Week 3 - infection and immunity Flashcards
Describe the presentation of fevers. (LO1)
- Rigors.
- Night sweats.
- Excessive sweating.
- Recurrent fever.
Accompanying features:
- Severe headache and photophobia.
- Delirium.
- Myalgia.
- Shock may accompany severe infections and sepsis.
Describe the history-taking aspect of investigations of fevers. (LO1)
- Presenting complaint.
- System review.
- Past medical history.
- Medication history.
- Allergy history.
- Family history.
- Contact history.
- Travel history.
- Occupation.
- Recreational pursuits.
- Animal exposures.
- Dietary history.
- Sexual history.
Describe the clinical investigations of fevers. (LO1)
- Full blood count (FBC).
- Urea and electrolytes (U+Es).
- Liver function tests (LFTs).
- Blood glucose.
- Muscle enzymes.
- Inflammatory markers: ESR, CRP.
- Test for HIV-1 antibodies.
- Autoantibodies, including antinuclear antibodies (ANA).
- Urinalysis and urine culture.
- Blood culture.
- Throat swab for culture or PCR.
- Other speciments as indicated by history - wound swab, sputum culture, stool culture, microscopy, C. diff toxin assay.
- Specific tests and their priority.
What disease specific tests would be done provided the history indicated it (fever)? (LO1)
- Malaria films on 3 consecutive days or malaria rapid diagnostic test (antigen detection).
- Test for non-structural protein (NS1) in dengue (antigen detection).
- Blood cultures for Salmonella typhi.
- Abdominal ultrasound standard test in many regions of Africa, Asia, Oceania and Central and South America.
Describe the features of factitious fever. (LO1)
- Patient who looks well.
- Bizarre temperature chart with absence of diurnal variation or temperature-related changes in pulse rate.
- Temperature >41°C.
- Absence of sweating during defervescence.
- Normal erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), despite high fever.
- Evidence of self-injection or self-harm.
- Normal temperature during supervised (observed) measurement.
- Infection with multiple commensal organisms (e.g. enteric or mouth flora).
Describe the initial management of fever. (LO1)
- Fever and associated symptoms can be treated with paracetamol.
- Tepid sponging to cool the skin.
- Replenishing of salt and water is important in patients with drenching sweats.
- Further management is focused on underlying cause.
Why is past medical history important when asking about a presenting complaint of fevers? (LO1)
- Defines the ‘host’ and likelihood of infection.
- Includes surgical and dental procedures involving prosthetic materials.
- Document previous infections.
- History of intravenous drug injection or receipt of blood products.
- Risks for blood-borne viruses, e.g. HIV-1, HBV, and HCV.
Why is medication history important when asking about a presenting complaint of fevers? (LO1)
- Include non-prescription drugs, use of antimicrobials and immunosuppressants.
- Identify medicines that interact with antimicrobials or that may cause fever.
Why is allergy history important when asking about a presenting complaint of fevers? (LO1)
- Could have an allergy to anti-microbials, noting allergic manifestations (e.g. rash vs. anaphylaxis).
Why is family and contact history important when asking about a presenting complaint of fevers? (LO1)
- Checking for transmissible diseases such as TB, HIV, etc.
Why is travel history important when asking about a presenting complaint of fevers? (LO1)
- Countries visited and whether they were a previous resident.
- Gives info about relevant exposure and likely vaccination history, e.g. likelihood of BCG vaccination in childhood.
Why is occupation important when asking about a presenting complaint of fevers? (LO1)
- Occupational hazard.
- e.g. Anthrax in leather tannery workers.
- Ask about occupation-related vaccines.
Why is recreational pursuit important when asking about a presenting complaint of fevers? (LO1)
- Could be caused by the activity.
- e.g. Leptospirosis in canoeists and windsurfers.
Why is animal exposure important when asking about a presenting complaint of fevers? (LO1)
- Including pets, e.g. dogs.
- Could be hydatid disease.
Why is dietary history important when asking about a presenting complaint of fevers? (LO1)
- Undercooked meats, shellfish, unpasteurised dairy products or well water.
- Establish who else was exposed, e.g. to food-borne pathogens.
- Risks for blood-borne viruses, e.g. HIV-1, HBV, and HCV.
Why is sexual history important when asking about a presenting complaint of fevers? (LO1)
- Explore in confidential manner.
- Most common mode of HIV-1 transmission is heterosexual.
- Vaccine history and use of prophylactic drugs.
- In a traveller or infection-predisposed patient, establish adherence to prophylaxis.
List the common differential diagnoses for bacterial infections causing fever. (LO1)
- Osteomyelitis.
- Pyelonephritis.
- Abscess.
- Infective endocarditis.
- Tuberculosis.
- Cat scratch disease.
- Typhoid fever.
