Immunology L18-26 Flashcards
What is immunity?
It is the ability of an organism to resist a particular infection or toxin by the action of specialised cells or molecules
What are the different types of immunity?
Active and passive
What is innate immunity?
Present from birth
Simple recognition system
Limited capacity
There before infection starts
Patrols for infection
Recognises common danger signals
Rapid response
No memory
What is adaptive/acquired immunity?
Not present from birth
Learns from invading organisms
Sophisticated, highly specific recognition
Specific memory
Slower respinse
Activated in immune organs
What is immunological memory?
Maintenance of memory B & T cells and high serum or mucosal antibody levels, protection against reinfection
What are the goals of the immune system?
TO clear potential pathogenic in a controlled and efficient process
With limited pathology in host
Appropriate duration leads to return to homeostasis
Potentially confer future protection
Not attack self
Remove any non-healthy cells
What are the factors that effect immunity?
General health
Infection
Nutrition
Adverse environmental conditions
State of microbiome
Pregnancy
Genetic disorders
Exams (stress)
How does herd immunity work?
By vaccinating most of the population it protects the individual and the population as disease declines if majority of population immune
How did herd immunity help measles?
Needed >95% population immune to prevent outbreaks
MMR vaccine introduced 1998
What are the 4 main types of vaccine?
Live
Killed (inactivated/attenuated)
Subunit
Nucleic acid
What are antibodies?
Proteins produced by plasma cells which are mature B cells
What is the process of clonal selection and expansion?
Single progenitor cell gives rise to a large number of lymphocytes, each with a different specificity
Then removal of a potentially self-reactive immature lymphocyte by clonal deletion
Then a pool of mature naive lymphocytes form
Then proliferation and differentiation of activated specific lymphocytes occurs to form a clone of effector cells
What are the primary lymphoid tissues?
Bone marrow - highly cellular tissue, fills internal cavity of bones
Thymus - specialised, highly cellular gland
What does bone marrow produce as a primary lymphoid tissue?
B & T cells continually
B cells mature, T cells are immature and leave to thymus
Clonal diverse - specific receptor
Cells are specific to antigen
What does the thymus produce as a primary lymphoid tissue?
T cells are educated here
Migrate to secondary lymphoid tissues
What are the secondary lymphoid tissues and how do they work?
Peripheral lymphoid tissues
e.g. lymph nodes, spleen, tonsils, mucosal associated lymphoid tissues (MALT)
Once developed B and T lymphocytes recirculate and, if they meet antigen, undergo clonal expansion and differentiation in the tissues
Circulate in fluids
Where do adaptive immune responses take place?
In the secondary lymphoid tissues
How do mucosal associated lymphoid tissues work?
Diffuse system of nonencapsulated, submucosal lymphoid tissue in the intestinal and respiratory tracts
Respiratory MALTs include nasopharyngeal lymphatic tissues
Intestinal MALTs: Peyer’s patches, appendix and isolated follicles in intestinal mucosae
What are the major innate defence mechanisms?
Barriers
Cellular defences
Molecular defences
What are the different physical and chemical barriers to infection?
Skin - physical, FAs, commensals
Mucus membranes - mucus, cilia, commensals, low pH
Lysozyme in tears
Acid in stomach
What are the different antibacterial enzymes in the body?
Lysozyme
Secretory phospholipase A2
Tears, saliva, phagocytes
Antimicrobial peptides (AMPs)
What are pattern recognition receptors (PRRs)?
Located on host cells: macrophages, neutrophils and dendritic cells
Allow identification of pathogens
Recognise simple molecules and regular patterns
‘lock and key mechanism’
What are the different subtypes of pattern recognition receptors (PRRs)?
Toll-like receptors - membrane surface
evolutionary conserved
10 in humans each has own repertoire of pathogen-associated molecular patterns
NOD- like receptors intracellular
(nucleotide-binding oligomerisation domain)
RIG-I-like helicases
What are the different pathogen-associated molecular patterns (PAMPs)?
Mannose-rich oligosaccharides
Peptidoglycans
Lipopolysaccharides
Unmethylated CpG DNA
What are leucocytes?
WBCs, produced from pluripotent haematopoietic stem cellist bone marrow
Inclue - lymphocytes, monocytes and granulocytes
Neutrophils
Eosinophils
Basophils - tissue mast cell
What are macrophages activated function?
Phagocytosis and activation of bacterial mechanisms, antigen presentation
What are dendritic cells activated functions?
Antigen uptake in peripheral sites, antigen presentation
What are neutrophils activated function?
