macrophages (M1) produce NO and ROS and upregulate lysosomal enzymes. These things help them kill ingested organisms, trigger inflammation via cytokines.
- Alternate macrophage activation is induced by cytokines other than IFN-gamma (IL-4 and IL-13), produced by T lymphocytes and other cells. The function of alternately activated macrophages (M2) is tissue repair. They secrete GF that stimulates angiogenesis, activate fibroblasts and stimulate collagen synthesis.
What are the main functions of macrophages in chronic inflammation?
- Ingest and eliminate microbes and dead tissue
- Initiate the process of tissue repair
- Secrete mediators of inflammation, such as cytokines (TNF, IL-1, chemokines) and eicosanoids
- Display antigens to T-lymphocytes and respond to signals from T cells
What are the roles of lymphocytes in chronic inflammation?
Microbes and other environmental antigens activate T and B lymphocytes, which amplify and propogate chronic inflammation.
Lymphocytes may be the dominant population in the chronic inflammation seen in autoimmune and other hypersensitivity diseases
There are three subsets of CD4+ T cells that secrete different cytokines and ellicit different types of inflammation. Tell me them…
- TH1 cells produce the cytokine IFN-gamma, which activates macrophages via the classical pathway
- TH2 cells secrete IL-4, IL-5 and IL-13, which recruit and activate eosinophils and are reponsible for the alternative pathway of macrophage activation.
- TH17 cells secrete IL-17 and other cytokines, which induce the secretion of chemokines, reponsible for the recruitment of neutrophils (and monocytes) to the reaction
Lymphocytes and macrophages interact in a bidirectional way, and these interactions play an important role in propogating chronic inflammation. Tell me about them…
- Macrophages display antigens to T cells, express membrane molecules (called costimulators), and produce cytokines (IL-12) that stimulate T cell responses.
- Activated T cells produce cytokines (IFN-gamma), which recruit and activate macrophages, promoting more antigen presentation and cytokine secretion
What is a tertiary lymphoid organ? What is the term used for it’s formation?
In some chronic inflammatory reactions, the accumulated lymphocytes, antigen-presenting cells, and plasma cells cluster together to form lymphoid tissues resembling - this is a tertiary lymphoid organ.
It’s formation is called lymphoid organogenesis
Apart from lymphocytes and macrophages, what other cells are involved in chronic inflammation?
- Eosinophils are abundant in immune reactions mediated by IgE and parasitic infections. Their recruitment is similar to neutrophils, via the chemokine eotaxin. They contain major basic protein, which is toxic to parasites but also mammalian epithelial cells
- Mast cells are present in chronic inflammatory reactions, and because they secrete a plethora of cytokines, they may promote inflammatory reactions in different situations.
- Although neutrophils are characteristic of acute inflammation, many forms of chronic inflammation still show large numbers of neutrophils
What is granulomatous inflammation?
A form of chronic inflammation characterised by collections of activated macrophages, often with T lymphocytes, and sometimes associated with central necrosis
There are two types of granulomatous inflammation, which differ in pathogenesis. What are they and what is their pathogeneses?
- Foreign body granulomas are incited by relatively inert foreign bodies, in the abscense of T cell-mediated immune responses. Typically occur around talc (IVDU) or sutures, things large enough to not be phagocytosed. Epitheliod cells and giant cells are opposed to the foreign body surface.
- Immune granulomas are caused by agents capable of inducing a persistent T-cell mediated immune response. The agent is often difficult to erradicate. In such responses, macrophages keep activatign T cells and T cells keep activating macrophages until giant cells epitheliod cells form.
What are epithelioid and giant cells?
- The activated macrophages may develop abundant cytoplasm and begin to resemble epithelial cells, these are called epithelioid cells
- Some activated macrophages may fuse, forming multinucleate giant cells
What are the two types of reaction that could occur in repair of damaged tissue?
- Regeneration by proliferation of residual (uninjured) cells and maturation of tissue stem cells
- The deposition of scar tissue to form a scar
How does regeneration occur in tissue repair?
Some tissues are able to replace the damaged components and essentially return to a normal state - this is called regeneration
It occurs by proliferation of cells that survive the injury and retain the capacity to proliferate
In what organs does regeneration often occur in tissue repair?
The rapidly dividing epithelia of the skin and intestines, and in some parenchymal organs, notably the liver.
In other cases, tissue stem cells may contribute to the restoration of the damaged tissues
When would scar tissue form in tissue healing?
If the injured tissues are incapable of complete restitution, or if the supporting structures of the tissue are severely damaged, repair occurs by the laying down of connective (fibrous) tissue.
Although the fibrous scar is not normal, it provides enough structural stability that the injured tissue can continue to function
What is fibrosis? and how it this linked with organisation?
The term fibrosis is often used to describe the extensive depostion of collagen that occurs in the lungs, liver, kidney and other organs as a consequence of chronic inflammation or in the myocardium after extensive ischaemic necrosis.
If fibrosis occurs in a tissue space occupied by an inflammatory exudate, it is called an organisation
The regeneration of injured cells and tissues involves cell proliferation. What is the driven by and dependent on?
It is driven by growth factors and is critically dependent on the integrity of the extracellular matrix, and by the development of mature cells from the stem cells.
Several cell types proliferate during tissue repair. What are they and why do they proliferate?
- Remnants of the injured tissue - which attempt to restore normal structure
- Vascular endothelial cells - to create new vessels that provide the nutrients required for tissue repair
- Fibroblasts - the source of the fibrous tissue that forms the scar to fill defects that cannot be corrected by regeneration
The ability of tissues to repair themselves is determined, in part, by their intrinsic proliferative capacity. Based on this, tissues are divided into three groups. What are the groups and tell me about them?
Not examples yet, that will be next …
- Labile (continuously dividing) tissues - cells of these tissues are constantly being lost and replaced by maturation from tissue stem cells and by proliferation of mature cells. They can readily regenerate after injury, as long as the stem cells aren’t damaged
- Stable tissues - cells of these tissues are quiescent (in the G0 stage of the cell cycle) and have only minimal proliferative activity in their normal state. However, they are capable of dividing in response to injury but have a limited capacity for regeneration
- Permanent tissues - The cells of these tissues are considered to be terminally differentiated and nonproliferative in post-natal life. Repair is normally dominated by scar formation
Give me some examples of labile, stable and permanent tissues…
- Labile - haematopoeitic cells in the bone marrow, stratified squamous epithelium of the skin, oral cavity, vagina, and cervix ; the cuboidal epithelia of the ducts draining exocrine organs (salivary glands, pancreas, biliary tract); columnar epithelium of GI tract, uterus, fallopian tubes and the transitional epithelium of the urinary tract
- Stable - the parenchyma of most solid tissues, such as liver, kidney and pancreas. They also include endothelial cells, fibroblasts, smooth muscle cells.
- Permanent - cardiac myocytes and neurons
Where and by what cells are growth factors released that drive tissue repair?
- They are typically produced by cells near the site of damage.
- The most important sources are macrophages activated by the tissue injury, but epithlial cells and stromal cells also produce some of them.
How do growth factors drive tissue repair?
- Several growth factors bind to ECM proteins and are displayed at high concentrations
- All growth factors activate signaling pathways that ultimately induce the production of proteins that are involved in driving cells through the cell cycle and other proteins that release blocks on the cell cycle.
In addition to growth factors, what else can stimulate cell proliferation in tissue repair?
Cells use integrins to bind to ECM proteins, and signals from the integrins can stimulate cell proliferation
What are the most important stem cells for regeneration after injury? Where are they and what do they do?
Tissue stem cells live in specialised niches, and it is believed that injury triggers signals in these niches that activate quiescent stem cells to proliferate and differentiate into mature cells that repopulate the injured tissue
How is the loss of blood cells corrected and what triggers this?
Loss of blood cells is corrected by the proliferation of haematopoietic stem cells in the bone marrow and other tissues, driven by growth factors called colony-stimulating factors, which are produced as the response to the reduced number of blood cells
What are the steps in scar formation that follow tissue injury and the inflammation response?
- Angiogenesis to supply nutrients and oxygen needed to support the repair process. Newly formed vessels are leaky because of incomplete interendothelial junctions and because VEGF, the growth factor that drives angiogenesis, increases vasular permeability
- Formulation of granulation tissue - migration and proliferation of fibroblasts and the deposition of loose connective tissue, together with the vessels and interspersed leukocytes, form granulation tissue. It progressively invades the site of injury
- Remodelling of the connective tissue - maturation and reorganising of the connective tissue produces the stable fibrous scar. The amount of connective tissue increases in the granulation tissue, eventually resulting in a scar
What is the histological appearance of granulation tissue?
Proliferation of fibroblasts and new thin-walled delicate capillaries (angiogenesis) in a loose extracellular matrix, often with admixed inflammatory cells, especially macrophages
How soon after injury do the steps of tissue repair take place?
- Repair begins within 24 hours of injury by the emmigration of fibroblasts and the induction of fibroblast and endothelial cell proliferation.
- By 3-5 days, the specialised granulation tissue that is characteristic of healing is apparent.
What is angiogenesis and when does it happen?
Angiogenesis is the process of new blood vessel development from existing vessels
It is critical:
* in healing at sites of injury
* in the development of collateral circulations at sites of ischaemia
* in allowing tumours to increase in size beyond the constraints of their original blood supply
What are the steps of angiogenesis?
- Vasodilation in response to NO and increased permeability induced by VEGF
- Seperation of pericytes from the abluminal surface and breakdown of the basement membrane to allow formation of a vessel sprout
- Migration of endothelial cells towards the site of tissue injury
- Proliferation of endothelial cells just behind the leading front (‘tip’) of migrating cells
- Remodelling into capillary tubes
- Recruitment of periendothelial cells (pericytes for small vessels, smooth muscle for large) to form the mature vessel
- Suppression of endothelial proliferation and migration and deposition of the basement membrane
The process of angiogenesis involves several signalling pathways, cell-cell interactions, ECM proteins and tissue enzymes.
What growth factors are required (name 5) and what do they do?
- Vascular endothelial growth factors (VEGF), mainly VEGF-A, stimulates both migration and proliferation of endothelial cells, initiating the process of capillary sprouting. It promotes vasodilation by stimulating the production of NO and contributes to the formation of the vascular lumen
- Fibroblast growth factors, mainly FGF-2, stimulates the proliferation of endothelial cells. It also promotes the migration of macrophages and fibroblasts to the area, and stimulates epithelial cell migration to cover epidermal wounds
- Angiopoietins 1 and 2 (Ang 1 & 2) are growth factors that play a role in angiogenesis and the structural maturation of new vessels. Ang1 interacts with an endothelial tyrosine kinase receptor called Tie1
- PGDF recruits smooth muscle cells
- TGF-beta suppresses endothelial cell proliferation and migration, and enhances the production of ECM proteins
What is Notch signalling?
Through ‘cross-talking’ with VEGF, the Notch signalling pathway regulates the sprouting and branching of new vessels and thus ensures that the new vessels that are formed have the proper spacing to effectively supply the healing tissue with blood.
The process of angiogenesis involves several signalling pathways, cell-cell interactions, ECM proteins and tissue enzymes.
What is the function of ECM proteins?
ECM proteins participate in the process of vessel sprouting, largely with interactions with integrin receptors in endothelial cells and providing the scaffold for vessel growth
The process of angiogenesis involves several signalling pathways, cell-cell interactions, ECM proteins and tissue enzymes.
What is the function of the tissue enzymes?
Enzymes in the ECM, notably the matrix metalloproteinases (MMPs), degrade the ECM to permit remodelling and extension of the vascular tube
What are the two steps in the laying down of connective tissue in tissue repair?
- Migration and proliferation of fibroblasts to the site of injury
- The deposition of ECM proteins produced by these cells
The process of laying down connective tissue is orchestrated from what? and where are these produced?
The proceses are orchestrated by locally produced cytokines and growth factors, including PDGF, FGF-2 and TGF-beta.
The major sources for these are inflammatory cells, particularly macrophages activated by the alternative pathway (M2). Mast cells and lymphocytes can also release them
What is the most important cytokine for the synthesis and deposition of the connective tissue proteins?
Transforming growth factor - beta
TGF-beta
Where is TGF-beta produced and what does it do?
- It is produced by most of the granulation tissue, including alternatively activated macrophages.
- It stimulates fibroblast migration and proliferation, increased synthesis of collagen and fibronectin, and decreased degradation of the ECM due to inhibition of metalloproteinases
What factors regulate TGF-beta production?
The levels in tissues are primary regulated not by the transcription of the gene but by the posttranscriptional activation of latent TGF-beta, the rate of secretion of the active molecule, and factors in the ECM (notably integrins), that enhance or diminish TGF-beta activity
What are the processes in which TGF-beta is involved in?
- Scar formation after injury
- Development in fibrosis in the lung, liver and kidney that follow chronic inflammation
- It serves as an anti-inflammatory cytokine that limits and terminates the inflammatory response
How does TGF-beta limit and terminate the inflammatory response?
It inhibits leukocyte production and the activity of other leukocytes
How do fibroblasts change as healing progresses and what is the outcome of this?
As healing progresses, the number of proliferating fibroblasts and new vessels decreases; however, the fibroblasts progressively assume a more synthetic phenotype, and hence there is an increased deposition of ECM.
When does collagen synthesis by fibroblasts begin after injury and how long does it last for?
It begins early in wound healing (3-5 days) and continues for several weeks, depending on the size of the wound.
What is a scar tissue composed of?
Largely inactive, spindle-shaped fibroblasts, dense collagen, fragments of elastic tissue and other ECM components
What happens to the scar as it matures?
There is progressive vascular regression, which eventually transforms the highly vascularised granulation tissue into a pale, largely asvascular scar.
Some of the fibroblasts also acquire features of smooth muscle cells, including the presence of actin filaments, and are called myofibroblasts.
How do myofibrils change a scar over time?
The contribute to the contraction of a scar over time
What affects the outcome of the repair process?
It is influenced by a balance between the synthesis and degradation of the ECM proteins
What family degrades the ECM proteins in scar formation?
The degradation of collagens and other ECM components is accomplished by a family of matrix metalloproteinases (MMPs), so called because they are dependent on metal ions (zinc) for their activity.
What are some examples of some matrix metalloproteinases (MMPs)?
- Interstitial collagenases which cleave fibrillar collagen (MMP-1, -2 and -3)
- Gelatinases which degrade amorphous collagen and firbonectin (MMP-2 and -9)
- Stromelysins which degrade a variety of ECM constituents, including proteoglycans, laminin, fibronectin and amorphous collagen (MMP -3, -10 and -11)
What cells produce MMPs and what regulates them?
- MMPs are produced by a variety of cell types (fibroblasts, macrophages, neutrophils, synovial cells, and some endotheliali cells)
- Their synthesis and secretion is regulated by growth factors, cytokines and other agents
MMPs have to be tightly controlled. What processes control this?
- They are produced as inactive precursors (zymogens) that must first be activated by proteases (e.g.plasmin) likely to be present only at the site of injury
- In addition, activated collagenases can be rapidly inhibited by specific tissue inhibitors of metalloproteinases (TIMPs), produced by most mesenchymal cells.
What are the factors that affect tissue repair?
- Infection prolongs inflammation
- Diabetes prolongs wound healing
- Nutritional status particularly protein deficiency and vitamin C deficiency
- Glucocorticoids (steroids) inhibit TGF-beta production and diminish fibrosis
- Mechanical factors such as increased local pressure or torsion
- Poor perfusion due to either atherosclerosis and diabetes or to obstructed venous damage
- Foreign bodies
- The type and extent of tissue injury - the type of cells, as explained earlier (you know this gurl)
- The location of the injury depends on the type of acute inflammation that happens and some of those repair better than others
What is healing by first intention vs second intention?
- First intention is when the injury only involves the epithelial layer, the principal mechanism of repair is epithelial regeneration
- Second intention is when cell of tissue loss is more extensive, such as in large wounds, abscesses, ulceration and ischaemic necrosis in parenchymal organs, the repair process involves a combination of regeneration and scarring
What are the steps of first intention healing?
- Inflammation
- Proliferation of epithelial and other cells
- Maturation of the connective tissue scar
What happens immediately during healing of first intention?
- Wounding causes the rapid activation of coagulation pathways, which results in the formation of a blood clot on the wound surface.
- In addition to trapped red cells, the clot contains fibrin, fibronectin, and complement proteins.
- The clot serves to stop bleeding and act as a scaffold for migrating cells, which are attracted by growth factors, cytokines and chemokines.
- Release of VEGF leads to increased vascular permeability and oedema.
- As dehydration occurs at the external surface of the clot, a scab covering the wound is formed.
What happens within 24 hours of healing with first intention?
- Neutrophils are seen at the incision margin, migrating towards the fibrin clot. They release proteolytic enzymes that begin to clear the debris.
- Basal cells at the cut edge of the epidermis begin to show increased mitotic activity.
What happens between 24-48 hours of healing with first intention?
- Epithelial cells from both edges have begun to migrate and proliferate along the dermis, depositing basement membrane components as they progress.
- The cells meet in the midline beneath the surface scab, yielding a thin but continuous epithelial layer that closes the wound
What happens by day 3 of healing with first intention?
- Neutrophils have been largely replaced by macrophages, and granulation tissue progressively invades the incision space.
- As you know, macrophages clear extracellular debris, fibrin and other foreign material, promoting angiogenesis and ECM deposition.
- Collagen fibres are now evident at the incision margins.
- Epithelial cells continue to proliferate, forming a covering approaching the thickness of the epidermis
What happens by day 5 of healing with first intention?
- Neovascularisation reaches its peak as granulation tissue fills the incisional space. These new vessels are leaky, allowing the passage of plasma proteins and fluid into the extravascular space. Thus, new granulation tissue is often oedematous.
- Migration of fibroblasts to the site on injury is driven by chemokines, TNF, PDGF, TGF-beta and FGF. Their subsequent proliferation is triggered by multiple growths factors, including PDGF, EGF, TGF-beta and FGF, and the cytokines IL-1 and TNF.
- The fibroblasts produce ECM proteins, and collagen fibrils become more abundant and begin to bridge the incision.
- The epidermis recovers it’s normal thickness as differentiation of surface cells yields a mature epiderminal architechture with surface keratinisation.
What happens during the second week of healing with first intention?
- There is continued collagen accumulation and fibroblast proliferation.
- The leukocyte infiltrate, oedema, and increased vascularity are substantially diminished.
- The process of ‘blanching’ begings, accomplished by increasing collagen deposition within the incisional scar and the regression of the vascular channels
What happens by the end of the first month of healing with first intention?
- The scar comprises a cellular connective tissue largely devoid from inflammatory cells and covered by an essentially normally epidermis
- However, the dermal appendages destroyed in the line of incision are permanently destroyed.
- The tensile strength of the wound increases with time
How does secondary intention differ from primary intention?
- The fibrin clot is larger, there is more exudate and necrotic debris, therefore the inflammatory response is larger and may cause inflammation-mediated injury
- Much larger amounts of granulation tissue, which generally results in a greater mass of scar tissue
- At first, a provisional matrix containing fibrin, plasma fibronectin, and type 3 collagen is formed, but in about 2 weeks this is replaced by a matrix composed primarily by type 1 collagen
- Wound contraction generally plays a larger role in secondary intention by myofibrils. Within 6 weeks, large skin defects may be reduced by 5-10% of their original size
What are the phases of cell cycle?
