Question
A 20 year old male develops redness and swelling over the skin after eating peanuts.
- (a) Define and classify hypersensitivity. 3 mark(s)
- (b) What is the type of hypersensitivity reaction seen in the above case. 1 mark(s)
- (c) Describe the mechanism. 3 mark(s)
- (d) Enumerate the clinical manifestations of this type of hypersensitivity and its management. e) Enumerate the clinical applications of this type of hypersensitivity with two suitable examples. 1 mark(s)
Answer
a) Definition and classification of hypersensitivity Hypersensitivity is an exaggerated, damaging immune response to an antigen (allergen) in a previously sensitized individual, causing tissue injury rather than protection.
Classification (Gell and Coombs):
- Type I — Immediate/anaphylactic, IgE-mediated.
- Type II — Antibody-mediated cytotoxic.
- Type III — Immune-complex-mediated.
- Type IV — Delayed-type, cell-mediated.
b) Type of hypersensitivity in this case: Type I (immediate/anaphylactic) hypersensitivity — redness and swelling developing shortly after eating peanuts (a classical food allergen) is consistent with an IgE-mediated reaction.
c) Mechanism
- Sensitization phase: on first exposure to peanut allergen, a susceptible (atopic) individual mounts 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 (as in this patient), 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 (producing the swelling/oedema), and pruritus/erythema, within minutes of exposure.
- A late-phase reaction may follow 4–8 hours later, driven by recruited eosinophils, neutrophils, and Th2 cells.
d) Clinical manifestations and management Manifestations: localized urticaria/angioedema (as in this case); allergic rhinitis; bronchial asthma; and, in severe cases, anaphylaxis (hypotension, bronchospasm, laryngeal oedema — a medical emergency).
Management: allergen avoidance (avoiding peanuts in this patient); antihistamines (H1 blockers) for mild-moderate reactions; adrenaline (epinephrine) as first-line emergency treatment for anaphylaxis; corticosteroids as an adjunct for more severe/prolonged reactions; allergen-specific immunotherapy for long-term desensitization in selected cases.
e) Clinical applications with two examples Type I hypersensitivity testing is used diagnostically to identify specific allergen sensitivities:
- Skin prick testing — a small amount of allergen extract is introduced into the skin; a wheal-and-flare reaction within 15–20 minutes indicates specific IgE sensitization, used to identify causative allergens in suspected food/environmental allergy.
- Serum specific IgE testing (RAST/ImmunoCAP) — measures allergen-specific IgE levels in blood, an alternative to skin testing, particularly useful when skin testing is contraindicated (e.g., severe eczema, dermatographism, or risk of anaphylaxis from direct skin exposure).