Phagocytosis and activation of bactericidal mechanisms
What are eosinophils activated functions?
Killing of antibody-coated parasites
What are basophils activated functions?
Promotion of allergic responses and augmentation of anti-parasitic immunity
What are mast cells activated functions?
Release of granules containing histamine and active agents
Which cell is he bridge between innate and adaptive immunity?
Dendritic cells
What is phagocytosis?
Ingestion and killing of microorganisms by specialised cells
What are the main phagocytic cells?
Neutrophils:
short-lived
multi-lobed nucleus
abundant in sites of acute inflammation
most common WBC in circulation
Mononuclear phagocytes:
blood monocytes, Kuppfer cells, alveolar macrophages etc
Monocytes when in blood
Macrophage when in tissue
longer lived cells
monocyte to macrophage
What is the main mechanism of action of phagocytosis?
Recognition receptors for
common bacterial components
complement
antibody
Internalisation - enclosing microbe in a membrane bound vacuole
Fusion - phagosome fuses with lysosome to form a phagolysosome
Killing
Digested products released
What are the different phagocytic mechanisms?
Acidification, toxic nitrogen oxides, enzymes, antimicrobial peptides, toxic oxygen products, competitors
How does oxygen-dependent killing take place?
Hexose monophosphate shunt generates NADPH
NAPDH oxidase generates reactive oxygen intermediates (either bacteriostatic/ bactericidal)
What are the additional functions of macrophages?
Can be activated by bacterial products or cytokines
Secrete soluble factors
Present antigen to lymphocytes
What are the different cells involved in extracellular killing?
Eosinophils
Natural killer cells
What are the different classes eosinophils can be separated into to enable killing of parasites?
Enzymes
Toxic proteins
Cytokines
Chemokines
Lipid mediator
How do natural killer cells help kill extracellularly?
Activated by IFNα, IFNβ and IL12 (interferon and interleukin = cytokines)
NK cells produce IFNγ (helps control infections)
Contain viral infections whilst adaptive/specific response kicks in
Deficiency leads to increased risk of herpes(rare)
What are the features of cytokines?
Both innate and adaptive immunity
Low molecular weight proteins secreted by cells that stimulate or inhibit the activity, proliferation or differentiation of other cells
Around 20
Sub groups: interferons, lymphokines, interleukins and chemokines
Have many different funtions
What is the complement system?
Protection from early infections
Major effector system of humeral branch of innate and adaptive response
~30 serum and membrane proteins
Act in concert and orderly sequence → amplification
Have initial activation → highly regulated enzymatic cascade
Main goal: antigen clearance & inflammatory response
What are the roles of components in the complement system?
Some activated proteins bind covalently to bacteria opsonising them → phagocytksed by cell with complement receptors
Some small fragments of complement recruit phagocytes to the site and regulate the inflammatory response
Some products activate B cells
Terminal component of system generates MAC → lysis of pathogen
What are the different pathways of the complement system?
Classical (adaptive), lectin (innate) and alternative (innate) pathway
How are the pathways of the complement system activated?
Classical: antigen-antibody complexes
Alternative: pathogen surfaces
Lectin: acute phase proteins bind glycoproteins/carbohydrates on micro-organisms
What are the components of the C1 protein?
C1q, C1r and C1s
2x molecules pf C1r and C1s bind to C1q
What does C1 bind to become activated?
2x IgG or 1x IgM and the pathogen, activates C1s
What are features of the C1q molecule?
It has 6 globular heads joined to common stem
Head binds to one Fc domain on antibody
~2 globular heads must bund to Fc domains for activation
Binds leads to conformational change revealing a proteolytic site on C1r
What does C1s now act on?
Cleaves C4 → C4a + C4b
C4b then binds covalently to pathogen surface
C2 binds to C4b on antigen/pathogen surface
C2 cleaved by C1a leaving C4b2a (C3 convertase)
What is the role of C3 convertase in the classical pathways?
Cleaves C3 molecules to C3b and C3a
What is the function of C3b in the classical pathway?
C3b → opsonin (lots on pathogen surface)
↓ (also activate alternate pathway)
Phagocytosis of pathogen cells with complement receptor
C3b and C4b2a bind forming C5 convertase
What is the function of C5 convertase?
Cleaves C5 → C5a + C5b
C5b binds to C6 + C7 + C8 + C9 forming C5bC6C7C8C9 = membrane attack complex (MAC)
What is the function of the membrane attack complex?
Displaces cell membrane phospholipids
Channel
Disrupts membrane
Cell lysis
Death
What is the action of C8?
