Which type of hypersensitivity is responsible for hay fever?

• A. Type II hypersensitivity (IgG-mediated)
• B. Type III hypersensitivity (immune complex)
• C. Type I hypersensitivity (IgE-mediated)
• D. Type IV hypersensitivity

Answer: (C)

Hay fever is an immediate-type allergic reaction driven by IgE antibodies. When someone is sensitized, pollen exposure causes B cells to switch to producing IgE, which binds to receptors on mast cells and basophils. On subsequent exposures, the pollen allergen cross-links these IgE molecules, triggering rapid degranulation of mast cells and release of histamine and other mediators. This leads to the early, classic symptoms—itchy eyes and nose, sneezing, nasal congestion, and watery rhinorrhea. There can also be a late-phase response with eosinophil recruitment a few hours later, contributing to ongoing nasal inflammation.

This mechanism is best described as Type I hypersensitivity, IgE-mediated. Other hypersensitivity types involve different pathways: cytotoxic reactions (Type II) use IgG/IgM against cell-surface antigens, immune complex–mediated inflammation (Type III) from circulating antigen–antibody complexes, and delayed-type hypersensitivity (Type IV) is T-cell–mediated with a slower onset. Hay fever specifically hinges on IgE binding and mast cell degranulation.