- G1 - presynthetic growth
- S - DNA synthesis
- G2 - premitotic growth
- M - mitotic
What are quiescent cells? What phase of the cycle are they in?
Cells that are not actively cycling. They are said to be in the G0 phase.
Where can cells enter G1 from?
Either from G0 or after completing a round of mitosis, as for continuously replicating cells.
What regulates the cell cycle?
It is regulated by activators and inhibitors.
- Cell cycle progression is driven by proteins called cyclins and cyclin-associated enzymes called cyclic-dependant kinases (CDKs)
How are cyclins and cyclic-dependant kinases linked?
- CDKs acquire the ability to phosphorylate protein substrates by forming complexes with the relevant cyclins.
- Transiently increased synthesis of a particular cyclin leads to increased kinase activity of the appropriate CDK binding partner.
- As the CDK completes its round of phosphorylation, the associated cyclin is degraded and the CDK activity abades.
- Thus, as cyclin levels rise and fall, the activity of associates CDKs likewise wax and wane.
- The different CDKs happen sequentially, like a relay race
Where are the cell cycle checkpoints and what do they check for?
- The G1-S checkpoint monitors the integrity of the DNA before irreversibly commiting cellular resources to DNA replication.
- The G2-M restriction point ensures that there has been accurate gene replication before the cell actually divides.
What happens if the checkpoints in the cell cycle detect an error in DNA replication?
- When cells do have DNA irregularities, checkpoint activation delays cell cycle progression and triggers DNA repair mechanisms
- If the genetic derangement is too severe to be repaired, the cells will undergo apoptosis.
- Alternatively, they may enter a non-replicative state called senescence - primarily throught the p53-dependant mechanisms
What enzyme ensures the checkpoints in the cell cycle do their job? How does it do that?
It is the job of the CDK inhibitors: they accomplish this by modulating the CDK-cyclin complex activity.
Some examples are p21, p27, and p57
What is the Warburg effect? Why is it important?
- It is the effect to ensure that there are enough resources to perform DNA replication
- It involves increased cellular uptake of glucose and glutamine, increased glycolysis, and (counter-intuitively) decreased oxidate phosphorylation
What are stem cells?
During development, stem cells give rise to all the various differentiated tissues.
In the adult organism, stem cells replace damaged cells and maintain tissue populations as individual cells within them undergo replicative senescence due to attirition of telemeres.
What are two important characteristics of stem cells?
- Self-renewal, which permits stem cells to maintain their numbers
- Asymmetric division, in which one daughter cell enters a differentiation pathway and gives rise to mature cells, while the other remains undifferentiated and retains its self-renewal capacity
What are the two varieties of stem cells?
- Embryonic stem cells are the most undifferentiated. They are present in the inner cell mass of the blastocyst, have virtually limitless cell renewal capacity, and can give rise to every cell in the body. They are totipotent.
- Tissue stem cells are found in intimate association with differential cells a given tissue. They are normally protected within specialised tissue microenvironments called stem cell niches. They have a limited repertoire of differentiated cells they can generate.
Where are mesenchymal stem cells found and what can they differentiate to?
Mesenchymal stem cells are found in bone marrow.
They can differentiate into a variety of stromal cells including chondrocytes (cartilage), osteocytes, adipocytes, and myocytes.
What is a bone fracture?
A fracture is defined as the loss of bone integrity due to mechanical injury and/or diminished bone strength
How can you describe fractures?
- Simple - the overlying skin is intact
- Compound - the bone communicates with the skin surface
- Comminuted - the bone is fragmented
- Displaced - the ends of the bone at the fracture site are not aligned
- Stress - a slowly developing fracture that follows a period of increased physical activity in which the bone is subjected to repetitive loads
- “Greenstick” - extending only partially through the bone, common in infants, where the bone is soft
- Pathological - involving bone weakened by an underlying disease process, such as tumour
What happens in the bone immediately after a fracture?
- Rupture of the blood vessels results in a haematoma, which fills the fracture gap and surround the area of bone injury.
- The clotted blood provides a fibrin mesh, sealing off the fracture site and at the same time creates a framework for the influx of inflammatory cells and ingrowth of fibroblasts and new capillaries.
- Simultaneously, degranulated platelets and migrating inflammatory cells release PDGF, TGF-beta, FGF and other factors, which activate osteoprogenitor cells in the periosteum, medullary cavity, and surrounding soft tissues and stimulate osteoclastic and osteoblastic activity.
What happens in the bone by the end of the first week after a fracture?
- Organisation of the haematoma, matrix production in adjacent tissues, and remodeling of the fractured ends of the bone
- This fusiform and predominantly uncalcified tissue - called soft tissue callus - provides some anchorage between the ends of the fractured bones but not structural rigidity for weight bearing
What happens in the bone by two weeks after a fracture?
- The soft tissue callus is transformed into a bony callus.
- The activated osteoprogenitor cells deposit subperiosteal trabeculae of woven bone that are orientated perpendicular to the cortical axis and within the medullary cortex.
- In some cases, the activated mesenchymal cells also form chondrocytes that make fibrocartilage and hyaline cartilage.
When does the bony cartilage after a bone fracture reach it’s maximal girth?
At the end of the second or third week. It helps stabilise the fracture site.
What happens to the newly formed cartilage after a bone fracture?
The newly formed cartilage along the fracture line undergoes endochondral ossification, forming a contigunous network of bone with newly deposited bone trabeculae in the medulla and beneath the periosteum.
In this fashion, the fracture ends are bridged, and as it mineralises, the stiffness and strength of the callus increases to the point that controlled weight bearing is tolerated.
How does the bony callus disappear in healed bone?
As the callus matures and is subject to weight-bearing forces, the callus portions that are not physically stressed are resorbed.
In this manner, the callus is reduced in size and the shape and the outline of the fractured bone are re-established as lamellar bone
The sequence of events in the healing of a fracture can be easily impeded or blocked. Give some examples of what by…
- Inadequate immobilisation - causes nonunion
- Infection
- Malnutrition and skeletal dysplasia
What is pathological calcification?
The abnormal tissue deposition of calcium salts, together with smaller amounts of iron, magnesium and other mineral salts
What are the two types of calcification?
- Dystrophic calcification occurs despite normal serum levels of calcium and in the absence of derangements of calcium metabolism
- Metastatic calcification - the deposition of calcium in otherwise normal tissues, almost always results from hypercalcaemia secondary to some disturbance in calcium metabolism
In what conditions would dystrophic calcification take place?
It is encountered in areas of necrosis, whether they are of coagulative, caseous or liquefactive type, and in foci of enzymatic necrosis of fat.
Commonly happens in atherosclerosis and heart valves
How does dystrophic calcification present macroscopically and histologically?
- Macroscopically, calcium salts appear as fine, white granules or clumps, often felt as gritty deposits
- Histologically, they have a basophilic, amorphous, granular, sometimes clumped appearance. You can see psammoma bodies, which are mineral deposits around necrotic cells
Are dystrophic and metastatic calcifications causes of organ dysfunction?
- Dystrophic calcification is often a cause of organ dysfunction, such as in calcific valvular disease
- Metastatic calcification if not often a cause of organ dysfunction, but on occasion massive involvement of the lungs produces some respiratory compromise
What are some common causes of transudates?
Heart failure
Liver failure
Kidney failure
Severe nutritional deficiency
What are some causes of increased hydrostatic pressure?
Increases in hydrostatic pressure are mainly caused by disorders that impair venous return. For example:
Congestive heart failure
Constrictive pericarditis
Ascites
Venous obstruction or compression (thrombosis, external pressures e.g. mass, lower extremity inactivity)
It is also sometimes caused by arteriolar dilation. For example in heat or neurohumoral dysregulation
What causes reduced plasma osmotic pressure?
Under normal circumstances albumin accounts for almost half of the total plasma protein; it follows that conditions leading to inadequate synthesis or increased loss of albumin from the circulation commonly cause reduced plasma oncotic pressure.
* Reduced synthesis occurs mainly in severe liver disease and protein malnutrition
* Increased loss occurs in nephrotic syndrome
How does sodium and water retention occur in oedema?
Increased salt retention (with obligate retention of water) causes both increased hydrostatic pressure and diminshes osmotic pressure (due to dilution).
Salt retention occurs whenever renal function is compromised, such as in primary kidney diseases and in cardiovascular diseases that cause hypoperfusion, resulting in the activation of the renin-angiotension-aldosterone axis.
How does lymphatic obstruction cause lymph-oedema?
Trauma, fibrosis, invasive tumours, and infection agents can all disrupt lymphatic vessels and impair the clearance of interstitial fluid, resulting in lymphoedema in the affected part of the body.
What are the different places you can get oedema?
Subcutaneous oedema
Pulmonary oedema
Pulmonary effusions
Peritoneal effusions (ascites)
Brain oedema
How is oedema recognised microscopically?
It is appreciated as clearing and separation of the extracellular matrix and subtle cell swelling
What is hyperemia vs congestion?
Hyperemia and congestion both stem from increased blood volumes within tissues.
Hyperemia is an active process in which arteriolar dilation (inflammation or exercising muscles) leads to increased blood flow. Affected tissues turn red (erythema) because of increased delivery of oxygenated blood
Congestion is a passive process resulting from reduced outflow of blood from a tissue. It can be systemic (heart failure) or localised (isolated venous obstruction)
What does chronic congestion lead to and why?
As a result of increased hydrostatic pressures, congestion commonly leads to edema.
The associated chronic hypoxia may lead to ischaemic tissue injury and scarring.
Capillary rupture can also produce small haemorrhagic foci; subsequent catabolism of extravasated red cells can leave clusters of haemosiderin-laden macrophages
Microscopically, what will you see in acute pulmonary congestion vs chronic pulmonary congestion?
- Acute pulmonary congestion exhibits engorged alveolar capillaries, alveolar septal emboli, and focal intraalveolar haemorrhage
- In chronic pulmonary congestion (often caused by congested heart failure) the septa are thickened and fibrotic, and the alveoli often contain numerous haemosiderin-laden macrophages called heart failure cells
Microscopically, what will you see in acute hepatic congestion vs chronic hepatic congestion?
- In acute hepatic congestion, the central vein and sinusoids are distended. Because the centrilobular area is distal end of the hepatic blood supply, centrilobular hepatocytes may undergo ischaemic necrosis with the periportal hepatocytes may only develop fatty change (better oxygenated)
- In chronic passive hepatic congestion, the centrilobular regions are grossly red-brown and slightly depressed and are accentuated against the surrounding zones of uncongested tan liver (nutmeg liver).
What is haemostasis?
A precisely organised process involving platelets, clotting factors, and endothelium that occurs at the site of the vascular injury and culminates in the formation of a blood clot, which serves to prevent or limit the extent of bleeding
What two groups are abnormal haemostasis divided into?
- Haemorrhagic disorders - characterised by excessive bleeding, haemostatic mechanisms are either blunted or insufficient to prevent abnormal blood loss.
- Thrombotic disorders - blood clots (thrombi) form within intact blood vessels or within the chambers of the heart.
Are thrombotic and haemorrhagic disorders always separate entities?
No, sometimes in generalised activation of clotting paradoxically produces bleeding due to the consumption of coagulation factors, as in DIC.
What are the four steps in haemostasis?
1) Arteriolar vasoconstriction
2) Primary haemostasis - the formation of the platelet plug
3) Secondary haemostasis - deposition of the fibrin
4) Clot stabilisation and resorption
What happens in arteriolar vasoconstriction during haemostasis?
- It occurs immediately and markedly reduced blood flow to the injured area.
- It is mediated by reflex neurogenic mechanisms and augmented by the local secretion of factors such as endothelin, a potent endothelium-derived vasoconstrictor.
- This effect is transient and bleeding would continue if it wasn’t for the next steps
What happens in primary haemostasis - formation of the platelet plug?
- Disruption of the endothelium exposes subendothelium vWF and collagen, which promote platelet adherence and activation.
- Activation of platelets results in a dramatic shape change (from small rounded discs to flat plates with spiky protrusions that markedly increase surface area), as well as the release of secretory granules (ADP, TxA2)
- Within minutes, the secreted products recruit additional platelets, which undergo aggregation to form a primary haemostatic plug
What happens in secondary haemostasis - deposition of fibrin?
- Tissue factor is exposed at the site of injury. Tissue factor is a membrane-bound procoagulant glycoprotein that is normally expressed by subendothelial cells in the vessel wall, such as smooth muscle cells or fibroblasts.
- Tissue factor binds and activates factor VII, setting in motion a cascade of reactions that generates thrombin.
- Thrombin cleaves circulating fibrinogen into insoluble fibrin, creating a fibrin mesh-work, and is also a potent activator of platelets, leading to additional platelet aggregation at the site of injury.
What happens during clot stabilisation and resorption during haemostasis?
- Polymerised fibrin and platelet aggregates undergo contraction to form a solid, permanent plug that prevents further haemorrhage.
- At this stage, counteregulatory mechanisms (e.g. tissue plasminogen factor t-PA) are set into motion that limit clotting to the side of injury and eventually lead to clot resorption and tissue repair.
What are platelets?
Disc-shaped anucleate cell fragments that are shed from megakaryocytes in the bone marrow into the bloodstream.
What do platelets need to function?
Their function depends on several glycoprotein receptors, a contractile cytoskeleton, and two types of cytoplasmic granules.
What cytoplasmic granules do platelets rely on to function?
- α-granules have the adhesion molecule P-selectin on their membranes and contain proteins involved in coagulation, such as fibrinogen, coagulation factor V, and vWF, as well as protein factors that may be involved in wound healing, such as fibronectin, platelet factor 4 (a heperin-binding chemokine), platelet-derived growth factor (PDGF), and transforming growth factor-β.
- Dense (or δ) granules contain ADP and ATP, ionised calcium, serotonin, and adrenaline
After a traumatic vascular injury, platelets encounter consistuents of the subendothelial connective tissue, such as vWF and collagen. On contact with these proteins, platelets undergo a sequence of reactions the culminate the formation of a platelet plug. What are they?
- Platelet adhesion
- Platelets rapidly change shape
- Secretion of granules content
- Platelet aggregation
How does platelet adhesion take place in plug formation?
- Platelet adhesion is mediated largely by interactions with vWF, which acts as a bridge between the platelet surface receptor glycoprotein 1b (Gp1B) and exposed collagen.
- Notably, the genetic deficiencies of vWF or Gp1B (Bernard-Soulier syndrome) result in bleeding disorders.
What happens when the platelets change shape in plug formation?
- The platelets change shape following adhesion, being converted from smooth discs to ‘sea urchins’ with greatly increased surface area.
- This change is accompanied by alterations in glycoprotein IIb/IIIa that increase it’s affinity for fibrinogen, and by the translocation of the negatively charged phospholipids (particularly phosphatidylserine) to the platelet surface.
- These phospholipids bind calcium and serve as nucleation sites for the assembly of coagulation factor complexes.
What is platelet activation in the plug formation? What triggers it?
Secretion of the granule contents occurs along with the changes in shape; these two events together are known as activation.
- Platelet activation is triggered by a number of factors, including the coagulation factor thrombin and ADP.
- Thrombin activates platelets through a special type of G-protein coupled receptor, protease-activated receptor (PAR), which is switched on by a proteolytic cleavage by thrombin.
- ADP is a component of dense body granules; thus, platelet activation and ADP release begets additional rounds of platelet activation, a phenomenon known as recruitment.
- Activated platelets also produce the prostaglandin thromboxane A2 (TxA2), a potent inducer of platelet aggregation.
What happens during platelet aggregation?
- Platelet aggregation follows their activation.
- The conformational change in glycoprotein 2b/3a that occurs with platelet activation allows a binding of fibrinogen, a large bivalent plasma polypeptide that forms bridges between adjacent platelets, leading to their aggregation
- The inital wave of aggregation is reversible, but concurrent activation of thrombin stabilises the platelet plug by causing further platelet activation and aggregation, and by promoting irreversible platelet contraction
- Platelet contraction is dependent of the cytoskeleton and consolidates the aggregated platelets.
- In parallel, thrombin also converts fibrinogen into insoluble fibrin, cementing platelets in place and creating the definitely secondary hemostatic plug.
- Entrapped red cells and leukocytes are also found in haemostatic plugs, in part due to adherence of leukocytes to P-selectin expressed on activated platelets
What is the coagulation cascade?
It is a series of amplifying enzymatic reactions that leads to the deposition of an insoluble fibrin clot.
What does each step in the coagulation cascade have in common? Where do they happen?
Each reaction step involves an enzyme (an activated coagulation factor), a substrate (an inactive proenzyme form of a coagulaton factor), and a cofactor (a reaction accelerator).
These components are assembled on a negatively charged phospholipid surface, provided by activated platelets.
Which ion and vitamin are important in the coagulation cascade?
- Calcium ions - assembly of reaction complexes depend on calcium, which binds to γ-carboxylated glutamic acid residues that are present in factors
- The enzymatic reactions that produce γ-carboxylated glutamic acid use Vitamin K as a cofounder and are antagonised by many drugs.
What are the two pathways in the coagulation cascade? What do we use to assess them?
Extrinsic and Intrinsic Pathways
* The prothrombin time (PT) assay assess the function of the proteins in the extrinsic pathway (factors VII, X, V, II and fibrinogen). In brief, tissue factor, phospholipids, and calcium are added to plasma and the time for a fibrin clot to form is recorded
* The partial thromboplastin time (PTT) assay screens the function of the proteins in the intrinsic pathway (factors XII, XI, IX, VIII, X, V, II, and fibrinogen). In this assay, clotting of plasma is initiated by addition of negative-charged particles (e.g. ground glass) that activate factor XII (Hageman factor) together with the phospholipids and calcium, and the time to fibrin clot formation is recorded
What do the deficiencys in the different clottin factors lead to? What does this show?
- Deficiencies in factors V, VII, VIII, IX and X are associated with moderate to severe bleeding disorders
- Prothrombin deficiency is incompatible with life
- Factor XI deficiency is only associated with mild bleeding
- Individuals with factor XII deficiency do not bleed and are in fact susceptible to thrombosis
This shows that the process in vivo and in the lab are different!
Which is the most important coagulation factor?
Thrombin, because its various enzymatic activities control divers aspects of haemostasis and link clotting to inflammation and repair
What are thrombin’s most important roles?
- Conversion of fibrinogen into crosslinked fibres - Thrombin directly converts soluble fibrinogen into fibrin monomers that polymerise into an insoluble clot, and also amplifies the coagulation process, not only by activating factor XI, but also by activating factors V and VIII.
- Stabilises the secondary haemostatic plug by activating factor XIII, which covalently cross-links fibrin.
- Platelet activation - thrombin in a potent inducer of platelet activation, and aggregation through its ability to activate PARs, therefore linking platelet function to coagulation
- Pro-inflammatory effects - PARs are also expressed on inflammatory cells, endothelium, and other cell types, and activation of these receptors by thrombin is believed to mediate proinflammatory effects that contribute to tissue repair and angiogenesis
- Anticoagulant effects - upon encountering normal endothelium, thrombin changes from a procoagulant to an anticoagulant. This reversal in function prevents clotting from extending beyond the site of vascular injury
Once initiated, coagulation must be restricted to the site of vascular injury. What are the factors that limit coagulation?