C8 = C8β + C8α-γ
C8β binds to C5b (allows C8α-γ binding)
C8α-γ inserts into lipid bilayer
C8α-γ induces polymerisation of 10-16 molecules C9 → ring structure
=MAC
How does C3 differ in the alternate pathway?
Undergoes spontaneous hydrolysis
C3b binds to surface
Factor B binds to C3b
Cleaved by factor D
C3bBb (C3 convertase)
What is the action of Factor P in the alternative pathway?
Factor P=Properdin
Stabilised C3 convertase
What is C3b2Bb in the alternative pathway?
C5 convertase
What is a summary of the alternative pathway?
Activated by action of lectin/classical pathway
Activated by spontaneous hydrolysis of C3
C3b binds to factor B
→ Factor D cleaves to Ba and Bb
C3bBb= C3 convertase
Properdin stabilises C3 convertase
What is the lectin pathway of the complement system?
Uses soluble receptors
Recognise microbial surfaces
Activates complement cascade
CHO on microbial surface
Get complexes of mannose binding lectin and MBL-associated serine proteases
(MASP1&2 → cleavage and activation, MASP2 cleaves C4 and C2)
How is C3 convertase initiated in the lectin pathway?
Activated MASP-2 associated with MBL/ ficolin cleaves C4→C4a + C4b binds to microbial surface
C4b binds to C2 cleaved by MASP-2 to C2a + C2b forms C4b2a (C3 convertase)
What do deficiencies in the complement system cause?
C3 deficiency affects opsonisation, inflammation and cytolysis which can cause life threatening infections with a range of bacteria
C6,C7,C8,C9 deficiency affects cytolysis which can cause problems with infection with Neisseria (e.g. meningitis)
What are the different regulatory proteins of the classical and alternative pathways?
C1 inhibitor (C1INH)
C4-binding protein (C4BP)
Complement receptor 1 (CR1)
Factor H (H)
Factor I (I)
Decay-accelerating factor (DAF)
Membrane cofactor protein (MCP)
CD59 (protectin)
What are the features of innate immunity?
Present from birth
Simple recognition systems
Limited capacity
There before infection starts
Patrols for infection
Recognises common danger signals
Rapid response
No memory
What are the features of adaptive immunity?
Not present from birth
Learns from invading organisms
Sophisticated, highly specific recognition
Specific memory
Slower response
Activated in immune organs
What is the biochemical nature of antibodies?
Glycoproteins (Igs)
Basic unit: 2 identical heavy chains + 2 identical light chains joined by non-covalent interactions and disulphide bridges
Each chain has N-terminal variable region and C-terminal constant region
Secreted by mature B cells
First expressed as membrane-bound B cell receptor in developing B cells
Where does enzymatic cleavage of immunoglobulins occur?
At hinge region
Fragment antigen binding (FAB) contains antigen-binding region
Fragment Crystalizable (Fc) interacts with Fc receptors on cells and with C1q
What are the main functions of antibodies?
Bind specifically to epitopes on pathogen/antigen that elicit immune response (neutralisation & opsonisation)
Recruit cells and molecules to destroy pathogen/antigen
(region involved in effector function is the constant (Fc))
What are the features of antibodies binding?
Ab Fab region binds to epitopes on pathogens/antigens
Epitopes: linear/ conformational components of antigen
Structure of FAB region and range of non-covalent forces allows antibody to bind to antigens with high affinities
What are the different non-covalent forces that allow binding of antibodies?
Electrostatic
Hydrogen bonds
Van der Waals forces
Hydrophobic forces
Cation-pi interaction
What are the 7 functions of antibodies?
Neutralise
Agglutinate
Opsonise
Activate complement
Improve phagocytosis
Antibody dependent cellular cytotoxicity
Degranulation
How do antibodies neutralise pathogens?
Bind to bacteria/virus surface or bacterial toxin
Prevent interaction with cell receptors
Prevents uptake of pathogen by target cells
Uptake and destruction by macrophages
How do antibodies opsonise?
Extracellular proteins binding to mark pathogen for destruction via phagocytosis
Direct = binding of antibody constant region to phagocyte receptors
Indirect = increasing complement deposition on pathogen and binding to complement receptors
How do antibodies activate the complement system?
Classical pathway, antigen-antibody complex binds to C1qrs
Acts on C4 and C2 generates C3 convertase
How do antibodies recruit phagocytosis?
Engluf pathogen/ toxin by phagocytosis
An endpoint for several mediated responses
How do antibodies have antibody dependent cellular cytotoxicity?
