Which hypersensitivity is IgE-mediated?

• A. Type I
• B. Type II
• C. Type III
• D. Type IV

Answer: (A)

IgE-mediated hypersensitivity is the immediate-type allergic reaction. It starts with sensitization to an allergen and production of allergen-specific IgE that binds to receptors on mast cells and basophils. When the allergen is encountered again, it cross-links the bound IgE, triggering rapid degranulation and release of mediators like histamine, leukotrienes, and prostaglandins. This cascade produces quick-onset symptoms such as itching, swelling, bronchoconstriction, and, in severe cases, anaphylaxis. The defining feature is the involvement of IgE and mast cell–basophil degranulation with a prompt response after exposure. Other hypersensitivity types involve different mechanisms: Type II uses IgG or IgM against cell-surface antigens; Type III involves immune complex deposition and complement activation; Type IV is T-cell–mediated and delayed. So the IgE-mediated, immediate allergic response corresponds to Type I hypersensitivity.