- Simple dilution - blood flowing past the site of injury washes out activated coagulation factors, which are rapidly removed by the liver
- The requirement for negatively charged phospholipids - which are mainly provided by platelets that have been activated by contact with the subendothelial matrix at sites of vascular injury.
- Factors that are expressed by intact endothelium adjacent to the injury
Activation of the coagulation cascade also sets into motion the fibrinolytic cascade that limits the size of the clot and contributes to it’s later dissolution. Tell me about this process…
- Fibrinolysis is largely accomplished through the enzymatic activity of plasmin, which breaks down fibrin and interferes with its polymerisation.
- There is an elevated level of breakdown products of fibrinogen (often called fibrin split products) e.g. d-dimer.
- Plasmin is generated by enzymatic catabolism of the inactive circulating precursor plasminogen, either by a factor XII-derived pathway or by plasminogen activators e.g. tPA.
- Once activated, plasmin is tightly controlled by counterregulatory factors such as α2-plasmin inhibitor, a plasma protein that binds and rapidly inhibits plasmin.
Normal endothelial cells express a multitude of factors that inhibit the procoagulant activities of platelets and coagulation factors and that augment fibrinolysis. What are the different antithrombotic properties of endothelium?
- Platelet inhibitory effects - Intact epithelium acts as a barrier that shields platelets from subendothelial vWF and collagen. It also releases NO, prostacyclin (PGI2), and ADP which inhibit platelet activation and aggregation. They also bind and alter the activity of thrombin, which is a potent activators of platelets.
- Anticoagulant effects - Intact epithelium shields coagulation factors from tissue factor. It also expresses multiple factors that actively oppose coagulation, such as thrombomodulin, endothelial protein C receptor, heparin-like molecules, and tissue factor pathway inhibitor.
- Fibrinolytic effects - normal endothelial cells synthesise t-PA, a key component of the fibrinolytic pathway
How do thrombomodulin and endothelial protein C receptor have anticoagulant effects?
They bind thrombin and protein C, respectively, in a complex on the endothelial cell surface. When bound in this complex, thrombin loses it’s ability to activate coagulation factors and platelets, and instead cleaves and activated protein C, a vitamin K-dependent protease that requires a cofactor, protein S. Activated protein C/protein S complex is a potent inhibitor of coagulation factors Va and VIIIa
How do Heparin-like molecules have anticoagulant properties?
Heparin-like molecules on the surface of endothelium bind and activate anti-thrombin III, which then inhibits thrombin and factors IXa, Xa, XIa, and XIIa.
The clinical utility of heparin and related drugs is based on their ability to stimulate antithrombin III activity.
How does tissue factor pathway inhibitor have anticoagulant properties?
TFPI, like Protein C, requires Protein S as a cofactor, and binds and inhibits tissue factor/factor VIIa complexes
What are the very broad causes of haemorrhagic disorders?
- Primary or secondary defects in vessel walls e.g. aortic dissection or arterial ruptures
- Platelet disorders
- Coagulation factor disorders e.g. haemophilia
How do defects of primary haemostasis present? Give an example
- Platelet defects or von Willebrand disease
- Often present with small bleeds in skin or mucosal membranes, which take the form of petechiae or purpura
- The capillaries of the mucosa and skin are more prone to rupture following minor trauma and under normal circumstances platelets seal these defects virtually immediately.
- Mucosal bleeding associated with disorders of primary haemostasis may present with epistaxis, GI bleeding, or menorrhagia. A fatal complication is intracerebral haemorrhage
What are petechiae, purpura and ecchymoses?
- Petechiae - minute 1-2mm haemorrhages
- Purpura - slightly larger (>3mm) haemorrhages
- Ecchymoses - haemorrhages of 1-2 cm in size
How do defects of secondary haemostasis present? Give an example
- Coagulation factor defects e.g. haemophilia
- Often present with bleeding into soft tissue (muscles) and joints
- Haemarthrosis after minor trauma is particularly characteristic of haemophilia
What does the clinical significance of haemorrhage depend on?
- Volume of the bleed
- Rate at which it occurs
- Location
What percentage of blood loss may lead to haemorrhagic shock?
Over 20%
What are the primary abnormalities that lead to thrombosis? (Virchow’s triad)
- Endothelial injury
- Stasis or turbulent blood flow
- Hypercoagulability of the blood
Where in the body is endothelial injury the most common cause of thrombosis and why?
Endothlial injury leading to platelet activation almost inevitably underlies thrombus formation in the heart and the arterial circulation, where the high rates of blood flow impede clot formation.
These clots are typically rich in platelets, which is why aspirin is good.
What is endothelial activation or dysfunction?
Severe endothelial injury may trigger thrombosis by exposing vWF and tissue factor. However, inflammation and other noxious stimuli also promote thrombosis by shifting the pattern of gene expression in endothelium to one that is ‘prothrombotic’. This is called endothelial activation or dysfunction.
It can be caused by physical injury, infection, abnormal blood flow, inflammatory mediators, metabolic abnormalities and toxins
What are the main endothelial changes that occur during endothelial activation?
- Procoagulant changes - endothelial cells activated by cytokines downregulate the expression of thrombomodulin, which results is sustained action of thrombin, which stimulates platelets and augments inflammation through PARs expressed on platelets and inflammatory cells. Endothelial cells also downregulate the expression of other anticoagulants, such as protein C and tissue factor pathway inhibitor
- Antifibrinolytic effects - activated endothelial cells secrete plasminogen activator inhibitors (PAIs), which limit fibrinolysis, and downregulate the expession of t-PA
What is laminar flow?
Normal blood flow is laminar such that the platelets (and other blood cellular elements) flow centrally in the vessel lumen, separated from endothelium by a slower moving layer of plasma
How do stasis and turbulent flow cause endothelial injury and thrombosis?
- Promote endothelial activation, enhancing procoagulant activity and leukocyte adhesion, in part through flow-induced changes in the expression of adhesion molecules and pro-inflammatory factors
- Disrupt laminar flow and bring platelets into contact with the endothelium
- Prevent washout and dilution of activated clotting factors by fresh flowing blood and the inflow of clotting factor inhibitors
What are some of the causes of stasis and turbulent blood flow in vessels?
- Ulcerated atherosclerotic plaques not only expose subendothelial vWF and tissue factor but also cause turbulence
- Aortic and arterial dilatations (aneurysms) result in local stasis
- Acute MIs are associated with areas of noncontractile myocardium and cardiac aneurysms, both are associated with stasis and flow abnormalities
- Hyperviscosity such as in polycythaemia vera causes increased resistance to flow and stasis
- Deformed red cells in sickle cell anaemia impede blood flow through small vessels, resulting in stasis
What is hypercoagulability? What groups can it be split into
Also called thrombophilia, hypercoagulability can be loosely defined as any disorder of the blood that predisposes to thrombosis.
It can be divided into primary (genetic) and secondary (acquired) disorders
What are some primary causes of hypercoagulability?
Common
* Factor V mutation (Arg to Glu substitution in amino acid residue 506 leading to resistance to factor C; Factor V Leiden
* Prothrombin mutation (G20210A noncoding sequence variant leading to increased prothrombin levels)
* Increased levels of factors VIII, IX, XI or fibrinogen
Rare
* Antithrombin III deficiency
* Protein C deficiency
* Protein S deficiency
Very rare
* Fibrinolysis defects
* Homozygous homocystinuria
How do the following secondary causes of hypercoagulability cause it?
* oral contraceptive
* disseminated cancers
* old age
- Oral contraceptive - increased hepatic synthesis of coagulation factors and reduced anticoagulant synthesis
- Disseminated cancers - release of various pro-coagulants from tumours
- Older age - reduced endothelial PGI2 production
What are some secondary causes of hypercoagulability?
High risk for thrombosis
* prolonged bed rest or immobilisation
* MI
* AF
* Tissue injury (fracture, burn, surgery)
* Cancer
* Prosthetic cardiac valves
* DIC
* Heparin-induced thrombocytopaenia
* Antiphospholipid syndrome
Lower risk for thrombosis
* Cardiomyopathy
* Nephrotic syndrome
* Hyperestrogenic states (pregnancy, oral contraceptive)
* Sickle cell anaemia
* Smoking
What is Heparin-Induced Thrombocytopaenic Syndrome?
- HIT occurs following the administration of unfractionated heparin, which may induce the appearance of antibodies that recognise complexes of heparin and platelet factor 4 on the surface of platelets, as well as complexes of heparin-like molecules and platelet factor 4-like molecules on endothelial cells.
- Binding of these antibodies to platelets results in their activation, aggregation and consumption. This produces a pro-thrombotic state.
- It happens less in LMWH
How may antiphospholipid antibody syndrome present?
- Recurrent thromboses
- repeated miscarriages
- cardiac valve vegetations
- thrombocytopaenia
- pulmonary hypertension from recurrent subclinical PEs
- stroke
- bowel infarction
- renovascular hypertension
What are the two types of antiphospholipid antibody syndrome?
- Secondary antiphospholipid antibody syndrome is when it occurs with individual with a well-defined autoimmune disease, such as SLE
- In primary antiphospholipid antibody syndrome, patients exhibit only the manifestations of a hypercoagulable state and lack evidence of other autoimmune disorders; occasionally it comes following drugs or an infection
Which way to arterial and venous thrombi occur and which ways do they grow?
- Arterial or cardiac thrombi usually begin at sites of turbulence and tend to grow retrograde
- Venous thrombi often begin at sites of stasis and extend in the direction of blood flow
What is a line of Zahn? Where does it happen and what does it mean?
- Thrombi often have grossly and microscopically apparent laminations called lines of Zahn, which are pale platelet and fibrin deposits alternating with darker red cell-rich layers
- They signify that a thrombus formed in flowing blood; they can therefore distinguish post-mortem to ante-mortem clots
What are mural thrombi?
Thrombi that form in heart chambers or in the aortic lumen.
What is different in the content or arterial vs venous clots?
Arterial thrombi often consist of a friable meshwork of platelets, fibrin, red cells, and degenerating leukocytes
Venous thrombi contain more enmeshed red cells and relatively few platelets - they are therefore known as red or stasis thrombi
If a patient survive the initial thrombus, in the ensuing days to weeks, thrombi can have one of four processes happen. What are they?
- Propagation - thrombi accumulate additional platelets and fibrin
- Embolisation - thrombi dislodge and travel to other sites in the vasculature
- Dissolution - the result of fibrinolysis, which can lead to shrinkage and total disappearance of the recent thrombi - only happens in the first few hours
- Organisation and recanalisation - older thrombi become organised by the ingrowth of endothelial cells, smooth muscle cells and fibroblasts. Capillary channels eventually form that reestablish the continuity of the original lumen, albiet to a variable degree. Eventually, with remodeling and contraction of the mesenchymal elements, only a fibrous lump may remain
What veins do superficial and deep venous thrombosis affect?
Superficial - in the saphenous veins - rarely embolise but same symptoms as DVT
Deep - in the popliteal, femoral or iliac veins - often embolise
What happens in DIC?
Widespread formation of thrombi in the microcirculation. These microvascular thrombi can cause diffuse circulatory insufficiency and organ dysfunction, particularly of the brain, lungs, heart and kidneys.
The runaway thrombosis ‘uses up’ platelets and coagulation factors and often activates fibrinolytic mechanisms . Thus, symptoms initially related to thrombosis can evolve into a bleeding catastrophe, such as haemorrhagic stroke or hypovolaemic shock.
What is an embolism?
An embolus is a detached intravascular solid, liquid, or gaseous mass that is carried by the blood from it’s point of origin to a distant site, where it often causes tissue dysfunction or infarction.
What is the most common thromboembolic disease?
Pulmonary emboli from the DVT
Where do pulmonary emboli travel?
Fragmented thrombi from DVTs are carried through progressively larger veins and the right side of the heart before slamming into the pulmonary arterial vasculature.
Depending on the size of the embolus, it can occlude the main pulmonary artery, straddle the pulmonary artery bifurcation (staddle embolus), or pass out into the smaller branching arteries.
Once you have one PE, what happens to your risk?
It increases significantly.
Frequently there are multiple emboli, occuring either sequentially or simultaneously as a shower of small emboli from a single large mass
What is a paradoxical embolism?
Rarely, a venous embolus passes through an interatrial or interventricular defect and gains access to the systemic arterial circulation.
At what percentage occlusion does a PE cause sudden death or Cor Pulmonale?
When emboli obstruct 60% or more of the pulmonary circulation
Why does embolic obstruction of medium-sized pulmonary arteries with subsequent vascular rupture not often cause an infarct? Why is this different in small end-arteriolar pulmonary branches
Because the lung is supplied by both the pulmonary arteries and the bronchial arteries, and an intact bronchial circulation is usually sufficient to perfuse the attached area.
This is not the case in small end-arteriolar pulmonary branches, so they often produce haemorrhage or infarction
Where do most systemic thromboembolisms come from?
80% arise from intracardiac mural thrombi, two thirds of which are associated with left ventricular wall infarcts and one quarter with left atrial dilatation and fibrillation.
The remainder originates from aortic aneurysms, atherosclerotic plaques, valvular vegetations, or venous thrombi
Where do systemic thromboembolisms often end up?
75% in the lower extremities
10% in the brain
May also involve the intestines, kidneys, spleen, and upper extremities
How do fat and marrow emboli occur?
Microscopic fat globules - sometimes associated with marrow- can by found in the pulmonary vasculature after fractures of long bones, soft tissue trauma and burns
Presumably these injuries rupture vascular sinusoids in the marrow or small venules, allowing marrow and adipose tissue to herniate into the vascular space and travel to the lung.
Do fat embolisms often give symptoms?
Fat embolism occurs in 90% of invidiuals with severe skeletal injuries, but less than 10% have any clinical findings.
What is fat embolism syndrome?
The term applied to the minority of patients who become symptomatic.
Characterised by pulmonary insufficiency, neurologic symptoms, anaemia and thrombocytopaenia, and is fatal in about 5 to 15% of cases
What are the symptoms of fat embolism syndrome?
Typically, 1 to 3 days after injury, there is a sudden onset of tachypnoea, dyspnoea, and tachycardia; irritability and restlessness can progress into delirium or come
Why does thrombocytopaenia occur in fat embolism syndrome?
It is attributed to platelet adhesion to fat globules and subsequent aggregation or splenic sequestration; anaemia can result from similar red cell aggregation and/or haemolysis.
A diffuse petechial rash is related to rapid onset of thrombocytopaenia and can be a useful diagnostic feature.
What is the pathogenesis of fat embolism syndrome?
- Fat microemboli and associated red cell and platelet aggregates can occlude the pulmonary and cerebral microvasculature.
- Release of fatty acids from the fat globules exacerbated the situation by causing local toxic injury to endothelium, and platelet activation and granulocyte recruitment (with free radical, protease, and eicosanoid release) complete the vascular assault.
What is an air embolism?
Gas bubbles within the circulation can coalesce to form frothy masses that obstruct vascular flow and cause distal ischaemic injury.
What volume of air is required to have an effect on the pulmonary circulation?
Generally more than 100cc
What is decompression sickness? Why does it happen?
It occurs when an individual experiences sudden decrease in atmospheric pressure.
When air is breathed at high pressure, increased amounts of gas (particularly nitrogen) are dissolved in the blood and tissues. If the diver then ascends too rapidly, the nitrogen comes out of the solution in the tissues and blood
What happens if you get an air embolus in your lungs?
Gas bubbles in the vasculature cause oedema, haemorrhage and focal atelectasis or emphysema, leading to a form of respiratory distress called the chokes
What happens in caisson disease? Who does it happen to?
Persistence of gas emboli in the skeletal system causes multiple foci of ischaemic necrosis; common in the femoral head, tibia and humeri
Happens in chronic decompression sickness, like workers in pressurised vehicles
What is an amniotic fluid embolis? What is it’s mortality?
It is the infusion of amniotic fluid or fetal tissue into the maternal circulation via a tear in the placental membranes or rupture of uterine veins.
It accounts for roughly 10% of maternal deaths in the US, the mortality rate is up to 80% and 85% of survivors are left with permanent neurological damage
What are the symptoms of an amniotic fluid embolus?
The onset is characterised by sudden severe dyspnoea, cyanosis and shock, followed by neurological impairment ranging from headache to seizures.
If the patient survives the initial crisis, pulmonary oedema typically develops, frequently accompanied by DIC.
How does an amniotic fluid embolus vary from a PE?
Much of the morbidity and mortality of an amniotic fluid embolism may stem from the biochemical activation of coagulation factors and components of the innate immune system by substances in the amniotic fluid, rather than the mechanical obstruction of a PE
What is an infarct?
An infarct is an area of ischaemic necrosis caused by occlusion or either the arterial supply or venous drainage.
Arterial thrombosis or arterial embolism underlies the vast majority of infarctions
How can infarcts be classified? Tell me about both groups
-
Red infarcts occur
1) with venous occlusions (e.g. testicular torsion)
2) in loose, spongy tissues (e.g. lung), where blood can collect in the infarcted zone
3) in tissues with dual circulations (e.g. lung and intestine) that allow to blood to flow from an unobstructed parallel supply in a necrotic zone
4) in tissues previously congested by sluggish venous outflow
5) when flow is reestablished to a site of previous arterial occlusion and necrosis - White infarcts occur with arterial occlusions in solid organs with end-arterial circulation (e.g. heart, spleen and kidney), and where tissue density limits the seepage of blood from adjoining capillary beds into the necrotic area.
What shape are infarcts?
Infarcts tend to be wedge-shaped, with the occluded vessel at the apex and the periphery of the organ forming the base.
How does the morphology differ over the first few days and based on the location of infarcts?
- When the base is a serousal surface, there may be an overlying fibrinous exudate resulting from an acute inflammatory response to mediators release from injured and necrotic cells
- Fresh infarcts are poorly defined and slightly haemorrhagic, but over a few days their margins become better defined by a narrow rim of congestion attributable to inflammation
Why are some infarcts red and some white?
- Over time, infarcts resulting from arterial occlusion without a dual blood supply typically become progressively paler and more sharply defined - white infarcts
- Extravasated red cells in haemorrhagic infarcts are phagocytosed by macrophages, which convert heme iron to haemosiderin, extensive haemorrhage can leave a firm, brown, haemosiderin-rich residuum.
What are septic infarctions?
They occur when infected cardiac valve vegetations embolise or when microbes seed necrotic tissue.
In these cases, the infarct is converted into an abscess, with a corresponding greater inflammatory response
What are the factors that influence development of an infarct?
- Anatomy of the vascular supply - is there an alternative blood supply?
- Rate of occlusion - slowly developing occlusions are less likely to cause infarction, because they provide time for collateral pathways of perfusion to form.
- Tissue vulnerability to hypoxia - neurons undergo irreversible damage within 3-4 minutes, myocardial cells last 20-30 minutes, fibroblasts within myocardium can last hours
- Hypoxaemia - the lower the oxygen, the more likely the infarct
What is shock?
Shock is a state in which diminished cardiac output or reduced effective circulating blood volume impairs tissue perfusion and leads to cellular hypoxia
What are the three main types of shock? Why do they happen? What are the other types of shock?