ADCC, detection of antibodies by FcR on several cell types including cell activation
Lyses target cells
Mainly NK cells
How do antibodies recruit degranulation in cells?
Degranulation via Fc receptors (FcR)
Degranulation of mast cells (allergy, parasites)
Killing by NK cells and eosinophils (ADCC)
Eosinophils attack a schistosome larva in presence from an infected patient
What is an example of antibodies being useful for lab/clinic?
If individual B cells are fused with B cell tumour line generates hybridoma which give unlimited supply of monoclonal antibody (mAb)
How can antibodies be used in the lab?
Lateral flow strips
Enzyme linked immunosorbent assay (ELISA)
Flow cytometry
Western blot
Immunofluorescence
How is western blot used with antibodies in the lab?
SDS-page used transfer of nitrocellulose and overlay with antiserum then detect bound antibody with enzyme linked anti-IgG
What are the general properties of cytokines?
Secretion is bried
Action often pleiotropic and redundant
Often influence synthesis and actions of other cytokines
Actions may be local and systemic
Bind to specific membrane receptors on target to be initiated
External signals regulate the expression of cytokine receptors and responsiveness
Response consists of changes in gene expression target cells, expression of new functions and sometimes proliferation target
What are the 3 major functional categories?
Mediators and regulators of innate immunity - produced by macrophages in response to infectious agents
Mediators and regulators of adaptive immunity - produced mainly by T lymphocytes response to specific recognition
Stimulators of haematopoiesis - produced by bone marrow stromal cells, leukocytes and other cells and stimulate growth and differentiation of immature leukocytes
What are examples of the most important cytokines?
IL2, IL1, TNFα and interferons
What are the 2 distinct groups of chemokines?
CXC, CC
What are the different cytokines produced by macrophages?
IL-1β: activates vascular endothelium, activates lymphocytes, local tissue destruction and increases access of effector cells → Fever, IL-6 produced
TNF-α: activates vascular endothelium and permeability, increased entry IgG, complement, cells to tissues and increased fluid drainage to lymph nodes → fever, mobilisation of metabolites, shock
IL-6: lymphocyte activation increased antibody production → fever, induces acute-phase protein production
CXCL8: chemotactic factor recruits neutrophils, basophils and T cells to site of infection
IL-12: Activated NK cells, induces differentiation of CD4 T cells into Th 1 cells
What are the effects of cytokines secreted by macrophages?
Inflammation (return to homeostasis) - chemo tactic, act on vascular endothelium
Fever
Acute phase response
Act on lymphocytes
What are antimicrobial peptides?
Innate, small peptides, different organisms, bacteria, fungi, parasites and viruses
Amipipathic →lipid bilayer → destabilise
Immunomodulatory effects on host cells
Produced by various cells
What are inflammatory triggers?
Pathogens
Tissue damage
Disruption/ inappropriate immune response
What are mediators of inflammation?
Complement products (C5a>C3a)
Macrophage/ T cell derived cytokines
Mast cells
Other mediators (prostaglandin, leukotrienes, thromboxane ETC)
What is endotoxic shock?
Excessive cytokine release in gram -ive bacterial infection
Widespread effects of cytokines on vascular endothelium - circulatory shock, disseminated intravascular coagulation (DIC)
What happens in cytokine storm?
Elevated levels of circulating cytokines and immune cell hyper-activation
Pro-inflammatory cytokines
Infection e.g. SARS CoV-2, Yersinia pestis, treatments, cancers autoimmune disease
Immune response is the problem
Organ failure
Disseminate intravascular coagulation
Lethal if severe
What are the GI host-immune interactions?
Tight junctions
Paneth cells secrete AMPs
IgA secretion
Complement
Saliva, pH, bile and peristalsis
Microbiome
Suppresses proliferation/colonisation of pathogens
What is an example of a disease that results in gut inflammation?
Crohn’s disease
Inflammatory bowl disease
Autoimmune
1 in 500 people
Ages 15-35
Autoreactive T cells against intestinal floral antigens
Fatigue, fever, abdominal pain, diarrhoea, weight loss, skin lesions…
Mouth to and can be affected
Treatment: immunomodulation, surgery, diet
What is MHC restriction?
When individual T cells only recognise specific peptides presented by specific MHC molecules
What are features of major hitocompatility complex (MHC) molecules?
Genes located in MHC section of genome
Cell surface glycoproteins
Enable T cell recognition of Ag on cell surfaces
MHC I - CD8+
MHC II - CD4+
Encoded by HLA (human leucocyte antigens) genes in HLA complex/cluster
What are features of MHC class I?
