Past Papers · SAQ
Hypersensitivity — Reaction Types
2015A Q15
Exam questionDescribe the different types of hypersensitivity reactions including an example of each.
CICMWrecks answer
Master answer
HYPERSENSITIVITY REACTIONS
| Type | Pathophysiology | Disease types |
|---|---|---|
| Type I Immediate Hypersensitivity IgE mediated | Sensitisation Plasma B cell production of IgE Mast cell proliferation → IgE binds to mast cells On re-exposure Allergenic antigen binds to mast cell (expressing IgE) → Mast cell degranulates Mast cell degranulation leads to Primary mediators: – Serotonin – Histamine Secondary mediators – Leukotrienes (SRSA) – Prostaglandins – ECFA, NCF | Anaphylaxis Atopy |
| Type II Cell Cytotoxicity IgG, IgM Mediated | Antibody attaches to antigen on target cell Complement release C5-9 “membrane attack complex” → Causes cell lysis | Blood Transfusions Goodpasteur’s syndrome Autoimmune cytopaenias |
| Type III Immune Complex IgG, IgM, IgA mediated | Circulating antibody-antigen complexes are deposited in basement membranes e.g. peritoneum, blood vessels, joints, glumeruli Complement cascade activation Attract granulocytes → inflammation | SLE Serum Sickness Necrotising vasculitis |
| Type IV Delayed hypersensitivity T-cell mediated | T cell mediated Cell mediated, as opposed to previous humoral Peak reaction 2-3 days CD4 helper T cells cause recruitment of Neutrophils Macrophages CD8 cytotoxic T cells | TB, Sarcoid Granulomatosis with polyangiitis (Wegener’s) Granulomatous vasculitis |
SRSA: “slow reacting substance” A – Mixture of Leukotrienes (LTC4, LTD4, LTE4)
ECFA: Eosinophil chemotactic factor of anaphylaxis
NCF: Neutrophil chemotactic factor of anaphylaxis
Key Points:
- “Anaphylaxis” may be:
- True anaphylaxis: a symptoms complex following exposure of a sensitised individual to an antigen, produced by a type I hypersensitivity reaction, associated with IgE mediated mast cell degranulation
- Anaphylactoid reactions: Indistinguishable from true anaphylaxis, however the immune nature of the reaction is either unknown, or not due to a type I hypersensitivity reaction, immediate generalised reaction is a better term.
Gladwin / Mooney 2016
Past papers
Exam appearances
| Exam | Exact exam wording | Candidate success |
|---|---|---|
| 2015A Q15 | Describe the different types of hypersensitivity reactions including an example of each. | 79% |
| 2013A Q16 | Classify and describe immune hypersensitivity reactions. Give examples for each reaction. | — |
| 2008B Q19 | Classify the hypersensitivity reactions, give an example for each reaction and describe the pathophysiological processes of each reaction. | — |
| 2007B Q09 | Classify the hypersensitivity reactions. Briefly describe the pathophysiological processes of each reaction. Give an example of each reaction. | 28% |