1) Cardiogenic shock results from low cardiac output due to myocardial pump failure. This can be due to intrinsic myocardial damage (infarction), ventricular arrythmias, extrinsic compression (tamponade), or outflow obstruction (PE)
2) Hypovolaemic shock results from low cardiac output due to low blood volume, such as can occur with massive haemorrhage or fluid loss from severe burns
3) Shock associated with systemic inflammation may be triggered by microbial infections, burns, trauma or pancreatitis. The common pathogenic feature is a massive outpouring of inflammatory mediators from innate and adaptive immune cells that produce vasodilation, vascular leaking and venous blood pooling. These result in tissue hypoperfusion, cellular hypoxia and metabolic derangements
4) Less commonly, shock can be neurogenic or anaphylactic, where acute vasodilation leads to hypotension and tissue hypoperfusion
What are most common triggers for septic shock?
Gram-positive bacterial infections, followed by gram-negative bacterial infections and then funghi
In broad terms, what are the factors believed to play a major role in the pathophysiology of septic shock?
- Inflammatory and counter-inflammatory responses
- Endothelial activation and injury
- Induction of a procoagulant state
- Metabolic abnormalities
- Organ dysfunction
What inflammatory responses have a play in the pathophysiology of septic shock?
- Microbial cell wall consituents engage receptors on innate immune cells.
- Likely initiators of inflammation are signalling pathways that lie downsteam of TLRs (which recognise PAMPs), and G-protein coupled receptors that detect bacterial pathogens.
- Upon activation, innate immune cells produce TNF,IL-1, IFN-γ, IL-12 and IL-18
- ROS, platelet activating factor and prostaglandins are also elaborated
- These effector molecules induce endothelial cell activation to upregulate adhesion molecule expression and further stimulate chemokines and cytokines production.
How is the complement cascade involved in the pathophysiology of septic shock?
- The complement cascade is also activated by microbial components, directly and indirectly by the proteolytic activity of plasmin
- This results in the production of anaphylotoxins (C3a, C5a), chemotactic fragments (C5a) and opsonins (C3b), all of which contribute to the pro-inflammatory state
How does the hyperinflammatory state initiated by sepsis also activate counter-regulatory immunosuppressive mechanisms? What does this mean?
- Septic patients may oscillate between hyperinflammatory and immunosuppressed states during their clinical course.
- Proposed mechanisms for the immune suppression include a shift from pro-inflammatory TH1 cells to anti-inflammatory TH2 cells, production of anti-inflammatory mediators (e.g. soluble TNF receptor, IL-1 receptor antagonist, and IL-10), lymphocyte apoptosis, the immunosuppressive act of apoptotic cells, and the induction of cellular energy
How does endothelial activation and injury contribute to the pathophysiology of septic shock?
- It leads to widespread vascular leakage and tissue oedema, which have deleterious effects on both nutrient delivery and waste removal.
- The oedema created impedes tissue perfusion
- Activated endothelium also upregulates production of NO and other vasoactive inflammatory mediators (e.g. C3a, C5a, and PAF), which may contribute to vascular smooth muscle relaxation and systemic hypotension
How does the induction of a procoagulant state contribute to the pathophysiology of septic shock?
- Pro-inflammatory cytokines increase tissue factor production by monocytes and possibly endothelial cells as well, and decrease the production of endothelial anti-coagulant factors (e.g. TFPI, thrombomodulin and protein C)
- They also dampen fibrinolysis by increasing plasminogen activator inhibitor 1 expression.
- The vascular leak and vasodilation causes stasis.
- All of these things together lead to systemic activation of thrombin and the deposition of fibrin-rich thrombi in small vessels throughout the body, often compromising tissue perfusion
How do metabolic abnormalities contribute to the pathophysiology of septic shock?
- Septic patients exhibit insulin resistance and hyperglycaemia.
- Cytokines such as TNF and IL-1, and stress hormones (glucagon, GH, and glucocorticoids) and catecholamines all drive gluconeogenesis.
- Pro-inflammatory cytokines suppress insulin release whilst promoting insulin resistance in the liver and other tissues, likely by impairing the expression of GLUT-4.
- Hyperglycaemia reduced neutrophil function - and causes increased endothelial cell adhesion molecule expression
- Cellular hypoxia and diminished oxidative phosphorylation leads to increased lactate production and lactic acidosis
How does organ dysfuction contribute to the pathophysiology of septic shock?
- Systemic hypotension, interstitial oedema and small vessel thrombosis all decrease the delivery of oxygen and nutrients to the tissue, which fail to properly utilise those nutrients that are delivered due to cellular hypoxia.
- High levels of cytokines and secondary mediators diminish myocardial contractility and cardiac output, and increased vascular permeability and endothelial injury can lead to the acute respiratory distress syndrome.
What affects the outcomes and severity of septic shock?
- The extent and virulence of the infection
- the immune status of the host
- the presence of other co-morbid conditions
- the pattern and level of mediation production
What are the three general phases of hypovolaemic and cardiogenic shock?
- An initial nonprogressive phase during which reflex compensatory mechanisms are activated and perfusion of vital organs is maintained
- A progressive stage characterised by tissue hypoperfusion and onset of worsening circulatory and metabolic imbalances, including a lactic acidosis
- An irreversible stage that sets in after the body has incurred cellular and tissue injury so severe that even if the haemodynamic defects are corrected, survival is not possible
What are the neurohumeral mechanisms that occur in the early non-progressive stage of shock that help to maintain cardiac output and blood pressure?
- Baroreceptor reflexes, catecholamine release, activation of the renin-angiotensin axis, ADH release, and generalised sympathetic stimulation.
- The net effect is tachycardia, peripheral vasoconstriction, and renal conservation of fluid
- Coronary and cerebral vessels are less sensitive to the sympathetic response and therefore maintain relatively normal caliber, blood flow and oxygen delivery
What happens during the progressive phase of shock?
- There is widespread tissue hypoxia
- In the setting of persistent oxygen deficit, intracellular aerobic respiration is replaced by anaerobic glycolysis with excessive production of lactic acid
- The lactic acidosis lowers the tissue pH and blunts the vasomotor response, arterioles dilate, and blood begins to pool in the microcirculation
- Peripheral pooling not only worsens the cardiac output, but also put the endothelial cells at risk of developing anoxic injury with subsequent DIC.
What happens during the irreversible stage of shock?
- Widespread cell injury is reflected in lysosomal enzyme leakage, further aggrevating the shock state.
- If ischaemic bowel allows intestinal flora into the circulation, bacterial septic shock may be superimposed
- At this point, the patient may develop anuria as a result of acute tubular necrosis and acute renal failure
- They may develop a coagulopathy
What are the morphological adrenal changes during shock?
There is cortical cell depletion. This does not reflect adrenal exhaustion but rather the conversion of the relatively inactive vacuolated cells to metabolically active cells that utilise stored lipids for the synthesis of steroids
Immunity is protection from infectious pathogens. The mechanisms of defense against microbes fall into two broad categories. What are they?
- Innate immunity refers to the mechanisms that are ready to react to infections even before the occur, and that have evolved to specifically recognise and combat microbes
- Adaptive immunity consists of mechanisms that are stimulated by microbes and are capable of recognising microbial and non-microbial substances.
When do innate immunity and adaptive immunity take place?
- Innate immunity is the first line of defence. It is mediated by cells and molecules that recognise products of microbes and dead cells and induce rapid protective host reactions.
- Adaptive immunity develops later, after exposure to microbes and other foreign substances, and is even more powerful that innate immunity in combating infections
What are the stages of innate immunity?
Innate immunity is always present, ready to provide defense against microbes and to eliminate damaged cells.
The three stages are: recognition of microbes and damaged cells, activation of various mechanisms, and elimination of unwanted substances
What are the major components of innate immunity?
Epithelial barriers that block entry of microbes, phagocytic cells (mainly neutrophils and macrophages), dendritic cells, natural killer cells, and several plasma proteins, including the proteins of the complement system.
What are the epithelial barriers in innate immunity?
- Epithelia of the skin and GI and resp tracts provide mechanical barriers to the entry of microbes from the external environment
- Epithalial cells also produce antimicrobial molecules, such as defensins, and lymphocytes located in the epithelia combat microbes at these sites
How do monocytes and neutrophils contribute towards innate immunity?
They are phagocytes in the blood that can rapidly be recruited to any site of infection; monocytes nter the tissues and mature to macrophages.
All tissues contain resident macrophages, which not only sense the presence of microbes and other offeding agents, but also ingest (phagocytose) these invaders and destroy them
How do dendritic cells contribute to innate immunity?
- They are a specialised cell population present in epithelia, lymphoid organs and most tissues.
- They capture protein antigens and display peptides for recognition by T lymphocytes.
- They are endowed with a rich collection of receptors that sense microbes and cell damage and stimulate the secretion of cytokines, mediators that play critical roles in inflammation and anti-viral defense
- Thus, dendritic cells are involved in the initiation of innate immune responses, but not destruction of microbes and other offending agents.
What are the roles of natural killer cells in innate immunity?
They provide early protection against many viruses and intracellular bacteria
What are pathogen-associated molecular patterns (PAMPs)?
Cells that participate in innate immunity are capable of recognising certain microbial components that are shared among related microbes and are often essential for infectivity (and thus cannot be mutated). These are called PAMPs
What are damage-associated molecular patterns?
Leukocytes recognise molecules released by injured and necrotic cells, which are called damage-associated molecular patterns (DAMPs).
What and where are pattern recognition receptors?
The cellular receptors that recognise PAMPs and DAMPs are often called pattern recognition receptors. They are located in all the cellular compartments where microbes may be present: plasma membrane receptors detect extracelluls microbes, endosomal receptors detect ingested microbes, and cytosolic receptors detect microbes in the cytoplasm
What are the different classes of pattern recognition receptors?
- Toll-like receptors
- NOD-like receptors
- C-type lectin receptors
- RIG-like receptors
- G protein-coupled recetprs
- Mannose receptors
Where are Toll-like Receptors (TLRs) and what do they do?
They are present in the plasma membrane and endosomal vesicles.
All these receptors signal by a common pathway that culminates in the activation of two sets of transcription factors:
1) NF-κB, which stimulates the synthesis and secretion of cytokines and the expression of adhesion molecules
2) Interferon regulatory factors (IRFs), which stimulate the production of antiviral cytokines, type I interferons
Where are NOD-like receptors (NLRs) and what do they recognise?
They are cytosolic receptors that recognise a wide variety of substances, including products of necrotic cells (uric acid and released ATP), ion disturbances (loss of K+), and some microbial products.
What is the inflammasome and how is it related to NLRs? What does it do?
Several of the NLRs signal via a cytosolic multiprotein complex - the inflammasome, which activates an enzyme (caspase-1) that cleaves a precursor form of IL-1 to generate the biologically active form.
Where are C-type lectin receptors (CLRs)? What do they do?
- They are expressed on the plasma membrane of macrophages and dentritic cells.
- They detect fungal glycans and ellicit inflammatory reactions to funghi.
Where are RIG-like receptors (RLRs)? What do they do?
- They are located in the cytosol of most cell types
- They detect nucliec acids of viruses that replicate in the cytoplasm of infected cells.
- They stimulate the production of antiviral cytokines
Where are G protein-coupled receptors? What do they do?
- They are on neutrophils, macrophages and most other types of leukocytes
- They recognise short bacterial peptides containing N-formylmethionyl residues, which is produced by all bacteral proteins and few mammalian proteins, which enables neutrophils to detect bacterial proteins and stimulate chemotactic responses of the cells
What do mannose receptors do?
They recognise microbial sugars and induce phagocytosis of the microbes
The innate immune system provides host defense by two main reactions. What are they?
- Inflammation - cytokines and products of complement activation are produced during innate immune reactions and trigger the vascular and cellular components of inflammation. The recruited leukocytes destroy microbes and ingest and eliminate damaged cells
- Antiviral defense - type I inferferons produced in response to viruses act on infected and uninfected cells and activate enzymes that degrade viral nucleic acids and inhibit viral replication.
How many different receptors does the innate immune system use compared to the adaptive immune system and why?
Innate immunity does not have memory or fine antigen specificity. It is estimated that innate immunity uses about 100 different receptors to recognise 1,000 molecular patterns.
In contrast, adaptive immunity uses two types of receptors (antibodies and T-cell receptors), each with millions or variations
What does the adaptive immune system consist of?
Lymphocytes and their products, inlduing antibodies.
There are two types of adaptive immunity. What are they and what are they mediated by?
- Humoral immunity, which protects against extracellular microbes and their toxins, is mediated by B lymphocytes and their secreted products, antibodies (AKA immunoglobulins).
- Cell-mediated immunity, which is responsible for defence against intracellular microbes, is mediated by T-lymphocytes.
Where are lymphocytes and why are they there?
They are not fixed in particular tissues but constantly circulate among lymphoid and other tissues via the vlood and the lymphatic circulation.
This promotes immune surveillance by allowing lymphocytes to home to any site of infection.
What are the different types of lymphocytes (5) and what do they do?
What are naive lymphocytes?
Mature lymphocytes that have not encountered the antigen for which they are specific are said to be naive (immunologically inexperienced).
After lymphocytes are activated by recognition of antigens, they differentiate into what? What do these cells do?
- Effector cells, which perform the function of eliminated microbes
- Memory cells, which live in a state of heightened awareness and are able to react rapidly and strongly to combat the microbe in case it returns
What is clonal selection?
Lymphocytes specific for a large number of antigens exist before exposure to antigen, and when an antigen enters, it selectively activates the antigen-specific cells. This fundamental concept is called clonal selection.
Antigen receptor diversity is generated by somatic recombination of the genes that encode the receptor proteins. How does this process happen?
All cells of the body contain antigen receptors genes in the germline configuration, in which the genes encoding these receptors consist of spatially separated segments that cannot be expressed as proteins. During lymphocyte maturation, these gene segments recombine in random sets and variations are introduced at the sites of recombination, forming many different geners that can be transcribed and translated into functional antigen receptors
What is the enzyme in developing lymphocytes that mediates recombination of the gene segments?
It is the product of RAG-1 and RAG-2 (recombination activating genes)
What cells contain recombined antigen receptor genes? How does this influence tumour identification?
Germline receptor genes are present in all cells in the body, but only T and B cells contain recombined antigen receptor genes (T-cell receptors in T cells and immunoglobulin in B cells).
Hence the presence of recombined T-cell receptors or Ig genes, is a marker of T- or B-lineage cells.
There are three major populations of T cells, which serve distinct functions. What are they and what do they do?
- Helper T lymphocytes stimulate B lymphocytes to make antibodies and activate other leukocytes (e.g. phagocytes) to destroy microbes
- Cytotoxic T lymphocytes kill infected cells
- Regulatory T lymphocytes limit immune responses and prevent reactions against self-antigens
Where do T cells develop and from what?
They develop in the thymus from precursors that arise from haematopoetic stem cells.
Where are mature T cells found and what percentage of lymphocytes do that constistute?
They are found in the blood, where they constitute 60-70% of lymphocytes, and in T-cell zones of peripheral lymphoid organs.
How do T cells recognise antigens and what is this thing made up of?
Each T cell recognises a specific cell-bound antigen by means of an antigen-specific TCR.
In approximately 95% of T cells, the TCR consists of a disulfide-linked heterodimer made up of an α and a β polypeptide chain, each having a variable (antigen-binding) region and a constant region.
The αβ TCR recognises peptide antigens that presented by what?
Major histocompatibility complex (MHC) molecules on the surfaces of antigen-presenting cells.
What is MHC restriction? What is its purpose?
By limiting the specificity of T cells for peptides displayed by cell surface MHC molecules, called MHC restriction, the immune system ensures that T cells see only cell-associated antigens (e.g. those derived from microbes in cells or from proteins ingested by cells
What forms the TCR complex?
Each TCR is noncovelently linked to six polypeptide chains, which form the CD3 complex and the ζ chain dimer. The CD3 and ζ proteins are invarient (identical) in all T cells. They are involved in the transduction of signals into the T cell that are triggered by binding of antigen to the TCR. Together, with the TCR, these proteins form the TCR complexes
In addition to CD3 and ζ proteins, T cells express several other proteins that assist the TCR complex in functional responses. These include CD4, CD8, CD28 and integrins. CD4 and CD8 are expressed on two mutually exclusive subsets of αβ T cells. What percentage are CD4 and what percentage are CD8. What do they do?
- Approximately 60% of mature T cells are CD4+ and about 30% are CD8+.
- Most CD4+ T cells function as cytokine-secreting helper cells that assist macrophages and B lymphocytes to combat infections.
- Most CD8+cells function as cytotoxic (killer) T lymphocytes to destroy host cells harboring microbes
What MHC molecules do CD4+ and CD8+ bind to? What happens after that?
- During antigen recognition, CD4 molecules bind to class II MCH molecules that are displaying antigen.
- CD8 molecules bind to class I MHC molecules
- The CD4 or CD8 coreceptor initiates signals that are necessary for activation of the T cells.
What cells are the only cells in the body capable of producing antibody molecules?
B lymphocytes
Where are B lymphocytes produced and where are they found?
B lymphocytes develop from precursors in the bone marrow.
Mature B cells consistute 10% to 20% of the circulating peripheral lymphocyte population and are also present in peripheral lymphoid tissues such as lymph nodes, spleen and mucose-associated lymphoid tissues.
How do B cells recognise antigens? How do they bind to them?
- B cells recognise antigen via the B-cell antigen receptor complex.
- Membrane bound antibodies of the IgM and IgD isotypes, present on the surface of all mature, naive B cells, are the antigen-binding component of the B-cell receptor complex.
What happens in B cells after they have bound to an antigen?
After stimulation by antigen and other signals, B cells develop into plasma cells, vertiable protein factories for antibodies.
How many antibody molecules can plasma cells secrete per second?
A single plasma cell can secrete hundreds to thousands of antibody molecules per second.
What are plasmablasts?
Antibody-secreting cells also detected in human peripheral blood.
What does the B-cell antigen receptor complex consist of?
It contains a heterodimer of Igα (CD79a) and Igβ (CD79b), which are essential for signal transduction through the antigen receptor
Apart from CD79a and CD79b, what are some other molecules that B cells also express that are essential for their reponses and what do they do?
- The type 2 complement receptor (CD2 or CD21), which recognises complement products generated during innate immune responses to microbes
- CD40, which receives signals from helper T cells
- CD21 is also used by the EBV as a receptor to enter and infect B cells
What is the purpose of dendritic cells?
They are the most important antigen-presenting cells for initiating T-cell responses against protein antigens.
Several features of dendritic cells account for their key role in antigen presentation. What are they?
- They are located at the right place to capture antigens - under epithelia, the common site of entry of microbes and foreign antigens, and in the interstitia of all tissues, where antigens may be produced
- They express many receptors for capturing and responsing to microbes (and other antigens), including TLRs and lectins
- In response to microbes, dendritic cells are recruited to the T-cell zones of lymphoid organs, where they are ideally located to present antigens to T cells.
- Dendritic cells express high levels of MHC and other molecules needed for presenting antigens to and activating T cells
What is a follicular dendritic cell? What do they do?
A second type of cell with dendritic morphologi present in the germinal centers of lypmhoid follicles in the spleen and lymph nodes and is called the follicular dendritic cell.
These cells bear Fc receptors for IgG and receptors for C3b and can trap antigen bound to antibodies or complement proteins. Such cells play a role in humoral immune responses by presenting antigens to B cells and selecting the B cells that have the highest affinity for the antigen, thus improving the quality of the antibody produced.
What is the function of natural killer cells?
The function of NK cells is to destory irreversibly stressed and abnormal cells, such as virus-infected cells and tumour cells
What is the morphology of natural killer cells?
