Which immunoglobulin mediates immediate hypersensitivity?

• A. IgG
• B. IgA
• C. IgE
• D. IgM

Answer: (C)

Immediate hypersensitivity is driven by IgE antibodies that sensitize mast cells and basophils. When an allergen enters, it binds to IgE already attached to FcεRI receptors on these cells, causing cross-linking. This triggers rapid degranulation and release of histamine and other mediators, leading to the quick symptoms of an allergic reaction such as vasodilation, increased vascular permeability, mucus production, and smooth muscle contraction. This direct link between IgE and rapid mediator release is what sets IgE apart as the mediator of immediate hypersensitivity.

Other immunoglobulins have different roles: IgG is mainly involved in opsonization and activities in other antibody-mediated processes, IgA protects mucosal surfaces, and IgM is the first antibody produced in a primary response but does not drive immediate allergic reactions.