Paper I — 2020 February (Supplementary) (2010 Scheme) — Q2
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Paper I
2020 February (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Type I hypersensitivity.
Q25 marksShort Essays
Answer
Type I (immediate/anaphylactic) hypersensitivity is an IgE-mediated reaction, occurring within minutes of re-exposure to an antigen (allergen) in a previously sensitized individual.
Mechanism:
Sensitization phase: on first exposure, susceptible (atopic) individuals mount a Th2-skewed response, producing allergen-specific IgE, which binds via its Fc portion to high-affinity FcεRI receptors on mast cells and basophils.
Effector phase: on re-exposure, the allergen cross-links adjacent IgE molecules on the sensitized cell surface, triggering degranulation — release of preformed mediators (histamine, tryptase) and newly synthesized mediators (leukotrienes, prostaglandins, platelet-activating factor).
These mediators cause vasodilation, increased vascular permeability, smooth muscle contraction (bronchospasm), and increased mucus secretion — producing the immediate response within minutes.
A late-phase reaction (4–8 hours later) follows, driven by recruited eosinophils, neutrophils, and Th2 cells, sustaining inflammation.
Laboratory diagnosis: skin prick testing, serum total and allergen-specific IgE (RAST/ImmunoCAP), peripheral eosinophil count.
Management: allergen avoidance, antihistamines for mild reactions, adrenaline as first-line emergency treatment for anaphylaxis, and allergen-specific immunotherapy for long-term desensitization in selected cases.