List the common differential diagnoses for viral infections causing fever. (LO1)
- Epstein-Barr virus.
- Cytomegalovirus.
- Enterovirus.
- Adenovirus.
List the common differential diagnoses for malignancies causing fever. (LO1)
- Leukaemia.
- Lymphoma.
- Neuroblastoma.
List the common differential diagnoses for autoimmune diseases causing fever. (LO1)
- Juveline idiopathic arthritis (JIA).
- Systemic lupus erythamatous (SLE).
- Inflammatory bowel disease (IBD).
List the common differential diagnoses for miscellaneous causes of fever. (LO1)
- Kawasaki disease.
- Drug fever.
- Periodic fever.
Describe the epidemiology of sepsis. (LO2)
- In 2017, 48.9 million cases and 11 million sepsis-related deaths worldwide, accounting for 20% of all global deaths.
- 80% of sepsis cases and sepsis-related deaths worldwide occured in low, middle income countries.
- In 2017, almost half of all global sepsis cases occured among children, 20 million cases and 2.9 million deaths in children <5 years.
- Increased incidence can be due to: immunosuppression therapy, invasive procedures, transplantation, chemotherapy, newborns, old age, pregnancy, chronic health conditions.
Define sepsis. (LO2)
The body’s amplified and dysregulated inflammatory response to infection (involving cytokine storm, e.g. tumour necrosis factor and IL-1). This leads to an imbalance between proinflammatory and anti-inflammatory responses. Also known as, systemic inflammatory response syndrome.
Define septicaemia. (LO2)
Bacterial infection in the blood.
Describe the pathophysiology of sepsis. (LO2)
- The inflammatory reaction is mediated by the release of cytokines, including TNF-α, interleukins and prostaglandins from neutrophils and macrophages.
- The cytokines activate the extrinsic coagulation cascade and inhibit fibrinolysis.
- Coagulation system activation leads to the consumption of endogenous anticoagulants (e.g. protein C and antithrombin).
- This leads to the development of microvascular coagulation (thrombosis being a potential factor for organ dysfunction).
- Toxins from bacteria or thrombi can damage small blood vessels and leak fluid into the surrounding tissues.
- This affects your heart’s ability to pump blood to your organs which means lower blood pressure (septic shock).
Define severe sepsis and septic shock. (LO2)
Sepsis can escalate to severe sepsis. Severe sepsis is the dysfunction of ONE of the major organ systems or unexplained metabolic acidosis.
Septic shock can follow severe sepsis. Septic shock is the severe drop in blood pressure contributing to late-stage multiple-organ failure.
Describe the presentation of initial sepsis. (LO2)
- Easily mistaken for other conditions particularly in post-operative patients, e.g. delirium, primary cardiac dysfunction, pulmonary embolism.
Initially:
- Fever.
- Tachycardia.
- Diaphoresis - excess sweating
- Tachypnoea.
- Normal blood pressure.
Describe the presentation of worsening sepsis/septic shock. (LO2)
- Confusion.
- Decreased alertness (particular in very young or the elderly).
- Blood pressure decrease.
- Skin paradoxically warm.
- Diarrhoea.
- Nausea.
- Vomiting.
Describe the presentation of late stage sepsis. (LO2)
- Extremities cool and pale.
- Peripheral cyanosis and mottling.
- Hypothermia.
- Organ dysfunction specific symptoms (e.g. oliguria, dyspnoea - shortness of breath).
- Leukocytosis (increased leukocytes in blood).
- Leukopenia (decreased leukocytes in blood).
Describe the investigations for sepsis. (LO2)
Abnormal increase in 2 or more of the following:
- White cell count (>12x10⁹).
- Body temperature (>38°C)
- Respiratory rate (>20/min).
- Heart rate (>90/min).
Lack of evidence of the probable cause of infection can make diagnosis challenging.
Lab tests:
- FBC.
- Lactate.
- C-reactive protein (CRP).
- Procalcitonin.
Describe the non-immediate management for sepsis. (LO2)
- IV antibiotics within 1 hour of suspecting sepsis.
- If patient in shock, use combination therapy (2 classes of antibiotics)
- Patients with hypoperfusion, give 30mL/kg of intravenous crystalloid within 3 hours and reassess frequently.
- Patients needing vasopressors - target mean arterial pressire is 65mm Hg. Give norepinephrine. Vasopressin or epinephrine can be added. For patients who remain unstable, dobutamine is recommended.
- Intravenous hydrocortisone for patients who are haemodynamically unstable despite fluids and vasopressors.
- Blood transfusion reserved for patients with haemoglobin concentration of <7.0g/dL or if haemorrhage or myocardial ischaemia.
- Platelets given if platelet count is <10,000mm³ or <20,000mm³ with bleeding.
Describe the immediate management of sepsis (within one hour of suspecting). (LO2)
- Oxygen to maintain SpO₂ at >94%.