They are somewhat larger than small lymphocytes, and they contain abundant azurophilic granules
What ability of natural killer cells makes then an early line of defense against viral infections and, perhaps, some tumours?
Natural killer cells are endowed with the ability to kill a variety of virus-infected cells and tumour cells, without prior exposure to or activation by these microbes or tumours.
What are the two cell surface molecules on natural killer cells? What do they do?
Two cell surface molecules, CD16 and CD56, are commonly used to identify NK cells.
CD16 is an Fc receptor for IgG, and it confers on NK cells the ability to lyse IgG-coated target cells. This process is known as antibody-dependent cell-mediated cytotoxicity.
How is the functional activity of NK cells regulated?
- By a balance between signals from activating and inhibitory receptors.
- There are many types of activating receptors, of which the NKG2D family is the best characterised - they recognise surface molecules that are induced by various kinds of stress, such as infections and DNA damage.
- NK cell inhibitory receptors recognise self class I MHC molecules, which are expressed on all healthy cells. The inhibitory receptors prevent NK cells from killing normal cells.
How does viral infection or neoplastic transformation trigger natural killer cells?
It often enhances expression of ligands for activating receptors and at the same time reduces the expression of class I MHC molecules.
As a result, the balance is tilted towards activation, and the infected or tumour cell is killed.
What cytokines are associated with natural killer cells?
Which ones do they secrete? Which ones stimulate proliferation and activate killing?
- Natural killer cells secrete cytokines such as interferon-γ, which activates macrophages to destroy ingested microbes, and thus NK cells provide early defense against intracellular microbial infections.
- The activity of NK cells is regulated by many cytokines, including the interleukins (IL-2, IL-15, and IL-12)
- IL-2 and IL-15 stimulate proliferation of NK cells
- IL-12 activated killing and secretion of IFN-γ
What are the peripheral lymphoid organs and what do they do?
- Lymph nodes - antigen-presenting cells in the nodes are able to sample the antigens of microbes that may enter through epithelia into tissues and are carried in the lymph. Dendritic cells transport antigens of microbes from epithelia and tissues via lymph vessels to the nodes. Thus, the antigens become concentrated in lymph nodes
- Spleen - Blood entering the spleen flows through a network of sinusoids. Bloodborne antigens are trapped by dendritic cells and macrophages in the spleen
- The cutaneous and mucosal lymphoid systems are located under the epithelia of the skin and the GI and resp tracts, respectively. Pharyngeal tonsils and Peyer’s patches of the intestine are two anatomically defined mucosal lymphoid tissues
Within the peripheral lymphoid organs, T lymphocytes and B lymphocytes are segregated into different regions. What are they?
- In lymph nodes the B cells are concentrated in discrete structures, called follicles, located around the periphery, or cortex, of each node. If the B cells in a follicle have recently responded to an antigen, this follicle may contain a central region called a germinal center.
- The T lymphoctyes are concentrated in the paracortex, adjacent to the follicles.
- The follicles contain the follicular dendritic cells, involved in B cell activation, and the paracortex contains the dendritic cells that present antigens to T lymphocytes
What is lymphocyte recirculation and why is it important?
Lymphocytes constantly recirculate between tissues and home to particular sites; naive lymphocytes traverse the peripher lympoid organs where immune responses are initiated and antigens are concentrated and effector T cells migrate to sites of infection and inflammation, to locate and eliminate microbes at any site of infection.
What is the purpose of MHC molecules?
To display peptide fragments of protein antigens for recognition by antigen-specific T cells.
What are human leukocyte antigens (HLA)?
In humans, the MHC molecules are called HLA because they were initially detected on leukocytes by the binding of antibodies.
On the basis of their structure, cellular distibution and function, MHC gene products are classified into two major classes. What are these classes and where are they found?
- Class I MHC molecules are expressed on all nucleated cells and platelets. They are heterodimers made of a polymorphic α heavy chain linked convalently to a smaller non-polymorphic β2-microglobulin.
- Class II MHC molecules are mainly expressed on cells that present ingested antigens and respond to T-cell help (macrophages, B lymphocytes, and dendritic cells).
What is the role of Class I and Class II MHC molecules?
- Class I MHC molecules display peptides that are derived from proteins, such as viral and tumour antigens, that are located in the cytoplasm and usually produced in the cell, and class I-associated peptides are recognised by CD8+ T lymphocytes
- Class II MHC molecules present antigens that are internalised into vesicles, and are typically derived from extracellular microbes and soluble proteins, class II-peptide complex is recognised by CD4+ T cells, which function as helper cells
MHC molecules play several key roles in regulating T cell-mediated immune reponses. What are they?
1) Because diferent antigenic peptides bind to different MHC molecules, it follows that an individual mounts an immune response against a protein antigen only if they inherit the genes for those MHC molecules that can bind peptides derived from the antigen and present it to T cells
2) By segregating cytoplasmic and internalised antigens, MHC molecules ensure that the correct immune reponse is mounted against different microbes - cytotoxic lymphocyte mediated killing of cells harboring cytoplasmic microbes, and helper T cell-mediated antibody and macrophage activation to combat extracellular microbes.
Cytokines contribute to different types of immune responses. How do they contribute to innate vs adaptive immune responses? Give examples for each type
- In innate immune responses, cytokines are produced rapidly after encounter with microbes and other stimuli, and function to include infammaltion and inhibit virus replication - TNF, IL-1, IL-2, type 1 IFNs, IFN-γ, and chemokines, produced mainly by macrophages, dendritic cells and natural killer cells
- In adaptive immune responses, cytokines are produced principally by CD4+ T lymphocytes activated by antigen and other signals, and function to promote lymphocyte proliferation and differentiation and to activate effector cells. - IL-2, IL-4, IL-5, IL-17, IFN-γ.
What cells secrete IL-2 and what does it do?
One of the earliest responses of CD4+ helper T cells is secretion of the cytokine IL-2 and expression of high-affinity receptors for IL-2. IL-2 is a growth factor that acts on these T lymphocytes and stimulates their proliferation, leading to an increase in the number of antigen-specific lymphocytes.
Some of the activated CD4+ T cells differentiate into effector cells that secrete different sets of cytokine and perform different functions. What are these sets and their functions?
- T helper 1 cells secrete the cytokine IFN-γ, which is a potent macrophage activator. The combination of CD40- ad IFN-γ mediated activation results in “classical” macrophage activation, leading to the induction of microbicidal substances in macrophages and the destruction of ingested microbes
- T helper 2 cells produce IL-4, which stimulates B cells to differntiate into IgE-secreting plasma cells, and IL-5, which activates eosinophils. Eosinophils and mast cells bind to IgE-coated microbes such as helminthic parasites, and function to eliminate helminths. They also induce the ‘alternative’ activation pathway of macrophages
- T helper 17 cells, which produce IL-17, recruit neutrophils and monocytes, which destroy some extracellular bacteria and funghi and are involved in some inflammatory diseases
What do activated CD8+ T lymphocytes differetiate into? What do they do?
They differentiate into cytotoxic T lymphocytes that kill cells harboring microbes in the cytoplasm. By destroying the infected cells, CTLs eliminate the reservoirs of infection
Antibody responses to most protein antigens require T cell help and are said to be T-dependent. In what way is this the case?
- B cells ingest protein antigens into vesicles, degrade them, and display peptides bound to class II MHC molecules for recognition by helper T cells.
- The helper T cells are activated and express CD40L and secrete cytokines, which work together to stimulate the B cells.
What antigens are T-independent? What happens here?
Many polysaccharide and lipid antigens cannot be recognised by T cells but have multiple identical antigenic determinants (epitopes) that are able to engage many antigen receptor molecules on each B cell and initate the process of B-cell activation; these responses are said to be T-independent
Each plasma cell is derived from an antigen-stimulated B cell and secretes antibodies that recognise the same antigen that initiated the reponse. What type of antibodies do different molecules stimulate the secretion of?
- Polysaccarides and lipids stimulate secretion mainly of IgM antibody
- Protein antigens, by virtue of CD40L- and cytokine-mediated helper T-cell actons, induce the production of antibodies of different classes, or isotypes (IgG, IgA, IgE)
What is affinity maturation and isotype switching? Where do they occur and what stimulates them?
- Isotype switching is induced by cytokines including IFN-γ and IL-4.
- Helper T cells also stimulate the production of antibodies with high affinities for the antigen. This process, affinity maturation, improved the quality of the humoral immune response
- They occur mainly in germinal centers, which are formed by proliferating B cells, especially in helper T cell-dependent response to protein antigens
What are follicular helper T cells? What do they do?
Some activated B cells migrate into follicles and form germinal centers, which are the major sites of isotype switching and affinity maturation. The helper T-cells that stimulate these process in B lymphocytes migrate to and reside in the germinal centers and are called follicular helper T cells.
The humoral immune response combates microbes in many ways. What are they?
- Antibodies bind to microbes and prevent then from infecting cells, thus neutralising the microbes
- IgG antibodies coat (opsonise) microbes and target them for phagocytosis as phagocytes express receptors for the Fc tails of IgG.
- IgG and IgM activate the complement system by the classical pathway, and complement products promote phagocytosis and destruction of microbes
- IgA is secreted from mucoseal epithelia and neutralises microbes inthe lumens of the respiratory and GI tracts
- IgG is actively transported across the placenta and protects the newborn until the immune system becomes mature
- IgE and eosinophils cooperate to kill parasites, mainly by release of eosinphil granules contents that are toxic to the worms.
- TH2 cytokines stimulate the production of IgE and activeate eosinophils, and thus the response to helminths is orchestrated by TH2 cells
There are several important general features of hypersensitivity disorders. What are they?
- Hypersensitivity reactions can be eilicited by exogenous environmental antigens (microbial and non-microbial) or endogenous self antigens
- Hypersensitivity usually results from an imbalance between the effector mechanisms of immune responses and the control mechanisms that serve to normally limit such responses
- The development of hypersensitivity diseases is often associated with the inheritance of a particular susceptibility genes (HLA genes)
- The mechanisms of tissue injury in hypersensitivity reactions are the same as the effector mechanisms of defense against infectious pathogens
What are the different types of hypersensitivity reactions?
1) Immediate hypersensitivity - caused by TH2 cells, IgE antibodies, and mast cells and other leukocytes. Mast cells release mediators that act on vessels and smooth msucle and proinflammatory cytokiens that recruit inflammatory cells
2) Antibody-mediated disorders - secreted IgG and IgM antibodies injure cells by promoting their phagocytosis or lysis and injure tissues by inducing inflammation
3) Immune complex-mediated disorders - IgG and IgM antibodies bind antigens usually in the circulation, and the antigen-antibody complexes deposit in tissues and induce inflammation. The leukocytes that are recruited produce tissue damage by release of lysosomal enzymes and generation of toxic free radicals
4) Cell-mediated immune disorders - sensitised T lymphocytes (Th1 and Th17 cells and CTLs) are the cause of the tissue injury. Th2 cells indue lesions that are part of immediate hypersensitivity reactions and are not considered a form of type IV hypersentivity
How is an immediate or type 1 hypersensitivity reaction triggered?
It is a rapid immunological reaction occurring in a previously sensitised individual that is triggered by the binding of an antigen to IgE antibody on the surface of mast cells
Local reactions are diverse and vary depending on the portal of entry of the allergen in type 1 hypersensitivity reaction?
Localised cutaneous rash or blisters (shin allergy, hives), nasal and conjunctival discharge (allergic rhinitis and conjunctivitis), hay fever, bronchial asthma, or allergic gastroenteritis
Many local type 1 hypersensitivity reactions have two well-defined phases. What are they and what happens during them?
1) The immediate reaction is characterised by vasodilation, vascular leakage, and depending on the location, smooth muscle spasm or glandular secretions. These changes usually become evident within minutes after exposure to an allergen and tend to subside in a few hours.
2) A second, late-phase reaction sets in 2-24 hours later without additiona exposure to antigen and may last for several days. This late-phase reaction is characterised by infiltration of tissues with eosinophils, neutrophils, basophils, monocytes and CD4+ T cells, as well as tissue destruction, typically in the form of mucosal epithelial cell damage
Most immediate hypersensitivity disorders are caused by excessive responses from what cells? What do they do?
Most are caused by excessive Th2 responses and these cells play a central role by stimulating IgE production and promoting inflammation.
The first step in the generation of Th2 cells is the presentation of the antigen (likely by dendritic cells) to naive CD4+ helper T cells. How do naive CD4+ helper cells respond to this?
- In response to antigen and other stimuli (IL-4), the T cells differentiate into Th2 cells.
- The newly minted Th2 cells produce a number of cytokines upon consequence encounter with the antigen (IL-4, IL-5, IL-13).
- IL-4 acts on B cells to stimulate class swtiching to IgE and promotes the development of Th2 cells
- IL-5 is involved in the development and activation of eosinophils.
- IL-13 enhances IgE production and acts on epithelial cells to stimualte mucus secretion
- In addition, Th2 cells produce chemokines that attract more Th2 cells, as well as other leukocytes to the reaction site
Where are mast cells? What do they contain?
- They are bone marrow-derived cells that are widely distributed in the tissues, being abundant near blood vessels and nerves and in subepithelial tissues
- Mast cells have cytoplasmic membrane-bound granules that contain a variety of biologically active mediators
How are mast cells activated in type 1 hypersensitivity reactions?
- They are activated by the cross-linking of high-affinity IgE Fc receptors
- They may also be triggered by complement components C5a and C3a (anaphylatoxins), both of which act by binding to receptors on the mast cell membrane.
- Other mast cell secretagogues include some chemokines (e.g. IL-8), drugs such as codeine, morphine, adenosine, bee stings and physical stimuli (heat, cold, sunlight)
How are basophils and mast cells similar and what makes them different?
They are similar in many ways, including the presence of cell surface IgE Fc receptors as well as cytoplasmic granules.
In contrast to mast cells, however, basophuls are not normally present in tissues but rather circulate in the blood in extremely small numbers.
Mast cells and basophils express a high-affinity receptor, called FcεRI. What is this specific for and what does this mean?
FcεRI is specific for Fc portion of IgE and therefore avidly binds IgE antibodies.
When are masts cells sensitised?
IgE-coated mast cells are said to eb sensitised, because they are sensitive to a subsequent encounter with the specific antigen.
When a sensitised mast cell is exposed to the same antigen, what happens?
The cell is activated, leading eventually to the release of an arsenal of powerful mediators responsible for the clinical features of immediate hypersensitivity reactions.
What are the steps of mast cell activation?
- The antigen binds to the IgE antibodies precious attached to the amst cells
- Multivalent antigens bind to and cross-link adjacent IgE antibodies.
- The underlying Fcε receptors are brought together, and this activates signal transudction pathways from the cytoplasmic portion of the receptors
- These signals lead to the production of mediators that are responsible for the initial, sometimes explosive, symptoms of immediate hypersensitivity, and they also set into motion the events that lead to the late-phase reaction
Mast cell activation leads to degranulation, with the discharge of preformed (primary) mediators that are stored in the granules, and de novo synthesis and release of secondary mediators, including lipid products and cytokines. Name some preformed mediators and what they do?
- Vasoactive amines - most important is histamine, causing smooth muscle contraction, increased vascular permeabilityk and increased mucus secretion
- Enzymes - contained in the granule matrix, include neutral proteases (chymase, tryptase) and several acid hydrolases. The enzymes cause tissue damage and lead to the generation of kinins and activated components of complement (C3a) by acting on their precursor proteins
- Proteoglycans - these include heparin and chondroitin sulfate, they seve to packes and store the amines in the granules
Mast cell activation leads to degranulation, with the discharge of preformed (primary) mediators that are stored in the granules, and de novo synthesis and release of secondary mediators, including lipid products and cytokines. Name some lipid mediators and what they do?
The major lipid mediators are arachidonic acid-derived products. Reactions in the mast cell membranes lead to activation of phospholipase A2.
* Leukotrienes C4 and D4 are the most potent vasoactive and spasmogenic agents known, they are several thousant times more active than histamine in increaseing vascular permeability and causing bronchial smooth muscle contraction. Leukotriene B4 is highly chemotactic for neutrophils, eosinophils and monocyes
* Prostaglandin D2 is the most abundant mediator produced in mast cells by the COX pathway, it causes intense bronchospasm and increased mucus secretion
* Platelet-activating factor is a lipid mediated produced by some mast cell populations but is not derived from AA. It causes platelet aggregation, release of histmine, bronchospasm, increased vascular permeability, and vasodilation.
Mast cell activation leads to degranulation, with the discharge of preformed (primary) mediators that are stored in the granules, and de novo synthesis and release of secondary mediators, including lipid products and cytokines. Name some cytokines and their roles.
- TNF, IL-1 and chemokines, which promote leukocytes recruitment
- IL-4, which amplified the Th2 response
On a morphological level, what happens during the late-phase reaction during type 1 hypersensitivity reactions?
In the late-phase reaction, leukocytes are recruited that amplify and sustain the inflammatory response without additional exposure to the tiggering antigen.
* Eosinophils are often an abundant leukocyte population in these reactions
How are eosinophils recruited to sites of immediate hypersensitivity?
By chemokines, such as eotaxin, and others that may be produced by epithelial cells, Th2 cells, and mast cells
The Th2 cytokine IL-5 is the most potent eosinophil-activating cytokine known.
Upon activation in late-phase immediate hypersensitivity reactions, what do eosinophils do?
They liberate proteolytic enzymes, as well as two unique proteins called major basic protein and eosinophil cationic protein, which damage tissues.
An increase propensity to develop immediate hypersensitivity reactions is called atopy. What are the morphological changes that atopic individuals tend to have?
They tend to have higher serum IgE levels and more IL-4 producing Th2 cells than does the general population.
Studies in patients with asthma reveal linkage to polymorphisms in several genes. What chromosome are these genes encoded, what are they and what do they do?
- Some of these genes are located in the chromosome 5q31 region
- These include genes encoding the cytokines IL-3, IL-4, IL-5, IL-9, IL-13 and GM-CSF.
What is nonatopic allergy?
It is estimated that 20-30% of immediate hypersensitivity reactions are triggered by non-antigenic stimuli, such as temperature extremes and exercsie, and do not invovle Th2 cells or IgE; such reactions are sometimes called nonatopic allergy
Give me an overview of what happens during antibody-mediated (type II) hypersensitivity reactions
Anibodies that react with antigens present on cell surfaces or in the extracellular matrix cause disease by destroying these cells, triggering inflammation, or interfering with normal functions.
What are the three steps that occur during antibody mediated (type II hypersensitivity) reactions?
1) Opsonisation and phagocytosis
2) Complement and Fc receptor-mediated inflammation
3) Antibody-mediated cellular dysfunction
What happens during opsonisation and phagocytosis in antibody mediated (type 2) hypersensitivity reaction?
- Cells opsonised by IgG antibodies are recognised by phagocyte Fc receptors.
- When IgM or IgG antibodies are deposited on the surfaces of cells, they may activated the complement system and C3b and C4b are deposited on the surfaces of the cells and recognised by phagocytes that express receptors for these proteins.
The net result is phagocytsis of the opsonised cells and their destruction.
Clinically, in what situations does antibody-mediated cell destruction and phagocytosis occur?
