Which class of immunoglobulin binds to basophils and mast cells to mediate immediate hypersensitivity reactions?

• A. IgA
• B. IgD
• C. IgE
• D. IgG

Answer: (C)

Immediate hypersensitivity is driven by IgE. IgE antibodies bind with high affinity to FcεRI receptors on the surface of mast cells in tissues and basophils in blood, effectively arming these cells with allergen-specific IgE. When an allergen enters, it cross-links adjacent IgE molecules on these receptors, triggering rapid degranulation and release of histamine, leukotrienes, and other mediators. This causes the quick vascular and smooth muscle changes seen in allergies. Other immunoglobulins don’t play this primary role: IgA mainly protects mucosal surfaces and doesn’t drive mast cell degranulation; IgD functions mainly as a B-cell receptor with limited serum activity; IgG has many roles in opsonization, neutralization, and complement activation but is not the primary mediator of immediate hypersensitivity.