Urticarial rash after pollen exposure is best classified as which hypersensitivity reaction?

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

Answer: (D)

Urticarial rash from pollen exposure is a rapid, IgE-mediated allergic reaction, which is characteristic of Type I hypersensitivity. In this process, pollen allergens cross-link IgE antibodies bound to FcεRI receptors on mast cells and basophils, triggering immediate degranulation and release of histamine and other mediators. The histamine causes the itching and the raised, swollen wheals seen in hives, typically occurring quickly after exposure.

The other hypersensitivity types don’t fit this pattern: Type II involves antibodies directed at cell-surface components causing cytotoxic damage; Type III involves immune complex deposition leading to inflammation; Type IV is a delayed T-cell–mediated reaction.