1) Transfusion reactions, in which cells from an incompatible donor react with and are opsonised by preformed antibody in the host
2) Heamolytic disease of the newborn, in which there is an antigenic difference between the mother and the fetus, and IgG antierythrocyte antibodies from the mother cross the placenta and cause destruction of fetal red cells
3) Autoimmune haemolytic anaemia, agranulocytosis, and thrombocytopaenia, in which individuals produce antibodies to their own blood cells, which are then destroyed
4) Certain drug reactions, in which a drug acts as a “hapten”, by attaching to plasma membrane proteins of red cells and antibodies are produced against the drug protein complex
What happens during the inflammation stage of antibody mediated (type II) hypersensitivity?
- When antibodies deposit in fixed tissues, such as basement membranes and extracellular matrix, the resultant injury is due to inflammation.
- The deposited antibodies activate complement, generating by-products, including chemotactic agents (C5a), which direct the migration of polymorphonuclear leukocytes and monocytes and anaphylatoxins (C3a and C5a), which increase vascular permeability
- The leukocytes are activated by engagement of their C3b and Fc receptors.
- This results in the production of other substances, that damage tissues, such as lysosomal enzymes, including proteases capable of digesting basement membrane, collagen, elastin, and cartilage, and ROS
Clinically, in what situations does antibody-mediated inflammation occur as the mechanism responsible for tissue injury?
In some forms of glomerulonephritis, vascular rejection in organ grafts
How does cellular dysfunction happen during antibody-mediated (type II) hypersensitivity reactions? Give examples
In some cases, antibodies directed against cell surface receptors impair or dysregulate function without causing cell injury or inflammation.
* In myasthenia gravis, antibodies reactive with acetylcholine receptors in the motor end plates of skeletal muscles block neuromuscular transmission and therefore cause muscle weakness
* In Graves disease, antibodies against the thyroid-stimulating hormone receptor on thyroid epithelial cells stimulate the cells, resulting in hyperthyroidism
Give a general overview of what happens in immune complex-mediated (type III) hypersensitivity?
Antigen-antibody complexes produce tissue damage mainly by eliciting inflammation at the sites of deposition
How is the pathological reaction usually initiated in immune complex-mediated (type III) hypersensitivity?
- It is usually initiated when antigen combines with antibody in the cirulation, creating immune complexes that typically deposit in vessel walls.
- Less frequently, the complexes may be formed at sites where antigen has been “planted” previously (called in situ immune complexes).
- The antigens that form immune complexes may be exogenous or endogenous
What organs do immune complex-mediated (type III) hypersensitivity reactions often take place in?
They tend to be systemic, but often preferentially involve the kidney (glomerulonephritis), joints (arthritis), and small blood vessels (vasculitis), all of which are common sites of immune complex deposition
The pathogenesis of systemic immune complex disease can be divided into three phases. What are they and what happens in each one?
1) Formation of immune complexes - the introduction of a protein antigen triggers an immune response that results in the formation of antibodies, typcically about a week after injection. These antibodies are secreted into the blood, where they react with the antigen still present and form antigen-antibody complexes
2) Deposition of immune complexes - the circulating antigen-antibody complexes are deposited in various tissues, often in the glomeruli and joints
3) Inflammation and tissue injury - once the complexes are deposited, they initiate an acute inflammatory reaction. The resultant inflammatory lesion is termed vasculitis if it occurs in blood vesels, glomerulonephritis if it occurs in renal glomeruli, arthritis if it occurs in the joints
What are the symptoms of immune complex-mediated (type III) reactions? When do thy happen?
During the inflammation and tissue injury phae (approx 10 days after antigen administration), clinical features such as fever, urticaria, joint pains, lymph node enlargement and proteinuria appear.
What antibodies are involved in immune complex-mediated (type III) hypersensitivity?
Complement-fixing antibodies (IgG and IgM) and antibodies that bind to leukocyte Fc receptors (some subclasses of IgG) induce the pathological lesions of immune complex disorders.
Give some examples of immune complex-mediated diseases
SLE
Post-strep glomerulonephritis
Polyarteritis nodosa
Reactive arthritis
Serum sickness
What is an Arthus reaction?
The Arthus reaction is localised area of tissue necrosis resulting from acute immune complex vasculitis, usually elicited in the skin.
These complexes precipitate in the vessel walls and cause fibrinoid necrosis, and superimposed thrombosis worsens the ischaemic injury.
Give a general overview of the process of T Cell-mediated (type IV) hypersensitivity?
The cell mediated type of hypersensitivity is caused by inflammation resulting from cytokines produced by CD4+ T cells and cell killing by CD8+ T cells
Give a general overview of what happens in CD4+ T cell-mediated hypersensitivity
In CD4+ T cell-mediated hypersensitivity reactions, cytokines produced by the T cells induce inflammation that may be chronic and destrucive.
What is the prototype of T cell-mediated inflammation? What happens during this?
Give an example
Delayed-type hypersensitivity, a tissue reaction to antigens give to immune individuals. An antigen administered into the skin of a previously immunised individual results in a detectable cutaneous reaction within 24-48 hours
An example of this is the tuberculin reaction
The inflammatory reactions stimulated by CD4+ T cells can be divided into sequential stages. What are these?
1) Activation of CD4+ T cells - naive CD4+ T cells recognise peptides displayed by dendritic cells and secrete IL-2, which functions as an autocrine growth factor to stimulate proliferation of the antigen-responsive T cells. The T cells differentiate into either Th1 (due to IFN-γ) or Th17 (via IL-1, IL-6 or IL-12).
2) Responses of differentiated effector T cells - upon repeat exposure to an antigen, Th1 cells secrete IFN-γ, which activates macrophages and they do their thing; if the activation is sustained, continued inflammation and tissue injury result
This produces granulomatous inflammation, which you know from before
Give some examples of T cell-mediated (type IV) diseases…
RA
MS
T1DM
TBD
Psoriasis
Contact sensitivity
What happens during CD8+ T Cell-mediated cytoxocity in T cell-mediated (type IV) hypersensitivity?
CD8+ CTLs kill antigen-expressing target cells. Tissue destruction by CTLs is important in T cell-mediated diseases, such as type 1 diabetes. The killing of infected cells leads to the elimination of the infection, but in some cases it is responsible for cell damage that accompanies the infection.
What is the principal mechanism of T cell-mediated killing of targets involves what?
Perforins and granzymes, preformed mediators contained in the lysosome-like granules of CTLs.
* CTLs that recognise the target cells secrete a complex consisting of perforin, granzymes, and other proteins which enters target cells by endocytosis.
* In the target cell cytoplasm, perforin facilitates the release of the granzymes from the complex. Granzymes are proteases that cleave and activate caspases, which induce apoptosis of teh target cells
* Activated CTLs also express Fas ligand, which can bind to Fas expressed on target cells and trigger apoptosis
What is the complement system?
It is a collection of soluble proteins and membrane receptors that function mainly in host defence against microbes and in pathological inflammatory reactions.
What does the complement system consist of?
More than 20 proteins, some of which are numbered C1 through C9
When does the complement system function?
In both innate and adative immunity for defense against microbial pathogens.
Broadly, what are the effects of complement activation?
Severeal cleavage products of complement proteins are elaborated that cause increased vascular permeability, chemotaxis, and opsonisation
Complement proteins are present in inactive forms in the plasma, what is the critical step in complement activation?
The proteolysis of the third (and most abundant) component, C3.
Cleavage of C3 in the complement pathway can occur by one of three pathways. What are these pathways?
- The classical pathway, which is triggered by fixation of C1 to antibody (IgM or IgG) that has combined with antigen
- The alternative pathway, which can be triggered by microbial surface molecules (e.g. endotoxin or LPS), complex polysaccharides, cobra venom, and other substances, in the absence of antibody
- The lectin pathway, in which plasma mannose-binding lectin binds to carbohydrates on microbes and directly activates C1
All three complement pathways converge lead to the same outcome. What is this and what happens next?
- They all lead to the formation of an active enzyme called the C3 convertase, which splits C3 into two functionally distinct fragments, C3a and C3b.
- C3a is released, and C3b becomes convalently attached to the cell or molecules where complement is being activated.
- More C3b then binds to the previously generated fragments to form C5 convertase, which cleaves C5 to release C5a and leave C5b attached to the cell surface.
- C5b binds the late components (C6-C9), culminating in the formation of the membrane attack complex (MAC)
The complement system has three main functions. What are they and how do they occur?
- Inflammation - C3a, C5a (anaphylatoxins) and, to a lesser extent, C4a stimulate histamine release from mast cells and thereby increase vascular permeability and cause vasodilation. C5a is also a chemotactic agent and activates the lipooxygenase pathway of AA, releasing leukotrienes.
- Opsonisation and phagocytosis - C3b and iC3b (inactive), when fixed to a microbial cell wall, act as opsonins and promote phagocytosis
- Cell lysis - the deposition of the MAC on cells makes these cells permaeblt ot water and ions and results in lysis of the cells. - this is particularly importnat in microbes with thin walls such as Neissaria bacteria
The activation of complement is tightly controlled by cell-associated and circulatin regulatory proteins. What are these and what do they do?
- C1 inhibitor (C1 INH) blocks the activation of C1
- Decay accelerating factor (DAF) and CD59 are two proteins that are linked to plasma memrbanes by a glycophosphatidyl anchor. DAF presents formation of C3 convertases and CD59 inhibits formation of the MAC
What are the ranges of cells that occur with malignant pleomorphisms?
Cells within the same tumour are not uniform, they range from small cells with an undifferentiated appearance, to tumour giant cells many time larger than their neighbours.
Some tumor giant cells possess only a single huge polymorphic nucleus, whilst others may have two or more large hyperchromatic nuclei.
What is abnormal about the nuclear morphology of malignant cells?
They have a nuclear-to-cytoplasm ratio that may approach 1:1 instead of the normal 1:4 or 1:6.
The nuclear shape is variable and often irregular, and the chromatin is often coarsely clumped and distributed along the nuclear membrane, or more darkly stained than normal (hyperchromatic)
What are the two phases of the metastatic cascade?
1) invasion of the extracellular matrix
2) vascular dissemination, homing of tumour cells and colonisation
Tumour cells must interact with the extracellular matrix at several stages in the metastatic cascade. What are they?
- A carcinoma must first breach the underlying basement membrane, then traverse the interstitial connective tissue, and ultimately gain access to the circulation by penetrating the vascular basement membrane.
- This process is repeated in revers when tumour cell emboli extravasate to a distant site.
Invastion of the ECM initiates the metastatic cascade and is an active process that can be resolved into several steps. What are they?
- “Loosening up” of tumour cell-tumour interactions
- Degradation of ECM
- Attachment to novel ECM components
- Migration and invasion of tumour cells
Degradation of the basement membrane and interstitial connective tissue is the second step in invasion. How does it occur?
Tumour cells may accomplish this by either secreting proteolytic enzymes themselves or by inducing stromal cells (e.g. fibroblasts and inflammatory cells) to elaborate proteases.
The third step of tumour invasion involves changes in attachment of tumour cells to ECM proteins. How does this happen?
- Loss of adhesion in normal cells leads to induction of apoptosis, but tumour cells are resistant to this form of cell death.
- Additionally, the matrix itself is modified in ways that promote invasion and metastasis - e.g. cleavage of the basement membrane proteins collagen IV and laminin by MMP2 or MMP9 generates novel sites that bind to receptors on tumour cells and stimulate migration.
What is the morphology of measles virus?
- The blotchy, reddish brown rash of measles virus infection on the face, trunk and proximal extremities is produced by dilated skin vessels, oedema, and a mononuclear perivascular infiltrate.
- Ulcerated mucosal lesions in the oral cavity near the opening of the Stensen ducts (the pathognomic Koplic spots) are marked by necrosis, neutrophilic exudate, and neovascularisation.
- The lymphoid organs typically have marked follicular hyperplasia, large germinal centers, and randomly distributed multinucleate giant cells, called Warthin-Finkeldey cells, which have eosinophilic nuclear and cytoplasmic inclusion bodies, found in the lung and sputum
There are eight types of human herpesviruses, belonging to three subgroups that are defined by the type of cell most frequently infected and the site of latency. What are the groups, give some examples and what cells do they infect?
- α-group viruses, including HSV-1, HSV-2 and VZV, which infect epithelial cells and produce latent infection in neurons
- Lymphotropic β-group viruses, including CMV, human herpesvirus-6 (which causes exanthem subitum, also known as roseola infantum and sixth disease, a benign rash of infants), and human herpesvirus-7, which infect and produce latent infection in a variety of cell types
- γ-group viruses EBV and KSHV/HHV-8, the cause of Kaposi sarcoma, which produce latent infection mainly in lymphoid cells.
What is the morphology of HSV-infected cells?
They contain large, pink to purple intranuclear inclusions (Cowdry type A) that consist of viral replication proteins and virions at various stage of assembly that push the host cell chromatin out to the edges of the nucleus. Due to cell fusion, HSVs also produc inclusion-bearing multinucleated syncytia.
What is gingivostomatitis? Is it HSV-1 or -2?
- It is usually encountered in children, and is caused by HSV-1
- It is a vesicular eruption extending from the tongue to the retropharynx and causing cervical lymphadenopathy.
What is herpetic whitlow in HSV? Who does it happen to?
Swollen, erythematous HSV lesions of the fingers of palm (herpetic whitlow) occur in infants and, occasionally, in health care workers
What are genital herpes? Is it caused by HSV-1 or -2?
- It is more often caused by HSV-2 than by HSV-1.
- It is characterised by vesicles on the genital mucous membranes as well as on the external genitalia that are rapidly converted into superficial ulcerations, rimmed by an inflammatory infiltrate
How long after a respiratory infection does the chickenpox rash present? How does it present?
- The chickenpox rash occurs approx 2 weeks after respiratory infection. Lesions appear in multiple waves centrifugally from the torso to the head and extremities.
- Each lesion progresses rapidly from a macule to a vesicle, which resembles a dewdrop on a rose petal.
- After a few days most chickenpox vesicles rupture, crust over, and heal by regeneration, leaving no scars. However, bacterial superinfection of vesicles that are ruptured by trauma may lead to destruction of the basal epidermal layer and residual scarring.
How does chickenpox appear histologically?
Chickenppox lesions show intraepithelial vesicles with intranuclear inclusions in epithelial cells at the base of the vesicles.
Histologically, what do the sensory ganglia appear like in shingles?
They contain a dense, predominantly mononuclear infiltrate, with herpetic intranuclear inclusions within neurons and their supporting cells.
How do CMV-infected cells appear?
CMV-infected cells exhibit gigantism of both the entire cell and its nucleus, which typically contains a large inclusion surrounded by a clear halo “owl’s eye)
How does EBV infect the body?
- EBV infects B cells and possibly epithelial cells of the oropharynx.
- An EBV envelope glycoprotein binds CD21 (CR2), the receptor for C3d component of complement, which is present on B cells.
- In a minority of B cells, infection is lytic, leading to viral replication and eventual cell lysis accompanied by release of virions, which may infect other B cells.
- In most B cells however, EBV establishes latent infection, during which the virus persists as an extrachromosomal episome.
A small number of EBV-encoded proteins are believed to be particularly important in the establishment of latency. What are they and what do they do?
- Epstein-Barr nucleur antigen 1 (EBNA1) binds the EBV genome to host cell chromosomes during mitosis, thereby ensuring that viral episomes are partitioned evenly to daughter cells when infected cells divide
- Latent membrane protein 1(LMP1) drives B-cell activation and proliferation. LMP1 does so by mimicking a constitutively active form of CD40, a B cell surface receptor. Like activating CD40, LMP1 binds to TNF receptor-assocated factors, adaptor molecules that trigger downstream events that activate NF-κB and the JAK/STAT signaling pathway. In addition, LMP1 prevents apoptosis by activating Bcl-2.
- EBNA2 also promotes B-cell activation and replication. It turns on the transcription of several host cell genes, inclduing genes that encode proteins that drive cell cycle antry, such as cyclin D.
- EBV produces a homologue of IL-10 (vIL-10), which inhibits its amcrophages and dendritic cells and suppresses anti-viral T cell responses.
What happens as a result of the actions of the EBV proteins?
- B cells that are latently infected with EBV are activated and begin to proliferate and to disseminate.
- This uncontrolled, expanding polyclonal population of EBV-infected B cells secretes antibodies with many specificities, inclduing antibodies that recognise sheep or horse red cells.
- These so-called heterophile antibodies are detected in diagnostic tests for mononucleosis.
Where in the body is EBV shed?
- In the saliva.
- It is not known whether the source of the virus is B cells, oropharyngeal epithelial cells, or both
EBV contributes to the development of which lymphoma? How?
It contributes to the development of some cases of Burkitt lymphoma, in which a chromosomal translocation (most commonly an 8:14 translocation) involving the MYC oncogene is the critical ongongenic event.
What are the morphological changes in EBV?
- The peripheral blood shows absolute lymphocytosis. Many of these are large, atypical lymphocytes, characterised by an abundant cytoplasm containing multiple clear vacuolations, an oval, indented, or folded nucleus, and scattered cytoplasmic azurophilic granules.
- The lymph nodes, histologically, have expansion of paracortical areas due to activation of T cells (immunoblasts). A minor population of EBV-infected B cells can also be detected in the paracortex.
- EBV-infected B cells resembling Reed-Sternberg cells may be found.
What are the common conditions caused by staphylococcal aureus?
- Skin lesions (boils, carbuncles, impetigo, and scalded-skin syndrome)
- Abscesses
- Sepsis
- Osteomyelitis
- Pneumonia
- Endocarditis
- Food poisoning
- Toxic shock syndrome
What is the morphology of S.aureus lung infections? What normally causes them
- They have a polymorphonuclear infiltrate similar to that of S.pneumoniae infections, but cause much more tissue destruction.
- They usually arise from a haematogenous source, such as an infected thrombus, or in the setting of a predisposing condition such as infleunza.
What causes Erysipelas? How does it present both clincically and histologically?
- It is caused by exotoxins from superficial infection with S.pyogenes
- Characterised by rapidly spreading erythematous cutaneous swelling that may begin on the face, or less frequently, on the body or an extremity. The rash has a sharp, well-demarcated, serpiginous border and may form a ‘butterfly’ distribution on the face.
- Histologically, there is a diffuse, oedematous, neutrophilic inflammatory reaction in the dermis and epidermis extending into the subcutaneous tissues. Microabscesses may be formed, but tissue necrosis is usually minor.
What bacteria causes scarlet fever? How does it present?
- Associated with pharyngitis cause by S.pyogenes.
- Most common between the ages of 3 and 15 years
- Manifested y a punctate erythematous rash that is most prominent over the trunk and inner aspects of the arms and legs.
- The face is also involved, but usually a small area about the mouth remains relatively unaffected, producing circumoral pallor.
- They skin usually becomes hyperkeratotic and scaly during defervescence.
What is the morphology of C.diphtheriae?
- Inhaled C.diphtheriae carried in respiratory droplets proliferate at the side of attachment on the mucosa of the nasopharynx, oropharynx, larynx, or trachea. The bacteria also form satellite lesions in the oesophagus or lower airways.
- Release of exotoxin causes necrosis of the epithelium, accompanied by an outpouring of a dense fibrinosuppurative exudate. The coagulaton of this exudate on the ulcerated necrotic surface created a tough, dirty gray to black, superficial membrane, sometimes called pseudo-membrane because it is not formed by viable tissue.
- There is an intense neutrophilic infiltration with marked vascular congestion, interstitial edema and fribin exudation.