Expressed by most nucleated cells
Three genes: HLA-A, HLA-B and HLA-C
Heterodimer: α chain MW 44 kDa
All pair with β2- microglobin MW 12 kDa
HLA-A, B and C each have own α chain with β2-microglobulin
Bind peptides 8-10 AAs long
What are the features of the MHC class I α chains?
α1 & α2 domains form peptide-binding region
-2 α helices, floor 8-stranded β-sheet
-Polymorphic AAs in α1 and α2 domains
α3 domain and β2-microglobulin are Ig-like
What are features of MHC class II?
Hetrodimer, α and β chains similar size and both transmembrane
Both chains polymorphic encoded by MHC
Similar structure to MHC I
Polymorphic α1 & β1 domains forming peptide binding site α2 and β2 domains Ig-like
Binds 13-24 AAs long
Expressed only on specialised immune cells
3 class II molecules: HLA-DP, GLA-DQ AND HLA-DR
Encoded by genes α and β forms of molecules
Where are the genes that encode MHC molecules?
Clustered together in a large genetic region/complex MHC (less energy to produce)
What are the properties of T cell recognition of MHC molecules?
Cytotoxic T cells (CD8) recognise peptide + class I MHC on any cell
Helper T cells (CD4) recognise peptide + class II MHC, specialised antigen presenting cell
How do cells generate peptides that end up in grooves of MHC molecules?
Processed
Intracellular Ag → Ag processing → peptide epitope presented on cell surface → TCR
What are the difference and properties between exogenous and endogenous antigens?
Endogenous: proteins found in cytoplasm
Processed then presented by MHC class I molecules
Exogenous: proteins found outside cell that have been taken up by specialised cells
Processed then presented by MHC class II molecules
What is the process of presentation by MHC class I?
Antigen synthesised in cytoplasm
Protein cleaved to peptides by proteasome
Peptides transported to ER by the pore-forming TAP complex
Peptides bind to MHC class I molecules
MHC-peptide complex transported to cell surface
What is the process of presentation by MHC class II?
MCH class II molecules produced in ER associate with invariant chain
MHC class II moves from ER to vesicles - invariant chain broken down
Antigen is engulfed by APC
Protein cleaved to peptide fragments by acid activated proteases
Vesicles with peptides fuse with vesicles containing MHC II molecules
MHC class II molecules binds peptide fragment
MHC-peptide complex transported to cell surface
How does the TCR signal?
Using ITAMs (immune receptor tyrosine-based activation motif)
How is the TCR stabilised by CD4?
Lck phosphorylates ITAMs in TCR upon co-receptor engagement which leads to nucleus expressing genes using TF (TH1/2/TFH/TH17 differentiation)
How are MHC the most polymorphic genes?
Extends range of peptides that can be presented to T cells
Responsible for graft rejection
Genetic influence on some diseases
Different pathogen targets
What are the different intracellular and extracellular sites of infections?
Extracellular: Interstitial spaces, blood, lymph, epithelial surfaces
Intracellular: cytoplasmic and vesicular
What are the different pathogenic mechanisms and infectious agents to disease in direct tissue damage?
Endotoxin: Streptococcus pyogenes → tonsilitis, scarlet fever
Endotoxin: Samonella typhi
Direct cytopathic effect: Influenza virus
What are the different pathogenic mechanisms and infectious agents to disease in indirect tissue damage?
Immune complexes
Anti-host antibody → streptococcus pyogenes
Cell-mediated immunity
What are the examples of immune response?
Infectious agent
Immunodeficiency
Immunopathology
Hypersensitivity (allergy and autoimmunity)
What are the differences between primary and secondary immunodeficiency?
Primary: intrinsic defect in immune system
Genetic
Conditions rare
Dominant/recessive, autosomal/ X-linked
Gene defect may/may not be identified
Secondary: Immune system initially intact
Consequence of another condition
What are the different hypersensitivity reactions?
Type I-IV responses
I-III are antibody mediated
IV is cell mediated
What happens in anaphylaxis?
Severe allergic reaction
IgE
Fc receptors on mast cells & basophils
↑release histamine
↑TNFα
↑vasodilation
Epinephrine:
↑Vasoconstriction
↑Blood glucose levels
What are the different types of vaccines and what do they treat?
Live: MMR, BCG, polio, cholera, yellow fever, nasal influenxa
Dead: Influenza, rabies and pertussis
Subunits: Diptheria, tetanus, cholera, homophiles, hepatitis, HPV, typhoid
mRNA: Covid
