Which antibody is primarily responsible for type I hypersensitivity reactions?

• A. IgG
• B. IgE
• C. IgA
• D. IgM

Answer: (B)

IgE is the antibody that drives type I hypersensitivity. In sensitized individuals, allergen-specific IgE binds to FcεRI on mast cells and basophils. When the allergen reappears, it cross-links these IgE molecules, causing rapid degranulation and release of histamine and other mediators. This mediator burst leads to the immediately recognizable symptoms such as vasodilation, increased vascular permeability, smooth muscle contraction, and in severe cases anaphylaxis. Other antibodies don’t mediate this immediate allergic pathway: IgG is typically involved in type II (cytotoxic) and type III (immune complex) reactions; IgA mainly protects mucosal surfaces; IgM is the initial antibody in a primary response but does not preferentially mediate the rapid allergic response.