- When the membrane sloughs off its inflamed and vascularised bed, bleeding and asphyxiation may occur.
- With control of the infection, the membrane is coughed up or removed by enzymatic digestion, and the inflammatory reaction subsides.
What is the pathogenesis of cholera?
- Cholera toxin is composed of five B subunits and a single A subunit.
- The B subunit binds GM1 ganglioside on the surface of intestinal epithelial cells, and is carried by endocytosis to the endoplasmic reticulum, a process called retrograde transport.
- Here, the A subunit is reduced by protein disulfide isomerase, and a fragment of the A subunit is unfolded and released.
- This peptide fragment is transported into the cytosol using host cell machinery that moves misfolded proteins from the endoplasmic reticulum to the cytosol. The A subnit refolds to void degradation.
- The refolded A subunit peptide then interacts with cytosolic ADP ribosylation factors (ARFs) to ribosylate and activate the stimulatory G protein Gsα, which stimulates adenylate cyclase and the resulting cAMP opens the cystic fibrosis transmembrane conductance regulator, CFTR, which releases chloride ions into the lumen.
- Chloride and sodium absoprtion are also inhibited by cAMP.
- The resulting accumulation accumulation of chloride, bicarbonate, and sodium within the lumen creates an osmotic driving force that draws water into the lumen and causes massive diarrhoea
What is the morphology of Campylobacter and how is it diagnosed?
- Diagnosis is primarily by stool culture, since biopsy findings are nonspecific, and reveal acute self-limited colitis with features ocmmon to many forms of infectious colitis.
- Mucosal and intraepithelial neutrophil infiltrates are prominent, particularly within the superficial mucosa; cryptitis (neutrophil infiltration of the crypts) and crypt abscesses may also be present.
- Importantly, crypt architecture is preserved, although this can be difficult to assess in cases with severe mucosal damage
What is the structure of HIV?
- The HIV-1 virion is spherical and contains an electron-dense, cone-shaped core surrounded by a lipid envelope derived from the host cell membrane
- The virus core contains 1) the major capsid protein 24; 2) nucleocapsid protein p7/p9; 3) two copies of viral genomic RNA; and 4) the three viral enzymes (protease, reverse transcriptase, and integrase).
- The viral core is surrounded by a matrix protein called p17, which lies underneath the virion envelope.
- Studding the viral envelope are two viral glycoproteins, gp120 and gp41, which are critical for HIV infection of cells.
What does the HIV-1 RNA genoma contain?
- The gag, pol and env genes, which are typical of retroviruses.
- The products of the gag and pol genes are large precursor proteins that are cleaved by the viral protease to yield the mature proteins.
- HIV contains several other accessory genes, including tat, rev, vif, nef, vpr and vpu, which regulate the synthesis and assembly of infectious viral particles and the pathogenicity of the virus
What does the life cycle of HIV consist of?
Infection of cells, integration of the provirus into the host cell genome, activation of viral replication, and production and release of infectious virus.
How does the infection of cells by HIV occur?
- HIV infects cells by using the CD4 molecule as a receptor and various chemokine receptors as corectpors.
- The initial step of infection is the binding of gp120 envelope glycoprotein to CD4 molecules, which leads to a conformation change that results in the formation of a new recognition site on gp120 for the coreceptors CCR5 and CXCR4.
- The conformational changes in gp41 result in the exposure of a hydrophobic region called the fusion peptide at the tip of gp41.
- This peptide inserts into the cells membrane of the target cells, leading to fusion of the virus with the host cell.
- After fusion, the virus core containing the HIV genome enters the cytoplasm of the cell
Once HIV is internalised into the cells, the RNA genome of the virus undergoes reverse trascription, leading to the synthesis of double-stranded complementary DNA (cDNA; proviral DNA). What are the options of what can happen next?
- In quiescent T cells, HIV cDNA may remain in the cytoplasm in a linear episomal form.
- In dividing T cells, the cDNA circularises, enters the nucleus, and is then integrated into the host genome.
- After this integration, the provirus may be silent for months of years, a form a latent infection.
- Alternatively, proviral DNA may be transcribed, with the formation of complete viral particles that bud from the cell membrane. Such productive infection, when associated with extensive viral budding, leads to death of infected cells.
What is the natural history of HIV infection?
- Virus typically enters through mucosal epithelia
- An acute retroviral syndrome
- A middle, chronic phase, in which most individuals are asymptomatic
- Clinical AIDS
Mucosal infection is followed by dissemination of the virus and the development of host immune responses. How does this happen?
- Dendritic cells in epithelia at sites of virus entry capture the virus and then migrate into the lymph nodes.
- Once in lymphoid tissue, dentritic cells may pass HIV on to CD4+ T cells through direct cell-t-cell contact.
- Within days after the first exposure to HIV, viral replication can be detected in the lymph nodes
- This replication leads to viremia. The virus disseminates throughout the body and infects helper T cells, macrophages, and dendritic cells in peripheral lymphoid tissues.
- As the infection. spreads, the individual mounts antiviral humoral and cell-mediated immune responses evidenced by seroconversion and by the development of virus-specific CD8+ cytotoxic T cells.
What immune responses to HIV infection occur at what time?
- A cytotoxic T lymphocyte response to HIV is detectable by 2-3 weeks after the initial ifnection, and it peaks by 9-12 weeks.
- Marked expansion of virus-specific CD8+ T cell clones occurs during this time, and up to 1-% of patients CTLs may be specific at 12 weeks.
- The humoral immune response to HIV peaks at about 12 weeks.
Because the loss of HIV immune containment is associated with declining CD4+ T cell counts, the CDC classification of HIV infection stratifies three categories on the basis of CD4+ cell counts. What are they and what levels are there?
What happens during the chronic phase of HIV?
- Lymph nodes and the spleen are sites of continuous HIV replication and cell destruction
- During this period, few or no clinical manifestations of the HIV infection are present.
- Although the majority of peripheral blood T cells do not harbor the virus, destruction of CD4+ T cells within lymphoid tissues continues and the numer of circulating blood CD4+ T cells steadily declines.
- Host defenses begin to wane, and the proportion of the surviving CD4+ cells infected with HIV increases, as does the viral burden per CD4+ cell.
What are the mechanisms of lymphoma development in AIDS?
- Unchecked proliferation of B cells infected with oncogenic herpesvirus in the setting of profound T cell depletion (AIDS). T cell immunity is required to restrain the proliferation of B cells infected with oncogenic viruses such as VBD and KSHV. AIDS patients are at high risk of developing aggressive B cell lymphomas composed of tumour cells infected by oncogenic viruses, particularly EBV.
- Germinal center B-cell hyperplasia in the setting of early HIV infection. In germinal centers, B cells diversify their immunoglobulin genes via lesions introduced into their DNA by the enzyme activation-induced deaminase (AID). AID can cause mutations in oncogenes implcated in B-cell lymphomagenesis.
What is the pathogenesis pathway of atherosclerosis?
- Endothelial injury and dysfunction, causing increased vascular permeability, leukocyte adhesion and thrombosis
- Accumulation of lipoproteins (mainly LDL and its oxidised forms) in the vessel wall
- Monocyte adhesion to the endothelium, followed by migration into the intima and transformation into macrophages and foam cells
- Platelet adhesion
- Factor release from activated platelets, macrophages and vascular wall cells, inducing smooth muscle cell recruitment, either from the media or from circulating precursors
- Smooth muscle cell proliferation, extrcellular matrix production and recruitment of T cells
- Lipid accumulation both extracellularly and within cells (macrophages and smooth muscle cells)
How does smooth muscle proliferation and matrix synthesis take place in atherosclerosis?
- Intimal smooth muscle cell proliferation and extracellular matrix deposition convert a fatty streak into a mature atheroma and contribute to the progressive growth of atherosclerotic lesions.
- Intimal smooth muscle cells have a proliferative and synthetic phenotype distinct from the underlying medial smooth muscle cells.
- Several growth factors are implicated in smooth muscle cell proliferation, including platelet-derived growth factor (PDGF), FGF and TGF-α. These factors also stimulate smooth muscle cells to syntehsis extracellular matrix (notably collagen), which stabilised atherosclerotic plaques.
- In contract, activated inflammatory cells in atheromas may increase breakdown of extracellular matrix components, resulting in unstabel plaques
What are fatty streaks in atherosclerosis?
- Fatty streaks are composed of lipid-filled foamy macrophages.
- Beginning as multiple minute flat yellow spots, they eventially coalesce into elongated streaks 1cm long or longer
- These lesions are no sufficiently raised to cause significant flow disturbances.
- Although fatty streaks can evolve into plaques, not all are destined to become advanced lesions
What are the key processes in atherosclerosis?
Intimal thickening and lipid accumulation, which together form plaques
How do atherosclerotic plaques and lesions present morphologically?
- Atheromatous plaques are white-yellow and encroach on the lumen of the artery; superimposed thrombu over ulcerated plaques is red-brown.
- Atherosclerotic lesions are patchy, usually involving only a portion of any given arterial wall adn are rarely circumferential; on cross-section, the lesions therefore appear “eccentric”. The focality of atherosclerotic lesions is attributable to the vagaries of vascular haemodynamics.
Atherosclerotic plaques are susceptible to which clinically important pathologic changes?
- Rupture, ulceration, or erosion of the surface of atheromatous plaques exposes highly thrombogenic substances and leads to thrombosis, which may partially or completely occlude the vessel lumen.
- Haemorrhage into a plaque - rupture of the overlying fibrous cap, or of the thin-walled vessels in the areas of neovascularisation, can cause intraplaque haemorrhage; a contained haemtoma may expand the plaque or induce plaque rupture
- Atheroembolism - plaque rupture can discharge atherosclerotic debris into the blood stream, producing microemboli
- Aneurysm formation - atherosclerosis-induced pressure or ischaemic atrophy of the underlying media, with loss of elastic tissue, causes weakness and potential rupture
The composition of atherosclerotic plaques is dynamic and can contribute to risk of rupture. What compositions increase the risk of rupture?
Plaques that contain large areas of foam cells and extracellular lipid, and those in which the fibrous caps are thin or contain few smooth muscle cells of have clusters of inflammatory cells, are more likely to rupture; there are referred to as “vulnerable plaques”
What is an aneurysm?
A localised abnormal dilation of a blood vessle or the heart that may be congenital or acquried
What are the different types of aneurysm? Give examples
- A ‘true’ aneurysm is when it involves an attenuated but intact arterial wall or thinned ventricular wall of the heart - atheroscleroric, syphilitic and congenital vascular aneurysms, ventricular aneurysms that follow MIs
- A false aneurysm is a defect in the vascular wall leading to extravascular haematoma that freely communicates with the intravascular space “pulsating haematoma” - ventricular rupture after MI or a leak at the sutured junction of a vascular graft with a natural artery
- An arterial dissection arises when blood enters a defect in the arterial wall and tunnels between its layers. Dissections are often but always aneurysmal.
All of these can rupture, often with catastrophic consequences
Descriptively, aneurysms are classified by macroscopic shape and size. How would you do this?
- Saccular aneurysms are spherical outpouchings involving only a portion of the vessel wall; they may vary from 5 to 20 cm in diameter and often contain thrombus
- Fusiform aneurysms are diffuse, circumferential dilations of a long vascular segment; they vary in diameter (up to 20cm) and in length and can involve extensive portions of the aortic arch, abdominal aorta or even the iliacs.
Histologically, what is cystic medial degeneration?
It is smooth muscle cell loss leading to scarring (and loss of elastic fibers), inadequate ECM synthesis, and production of increasing amounts of amorphous ground substance (glycosaminoglycan) that occurs in Marfan syndrome and scurvy
Where in the aorta is an AAA? How big can they be?
Usually positioned below the renal arteries and above the bifurcation of the aorta. They can occasionally affect the renal and superior or inferior mesenteric arteries, either by direct extensior of by occluding vessel ostia with mural thrombi.
AAA can be saccular or fusiform, up to 15cm in diameter and up to 25cm in length
What are the classifications of aortic dissections?
- The more common (and dangerous) proximal lesions (called type A dissections), involving either both the ascending and descending aorta (type 1 Debakey) or just the ascending aorta only (type II DeBakey)
- Distal lesions not involving the ascending part and usually beginning distal to subclavian artery (called type B dissections or DeBakey type III)
What is the treatment for type A dissections?
Rapid diagnosis and institution of intensive antihypertensive therapy coupled with surgical plication of the aortic intimal tear can save 65-85% of patients.
What is the prognosis of type A dissections?
Mortality approaches 70% in those who present with haemorrhage or symptoms related to distal ischaemia, and the overall 10-year survival is only 40-60%.
What is the prognosis and treatment of type B dissections?
Most type B dissections can be managed conservatively; patients have a 75% survival rate whether they are treated with surgery or antihypertensive medication only/
What is the myocardial response to coronary arterial obstruction?
Coronary arterial obstruction diminishes blood flow to a region of myocardiam, causing ischaemia, rapid myocardial dysfunction, and eventually - with prolonged vascular compromise - myocyte death.
What does the outcome of myocardial infarction depend on?
Predominantly on the severity and duration of flow deprivation
What are the early biological consequences of MI? How does this affect myocardial function?
- The cessation of aerobic metabolism within seconds, leading to inadequate production of high energy phosphates (e.g. creatine phosphate and ATP) and accumulation of potentially noxious metabolites (e.g. lactic acid)
- Because of the exquisite dependence of myocardial function on oxygen and nutrients, myocardial contractility ceases within a minute or so of the onset of severe ischaemia. Such loss of function actually precipitates heart failure long before myocyte death occurs
What are the ultrastructural changes that occur in MI? How long do they take? are they irreversible?
- Ultrastructural changes - myofibrillar relaxation, glycogen depletion, cell and mitochondrial swelling - develop within a few minutes of the onset of ischemia
- There early manifestations of ischaemic injury are potentially reversible
- Only severe ischemia (blood flow <10% of normal) lasting 20-30 minutes or longer leads to irreversible damage (necrosis) of cardiac myocytes.
Where is ischaemia most pronounced in the heart?
- Due to the myocardial perfusion pattern from epicardium to endocardium, ischaemia is most pronounced in the subendocardium; thus, irreversibly injury of ischaemic myocytes occurs first in the subendocardial zone.
- With more extended ischaemia, a wavefront of cell death moves through the myocardium to encompass progressively more of the transmural thickness and breadth of the ischaemic zone.
How long does it take for necrosis to involve half of the thickness of the myocardium and the full way through? What may change this?
- Necrosis involves approximately half of the thickeness of the myocardium in 2-3 hours of the onset of severe MI, and is usually transmural within 6 hours.
- However, in instances where chronic sublethal ischaemia has induced a more well-developed coronary collateral circulation, the progression of necrosis may follow a more protracted course (12 hours or longer)
What does the LAD branch of the coronary artery supply?
Most of the apex of the heart, the anterior wall of the left ventricle, and the anterior two thirds of the ventricular septum.
What is the “dominant” coronary artery?
Either RCA or LCX - that perfuses the posterior third of the septum is called ‘dominant’ even though the LAD and PCX perfuse the majority of the left ventricular myocardium
What proportion of the population are right dominant circulation and what does this mean?
- Present in 80% of individuals
- The RCA supplies the entire right ventricular free wall, the posterobasal wall of the left ventricle, and the posterior third of the ventricular septum, while the LCX generally perfuses only the lateral wall of the left ventricle
- Thus, RCA occlusions can potentially lead to left ventricular damage.
The distribution of myocardial necrosis correlates with the location and cause of the decreased perfusion. How does the location affect myocardial necrosis?
- Transmural infarction. MI caused by an epicardial vessel (in the absence of any therapeutic intervention) are typically transmural - the necrosis involves virtually the full thickness of the ventricular wall in the distribution of the affected coronary.
- Subendocardial (nontransmural) infarction. As the subendocardial zone is normally the least perfused region of myocardium, this area is most vulnerable to any reduction in coronary flow. It typically involves roughly the inner third of the ventricular wall.
- Multifocal microinfarction pattern is seen when there is pathology involving only smaller intramural vessels.
What usually causes the different patterns of myocardial infarction?
- Transmural infarction is usually associated with a combination of chronic coronary atherosclerosis, acute plaque change, and superimposed thrombosis
- Subendocardial (nontransmural) infarction can occur as a result of plaque disruption followed by a coronary thrombus that becomes lysed (therapeutically or spontaneously) before myocardial necrosis extends across the full thicckness of the wall. They can also result from prolonged, severe reduction in systemic blood pressure, as in shock on superimpsoed on chronic, otherwise noncritical, coronary stenoses
- Multifocal infarction may occur in the setting of microembolisation, vasculitis, or vascular spasm, for example, due to endogenous catechols (adrenaline) or drugs (cocaine)
In the subendocardial infarcts that occur as a result of global hypotension rather than an acute plaque, how is the pattern of infarction different?
The myocardial damage is usually circumferential, rather than being limited to the distribution of a singele major coronary artery
What proportion of the artery perfusion zone do transmural myocardial infarcts effect?
Nearly all transmural infarcts involve at least a portion of the left ventricle (comprising the free wall and ventricular septum) and encompass nearly the entire perfusion zone of the occluded coronary artery save for a narrow rim (approx 0.1mm) of preserved subendocardial myocardium that is preserved by diffusion of oxygen and nutrients from the ventricular lumen.
Of MIs caused by a right coronary obstruction, what proportion affect which parts of the heart?
- 15-30% extend from the posterior free wall of the septal portion of the left ventricle into the adjacent right ventricular wall.
- Isolated infarction of the right ventricle is unusual (only 1-3% of cases), as is infarction of the atria.
What are the frequencies of involvement of each of the three main arterial trunks and the correspondint sites of myocardial lesions resulting in infarction (in the typical right dominant heart)?
- Left anterior descending coronary artery (40-50%): infacrts involving the anterior wall of left ventricle near the apex, the anterior portion of the ventricular septum; and the apex circumferentially
- Right coronary artery (30-40%): infarcts involving the inferior/posterior wall of left ventricle; posterior portion of ventricular septum; and the inferior/posterior right ventricular free wall in some cases
- Left circumflex coronary artery (15-20%): infarcts involving the lateral wall of left ventricle except at the apex
Early morphological recognition of acute MI can be difficult. But if the infarct preceded death by 2-3 hours, how is it possible to highlight the area of necrosis? What does this show?
By immersion of tissue slices in a solution of triphenyl-tetrazolium chloride. This stain imparts a brick-red colour to intact, noninfarcted myocardium where LDH activity is preserved. Because dehydrogenases leak out through the damaged membranes of dead cells, an infarct appears as an unstained pale zone
Reperfused MIs are usually hemorrhagic. Why?
Because the vascular is injured during ischaemia and there is bleeding after flow is restored.
What is reperfusion injury?
This term encompasses various forms of damage that can occur after restoration of flow to “vulnerable” myocardium that is ischaemic but not yet irreversibly damaged
The laboratory evaluation of MI is based on measuring the proteins that leak out of irreversibly damaged myocytes. What are these?
- The most useful are cardiac specific troponins T and I, and the MB fraction of CK.
- The diagnosis of MI is established when blood levels of these cardiac biomarkers are elevated.
What does the rate of appearance of cardiac markers in the peripheral circulation following MI depend on?
- their intracellular location and molecular weight
- the blood flow and lymphatic drainage in the area of the infarct
- the rate of elimination of the marker from the blood
Following MI, how does the troponin level change over time?