- Take blood cultures and consider infective source.
- Administer intravenous antibiotics.
- Consider intravenous fluid resuscitation.
- Check serial lactates.
- Commence hourly urine output measurement.
Describe the prognosis of sepsis. (LO2)
- Mortality in sepsis is frequent due to organ failure.
- Risk of mortality increases with number of failing organs.
- Sepsis with 3 or more failing organs have 70% mortality rate.
- Sepsis and no organ failure have 15% mortality rate.
Describe the basic process of B cell function. (LO3)
- Pathogen with antigen invades body.
- Innate immune cells such as phagocytes and antigen presenting cells (APCs) present antigens to T helper cells.
- T helper cells signal for and recruit B cells and present antigens to B cells.
- B cells duplicate by a process called clonal expansion and are now called plasma cells.
- Plasma cells produce antibodies which carry out the adaptive immune response.
Describe the structure of antibodies. (LO3)
- Y-shaped molecule.
- Constant region with Fc receptors for binding to immune cells.
- Variable region for binding to antigens.
Describe the function of antibodies. (LO3)
- Antibody produced by B cell binds to a pathogen via the variable region which is complementary to the antigen.
- By binding the pathogens, antigens can cluster and the exposed Fc receptors signal for other immune cells.
- The Fc region can trigger the complement cascade which has a membrane attack complex (MAC) which is able to lyse the membranes of pathogens and kill them.
- Opsonisation: Fc receptors bind macrophages. The antibody bound to the pathogen allows phagocytic cells to recognise pathogens and engulf them. The pathogen lysed by proteases in the lysozyme. Alternatively, phagocytes can produce ROS (reactive oxygen species) which kills pathogens.
Describe the functions of the Fc receptor of antibodies. (LO3)
- The Fc region can trigger the complement cascade which has a membrane attack complex (MAC) which is able to lyse the membranes of pathogens and kill them.
- Opsonisation: Fc receptors bind macrophages. The antibody bound to the pathogen allows phagocytic cells to recognise pathogens and engulf them. The pathogen lysed by proteases in the lysozyme. Alternatively, phagocytes can produce ROS (reactive oxygen species) which kills pathogens.
List the types of antibodies produced by B cells. (LO3)
- IgA.
- IgE.
- IgG.
Describe IgA. (LO3)
- Dimer of 2 IgA molecules bound by joining protein (J protein).
- Produced in response to mucosal infection.
- Secreted across epithelial surfaces.
- IgA tail binds to IgA receptors allowing it to cross epithelial cells in mucosal tissue (lungs/stomach).
- IgA can bind pathogens which have not yet entered the body.
Describe IgE. (LO3)
- Like IgG but with different Fc domain.
- Can bind IgE receptors on mast cells and eosinophils.
- Binding causes degranulation of these cells leading to histamine release which triggers inflammatory response.
- Mostly defends against parasitses due to binding of eosinophils which can release peroxidase, ribonuclease, deoxyribonuclease, lipase, plasminogen which damage parasites.
- Also the cause of allergies like hayfever and asthma (atopy). Binding of IgE to mast cells in response to allergens like dust or pollen can prime the immune system which triggers inflammation whenever the allergen is encountered.
Describe IgG. (LO3)
- Carries out main function within the plasma by these mechanisms, e.g. complement cascade, opsonisation.
Explain what is meant by class-switching. (LO3)
- The variable region can be joined to the specific constant regions to create different classes of antibodies which can all bind to the same antigen.
- Depending on the pathogen and location of immune response, a specific type of antibody is made.
- VDJ recombination is a genetic mechanism allowing for the creation of a huge range of variable regions to fight off almost all pathogens.
- Various classes are: IgG, IgE, IgA which are class-switched from the basic class, IgM.
Explain what is meant by differentiation of B cells. (LO3)
- Formation of memory cells or plasma cells.
- Memory cells circulate through lymphatics and plasma and contain the ability to become plasma cells for antigens from previous infections.
- Presence of memory cells allows for immunity to certain pathogens.
Describe how a vaccine works to provide immunity. (LO3)
- Antigen in the vaccine is recognised and helps in the production of memory B cells. They produce specific antibodies if a pathogen with that antigen invades the body.
- Subsequent immune response to the infection is more rapid and disease can be prevented.
Describe the potential life-cycle of a mature B cell. (LO3)
- Initially a mature B cell can become a short-lived plasma cell producing IgM when activated by T helper cells or directly by antigens.
- Or it can become activated with interacting with T helper cells or antigens, causing proliferation by clonal expansion.
- B cells can undergo hypermutations which can either increase or decrease affinity for antigens.
- Those with decreased affinity are apoptosed. Those with increased affinity survive and undergo class-switching.
- These can further differentiate into memory cells or plasma cells which produce antibodies.
- Antibodies work by three different mechanisms: neutralisation, opsonisation and the complement cascade.
Describe the three mechanisms of action of antibodies against pathogens. (LO3)
- Neutralisation - antibodies can directly inhibit toxins produced by pathogens.
- Opsonisation - antibodies use their Fc receptor on the constant region to attract phagocytes which lyse pathogens.
- Complement cascade - activated complement which forms the membrane attack complex (MAC) which makes holes in the membrane, killing the pathogen.
Describe B cell antigen presentation. (LO3)
- Gene rearrangements hook variable regions to constant regions which are anchored to the cell membrane, forming a B cell receptor.
- B cell receptor binds the antigen and internalises it (endocytosis).
- The antigen is prepped for presentation on MHC II to T helper cells via the antigen presenting pathway.
- Binding of the antigen to T helper cells leads to antigen receptor activation of T cells.
- T cells release CD40 ligand which binds CD40 surface receptors on B cells.
- The costimulatory signal leads to T cell mediated cytokine release.
- B cells proliferate by clonal expansion in response to cytokines.
- B cells can become plasma cells or memory cells.
List the 3 types of T cells. (LO4)
- T cytotoxic cells - destroy infected cells.
- T helper cells - recognise antigen presenting cells (APCs) and active B cells.
- T memory cells - remain in blood stream for secondary response.
Describe the function of T cells. (LO4)
- Antigen presentation lies at the interface between innate and acquired immunity.
- T cells have T cell receptors that recognise broken down antigens presented by APCs.
- If the T-cell sees the pieces of virus as ‘foreign’, it will send a signal back to B cells which will make antibodies to the virus.
- This signal is called T-cell help and is crucial to the production of correct antibodies.
- The recognition of the antigen by the T cell will also cause T cell proliferation, which in turn, triggers proliferation of B cells (clonal expansion).
Describe the action of T cytotoxic cells. (LO4)
- Recognise infected cell due to antigen presentation.
- They release perforins which destroy the cell.
- This leaves the pathogen with nowhere to live - more important for viruses.
- Can help kill tumour cells.
Describe how T cell diversity occurs. (LO4)
- Allowed by VDJ recombination.
- Exons of V, D and J region of genes are repeated.
- A loop forms in D and J cluster - one repeat from each selected for use.
- The RAG protein removes unused repeats.
- The same thing happens in the V region after.
- You end up with a gene that codes for a T cell receptor.
Describe the action of T helper cells. (LO4)
- T cell receptor binds to antigen being presented by B cell.
- If the B cell has the correct antigen, the T cell expresses CD40 ligand which binds to CD40 on the B cell which sends more signals back to the T cell - COSTIMULATORY SIGNALS.
- This causes T cell to release cytokines which cause the B cell to duplicate (clonal expansion).
- Also activates antibody class-switching.
- B cell released Ig (if differentiated into plasma cell), which helps destroy pathogens.
Explain the important of T cells. (LO4)
- If T cells aren’t made - B cells can’t survive and also can’t produce the correct antibodies.
- Shown in HIV where T cells are not produced.
- This vastly weakens the immune system - leaves us susceptible to opportunistic infections.
What are costimulatory signals between T and B cells and why are they important? (LO4)
- When a T cell binds to the antigen presented by B cells, it expresses CD40 ligand if the B cell has the correct antigen.
- This then binds to CD40 on the B cell which sends more signals back to the T cell.
- THIS IS COSTIMULATORY SIGNALS.
- This causes T cells to release cytokines which triggers clonal expansion of B cells.
What is phagocytosis? (LO5)
- The process by which certain living cells called phagocytes ingest or engulf other cells or particles.
- Phagocytes are derived from the bone marrow - myeloid cells.
List the cells that can engage in phagocytosis. (LO5)
- Macrophages.
- Monocytes.
- Neutrophils.
- Dendritic cells - unlike the others, dendritic cells have the means to “preserve useful information” due to less degradation and the preservation of antigens, allowing better antigen presentation.
What is netosis? (LO5)
- Neutrophil extracellular traps (NET) formation activated by immune receptors that include reactive oxygen species (ROS).
- NETs can trap and kill pathogens such as viruses, fungi and parasites.
- If unregulated, NETs can damage healthy tissue and are therefore sometimes associated with autoimmune disorders.
Describe neutrophils in terms of phagocytosis. (LO5)
- Produced daily during homeostasis.
- Short lived - not more than few hours.
- During infection, the production increases.
- After neutrophils kill a pathogen, they die.
- Dead neutrophils are a major component of pus.
- Only migrate to inflamed tissue.
- Can kill pathogens via phagocytosis, via the release of toxic chemicals/enzymes and via NETs.
- Recruited by chemokines (small proteins that influence the immune system.)