- Troponins I and T are not normally detectable in the circulation.
- Following an MI, levels of both begin to rise at 3-12 hours
- cTnT levels peak somewhere between 12-46 hours
- cTnI levels are maimal at 24 hours
Creatinine kinase is an enzyme expressed in brain, myocardium and skeletal muscle. It is a dimer composed of two isoforms. What are they and where are they found?
- It is a dimer composed of two isoforms designated “M” and “B”.
- MM homodimers are found predominantly in cardiac and skeletal muscle
- BB homodimers in brain, lung and may other dissues
- MB heterodimers are principally localised to cardiac muscle. Thus, the MB form of CK (CK-MB) is sensitive but not specific, sincis can also be elevated after skeletal muscle injury
When do CK-MB, cTnI and cTnT return to normal following an MI?
- CK-MB returns to normal in 48-72 hours
- cTnI in 5-10 days
- cTnT in 5-14 days
What are the different types of cardiac rupture that occur post-MI?
- Rupture of the ventricular free wal (most common), with haemopericardium and cardiac tamponade (A)
- Rupture of the ventricular septum (less common), leading to an acute VSD and left-to-right shunting (B)
- Papillary muscle rupture (least common), resulting in the acute onset of severe mitral regurgitation (C)
When does pericarditis often occur after MI?
About the second or third day following a transmural infarct as a result of underlying myocardial inflammation (Dressler sydrome)
Vegetations on heart valves are the classic hallmark of IE. How do they present morphologically?
Vegetations on heart valves are the classic hallmark of IE; these are friable, bulky, potentially destructive lesions containing fibrin, inflammatory cells, and bacteria or other organisms.
What valves are the most common sites of infective endocarditis? Is it only one valve they happen in?
The aortic and mitral valves are the most common sites of infection, although the valves of the right heart may also be involved, particularly in IVDUs.
Vegetations can be single or multiple and may involve more than one valve.
What are some of the consequelae of infective endocarditis vegetations?
They can occasionally erode into the underlying myocardium and produce an abscess (ring abscess Fig B).
Vegetations are prone to embolisation; because the embolic fragments often contain virulent organisms, abscesses frequently develop where they lodge, leading to sequelae such as septic infarcts or mycotic aneurysms.
How do the vegetations of subacute endocarditis differ from acute endocarditis? Both in terms of destruction and microscopically
- The vegetations of subacute endocarditis are associated with less valvular destruction that those of acute endocarditis, although the distinction can be subtle.
- Microscopically, the vegetations of subacute IE typically exhibit granulation tissue at their bases indicative of healing. With time, fibrosis, calcification, and a chronic inflammatory infiltrate can develop.
What are the three pathological patterns of cardiomyopathy? and which ones are most/least common?
- Dilated cardiomyopathy (including arrhythmogenic right ventricula cardiomyopathy) - 90% of cases
- Hypertrophic cardiomyopathy
- Restrictive cardiomyopathy - least common
What is the left ventricular ejection fraction for the following cardiomyopathies:
* Dilated
* Hypertrophic
* Restrictive
Dilated < 40%
Hypertrophic - 50-80%
Restrictive - 45-90%
What are the causes of the following cardiomyopathies:
* Dilated
* Hypertrophic
* Restrictive
- Dilated - genetic, alcohol, peripartum, myocarditis; haemochromatosis; chronic anaemia; sarcoidosis; idiopathic
- Hypertrophic - genetic ; Friedrich ataxia; storage disease; infants of diabetic mother
- Restrictive - amyloidosis; radiation-induced fibrosis; idiopathic
What percentage of dilated cardiomyopathy are familial? What genes are specific for this?
- DCM is familial in at least 30-50% of cases, in which it is caused by mutations in a diverse group of more than 20 genes encoding porteins involved in the cytoskeleton, sarcolemma and nuclear envelope.
- In particular, mutations in TTN, a gene that encodes titin, may account for approximately 20% of all cases of dilated cardiomyopathy
What is the gross morphology of dilated cardiomyopathy?
- In dilated cardiomyopathy, the heart is usually enlarged, heavy (often weighing 2-3 times normal), and flabby, due to dilation of all chambers.
- Mural thrombi are common and may be a source of thromboemboli
- There are no primary valvular alterations; if mitral (or tricuspid) regurgitation is present, it results from left (or right) ventricular chamber dilation (functional regurgitation)
- Either the coronary arteries are free of significant narrowing or the obstructions present are insufficient to explain the degree of cardiac dysfunction.
What are the histological changes of dilated cardiomyopathy?
- The histological abnormalities in DCM are nonspecific and usually do not point to a specific etiology
- Most muscle cells are hypertrophied with enlarged nuclei, but some are attenuated, stretched, and irregular
- Interstitial and endocardial fibrosis of variable degree is present, and small subendocardial scars may replace individual cells or groups of cells, probably reflecting healing of previous ischaemic necrosis of myocytes caused by hypertrophy-induced imbalance between perfusion and demand
- Morever, the severity of morphological changes may not reflect either the degree of dysfunction or the patient’s prognosis
What is the morphology of arrhythmogenic right ventricular cardiomyopathy?
- Morphologically, the right ventricular wall is severely thinned due to loss of myocytes, accompanied by extensive fatty infiltration and fibrosis.
- Although myocardial inflammation may be present, ARVC is nor considered an inflammatory cardiomyopathy.
What are the genetic links associated with arrhythmogenic right ventricular cardiomyopathy?
- Classic ARVC has autosomal dominant inheritance with a variable penetrance.
- The disease has been attributed to defective cell adhesion proteins in the desmosomes that link adjacent cardiac myocytes.
- Naxos syndrome is a disorder characterised by arrhythmogenic right ventricular cardiomyopathy and hyperkeratosis of plantar palmar skin surfaces specifically associated with mutations in the gene encoding the desmosome-associated protein plakoglobin.
What are the genetic links associated with hypertrophic cardiomyopathy?
- In most cases, the pattern of transmission is autosomal dominant with variable penetrance.
- HCM is caused by mutations in any one of the several genes that encode sarcomeric proteins; there are more than 400 different known mutations in nine different genes, most being missense mutations
- Mutations causing HCM are found most commonly in the gene encoding β-myosin heavy chains, following by genes coding for cardiac TnT, α-tropomyosin, and myosin-binding protein C.
- The prognosis of HCM varies widely and correlates strongly with specific mutations
What are the most important histological features of the myocardium in hypertrophic cardiomyopathy?
1) massive myocyte hypertrophy, with transverse myocyte diametes frequently greater than 40μm (normal, approx 15μm)
2) haphazard disarray of bundles of myocytes, individual myocytes, and contractile elements in sarcomeres within cells (termed myofiber disarray)
3) interstitial and replacement fibrosis
What is the histological morphology of myocarditis?
- Active myocarditis is characterised by an interstitial inflammatory infiltrate associated with focal myocyte necrosis.
- A diffuse, mononuclear, predominantly lymphocytic infiltrate is most common.
- Although endomyocardial biopsies are diagnostic in some causes, they can be spuriosly negative because inflmmatory involvement of the myocardium may be focal or patchy.
- If the patient survives the acute phase, the inflammatory lesions either resolve, leaving no residual changes, or heal by progressive fibrosis
What is the histological morphology of giant-cell myocarditis? What is its prognosis?
- It is characterised by a widespread inflammatory cellular infiltrate containing multinucleate giant cells (fused macrophages) interspersed with lymphocytes, eosinophils, plasma cells, and macrophages.
- Focal to frequently extensive necrosis is present.
- This variant likely represents the fulminant end of the myocarditis spectrum and carries a poor prognosis
What is the histological morphology of Chagas disease myocarditis?
It is distinctive by virute of the parasitisation of scattered myofibers by trypanosomes accompanied by a mixed inflammatory infiltrate of neutrophils, lymphocytes, macrophages, and occasional eosinophils
What are the gross morphological features or purulent pericarditis?
- The exudate ranges from a thin cloudy fluid to frank pus up to 400-500mL in volume
- The serosal surfaces are reddened, granular, and coated with the exudate.
What is the gross morphology of aortic stenosis?
The gross morphologic hallfmark of non-rheumatic calcific aortic stenosis is mounded calcified masses within the aortic cusps that ultimately protrude through the outflow surfaces into the sinuses of Valvalva and prevent cuspal opening. The free edges of the cusps are usually not involved.
What is the pathology of a congenitally bicuspid aortic valve? What area is a major site for calcific deposits?
- There are only two functional cusps, usually of unequal size, with the larger cusp having a midline raphe resulting from incomplete commissural separation during development; less frequently the cusps are of equal size and the raphe is absent.
- The raphe is frequently a major site of calcific deposits.
Where do calcific deposits usually deposit in the mitral valve? How do they appear?
- As opposed to the predominantly cuspal involvement in aortic valve calcification, degenerative calcific deposits in the mitral valve typically develop in the fibrous annulus.
- Grossly, these appear as irregular, stony hard, occasionally ulcerated nodules (2-5mm in thickness) at the base of the leaflets (fig C and D).
What are the gross morphological changes in mitral valve prolapse?
- The characteristic anatomic change in MVP in interchordal ballooning (hooding) of the mitral leaflets or portions thereof.
- The affected leaflets are often enlarged, redundant, thick and rubbery.
- The associated tendinous cords may be elongated, thinned, or even ruptured, and the annulus may be dilated.
- The tricuspid, aortic or pulmonary valves may also be affected.
What are the key histological changes associated with mitral valve prolapse?
- Marked thickening of the spongiosa layer with deposition of mucoid (myxomatous) material, called myxomatous degeneration
- There is also attenuation of the collagenous fibrosa layer of the valve, on which the structural integrity of the leaflet depends.
In mitral valve prolapse, secondary changes reflect the stresses and tissue injury indicent to the billowing leaflets. What are these changes?
1) fibrous thickening of the valve leaflets, particularly where they rub against each other
2) linear fibrous thickening of the left ventricular endocardial surface where the abnormally long cords snap or rub against it
3) thickening of the mural endocardium of the left ventricle or atrium as a consequence of friction-induced injury induced by the prolapsing, hypermobile leaflets
4) thrombi on the atrial surfaces of the leaflets or the atrial walls (B)
5) focal calcifications at the base of the posterior mitral leaflet (C)
What are the main types of acquired atelectasis, which is encountered principally in adults? What causes each one each?
- Resorption atelectasis stems from complete obstruction of an airway. It is most often caused by excessive secretions (mucus plugs) or exudates within smaller bronchi, as may occur in bronchial asthma, chronic bronchitis, bronchiectasis, and post-op states. Aspiration, foreign bodies and rarely tumours may also lead to airway obstruction
- Compression atelectasis results whenever significant volumes of fluid (transudate, exudate, or blood), tumor or air accumulated within the pleural cavity.
- Contraction atelectasis occurs when focal or generalised pulmonary or pleural fibrosis prevents full lung expansion
How do resorption and compression atelectasis effect the mediastinum?
In resorption atelectasis, the lung volume is diminshed and the mediastinum shifts toward the atelectatic lung
In compression atelectasis, the mediastinum is pushed away from the affected lung
Based on the segments of the respiratory units that involved emphysema is classified into four major types. What are they? Which ones cause significant obstruction? Which is most common?
1) Centriacinar
2) Panacinar
3) Paraseptal
4) Irregular
Of these, only the first two cause clinically significant airflow obstruction.
Centriacinar emphysema is the most common form, consistuting more than 95% of clinically significant cases.
What parts of the acini (the terminal respiratory units) does centriacinar emphysema effect? Where in the lung are the lesions commonly found?
- The central or proximal parts of the acini, formed by respiratory bronchioles, are affected, whereas distal alveoli are spared.
- Thus, both emphysematous and normal arispaces exist within the same acinus and lobule.
- The lesions are more common and usually more severe in the upper lobes, particularly in the apical segements.
- Inflammation around bronchi and bronchioles is common.
What parts of the acini are affected in panacinar emphysema? Where does it tend to happen?
- The acini are uniformly enlarged from the level of the respiratory bronchiole to the terminal blind alveoli.
- In contrast to centriacinar emphysema, panacinar emphysema tends to occur more commonly in the lower zone and in the anterior marghins of the lungs, and it is usually most severe at the bases
What part of the acini does distal acinar (paraseptal) emphysema usually happen? Where in the lungs does it happen?
- The proximal portion of the acinus is normal, and the distal part is predominantly involved.
- The emphysema is mroe striking adjacent to the pleura, along the lobular connective tissue septa, and at the margins of the lobules.
- It occurs adjacent to areas of fibrosis, scarring or atelectasis and is usually more severe in the upper half of the lungs
What diseases are often associated with centriacinar, panacinar and distal acinar emphysema?
- Centriacinar emphysema occurs predominantly in heavy smokers, often in association with chronic bronchitis (COPD)
- Panacinar emphysema is associated with α1-antitrypsin deficiency
- Distal acinar emphysema probably underlies many cases of spontaneous pneumothorax in young adults
In broad terms, what is the pathogenesis of emphysema?
- Inhaled cigarette smoke and other noxious particles cause lung damage and inflammation
- This result in parenchymal destruction (emphysema) and airway disease (bronchiolitis and chronic bronchitis).
What are the factors that influence the development of emphysema?
- Inflammatory mediators and leukocytes including leukotriene B4, IL-8, TNF, are released by resident epithelial cells and macrophages and attract inflamatory cells, amplify the inflammatory process and induce structural changes
- Protease-antiprotease imbalance - several proteases are released from the inflammatory cells and epithelial cells that break down connective tissue components
- Oxidative stress - substances in tobacco smoke, alveolar damage, and ifnlammatory cells all produce oxidants, which may beget more tissue damage and inflammation
- Infection may exacerbate the associated inflammation and chronic bronchitis
What is the gross morphology of advanced emphysema?
- Advanced emphysema produces voluminous lungs, often overlapping the heart and hiding it when the anterior chest wall is removed.
- Generally, the upper two thirds of the lungs are more severely affected.
- Large apical blebs or bullae are more characteristic of irregular emphysema secondary to scrring and ofdistal acinar emphysema
- Large alveoli can easily be seen on the cut surface of fixed lungs
Between predominantly bronchitis and predominantly emphysema, how do the following things differ:
* Age
* Dyspnea
* Cough
* Infections
* Respiratory insufficiency
* Cor pulmonale
* Airways resistance
* Elastic recoil
* CXR
* Appearance
What is bullous emphysema?
This is a descriptive term for large subpleural blebs or bullae (spaces greater than 1cm in diameter in the distended state) that can occur in any form of emphysema
These localised accentuations of emphysema occur near the apex, sometimes near old TB scarring.
On occasion, rupture of the bullae may give rise to PTx
A fundamental abnormality in asthma is an exaggerated TH2 reponse to normally harmless environmental antigens. What happens in this process?
- TH2 cells secrete cytokines that promote inflammation and stimulate B cells to produce IgE and other antibodies.
- These cytokines include IL-4, which stimulates the production of IgE; IL-5, which activates locally recruited eosinophils; and IL-13, which stimulates mucus secretion from bronchial submucosal glands and also promotes IgE production by B cells.
- The T cells and epithelial cells secrete chemokines that recruit more T cells and eosinophils, thus exacerbating the reaction.
- IgE binds to the Fc receptors on submucosal mast cells, and repeat exposure to the allergen triggers the mast cells to release granule contents and produce cytokines and other mediators, which collectively induce the early-phase (immediate hypersnesitivity) reaction and the **late-phase reaction.
What happens during the early reaction in asthma?
- The early reaction is dominated by bronchoconstriction, increased mucus production, variable degrees of vasodilation, and increased vascular permeability
- Bronchoconstriction is triggered by direct stimulation of subepithelial vagal (parasympathetic) receptors through both central and local reflexes trigerred by mediators produced by mast cells and other cells in the reaction.
What happens during the late-phase reaction in asthma?
The late-phase reaction is dominated by recruitment of leukocytes, notably eosinophils, neutrophils, and more T cells.
What happens during “airway remodelling” in asthma?
Over time, repeated bouts of allergen exposure and immune reactions result in structural changes in the bronchial wall, referred to as “airway remodelling”. These changes include:
* Thickening of the airway wall
* Subbasement membrane fibrosis (due to deposition of type I and III collagen)
* Increased vascularity
* An increase in the size of submucosal glands and number of airway goblet cells
* Hypertrophy and/or hyperplasia of the bronchial wall muscles
ALI/ARDS is initiated by injury of pneumocytes and pulmonary endothelium, setting in motion a vicious cycle of increasing inflammation and pulmonary damage. What are the steps involved in this?
Endothelial activation
Adhesion and extravasation of neutrophils
Accumulation of intraalveolar fluid and formation of hyaline membranes
Resolution of injury
What happens in endothelial activation during ARDS?
- In some instances, endothelial activation is secondary to pneumocyte injury, which is sensed by resident alveolar macrophages.
- In response, these immune sentinels secrete mediators such as TNF that act on the neighbouring endothelium.
- Alternatively, circulating inflammatory mediators may activate pulmonary endothelium directly in the setting of severe tissue injury or sepsis.
- Some of these mediators injure endothelial cells, while others (notably cytokines) activate endothelial cells to express increased levels of adhesion molecules, procoagulant proteins and chemokines
What happens during adhesion and extravasation of neutrophils in ARDS?
- Neutrophils adhere to the activated endothelium and migrate into the interstitium and the alveoli, where they degranulate and release inflammatory mediators, including proteases, ROS and cytokines.
- Macrophage migration inhibitory factor (MIF) released into the local milieu also helps to sustain the ongoing pro-inflammatory response.
- The result is increased recruitment and adhesion of leukocytes, causing more endothelial injury, and local thrombosis.
- This cycle of inflammation and endothelial damage lies at the heart of ALI/ARDS.
What happens regarding accumulation of alveolar fluid and formation of hyaline membranes during ARDS?
- Endothelial activation and injury make pulmonary capillaries leaky, allowing interstitial and intraalveolar oedema fluid to form.
- Damage and necrosis of type II alveolar pnuemocytes leads to surfactant abnormalities, further compromising alveolar gas exchange.
- Ultimately, the inspissated protein-rich oedema fluid and debris from dead alveolar epithelial cells organise into hyaline membranes, a characteristic feature of ALI/ARDS.
What happens regarding resolution of injury during ARDS?
- It is impeded in ALI/ARDS due to epithelial necrosis and inflammatory damage that impairs the ability of remaining cells to assist with oedema resorption
- Eventually, however, if the inflammatory stimulus lessens, macrophages remove intraalveolar debris and release fibrogenic cytokines such as transforming growth factor β and platelet-derived growth factor (PDGF).
- These factors stimulate fibroblast growth and collagen deposition, leading to fibrosis of alveolar walls.
- Bronchiolar stem cells proliferate to replace pneumocytes. Endothelial restoration occurs through proliferation of uninjured capillary endothelium
What is the morphology in the acute stage on ARDS?
In the acute stage, the lungs are heavy, firm, red and boggy.
They exhibit congestion, interstitial and intra-alveolar oedema, inflammation, fibrin deposition, and diffuse alveolar damage.
The alveolar walls become lined with waxy hyaline membranes that are morphologically similar to those seen in hyaline membrane disease of neonates.
Alveolar hyaline membranes consist of fibrin-rich oedema fluid mixed with the cytoplasmic and lipid remnants of necrotic epithelial cells.
