Define hypersensitivity Type I and name a classic example and mediator.

• A. Type I is IgG-mediated cytotoxic; example autoimmune hemolytic anemia; mediator histamine
• B. Type I is immune complex-mediated with serum sickness; mediator complement
• C. Type I is IgE-mediated immediate hypersensitivity; examples include allergic rhinitis and anaphylaxis; mediator histamine released from mast cells/basophils
• D. Type I is cell-mediated delayed-type hypersensitivity; mediator IL-2

Answer: (C)

Type I hypersensitivity is an IgE-mediated immediate reaction. IgE antibodies bind to mast cells and basophils; when an allergen cross-links these IgE molecules, the cells rapidly degranulate and release histamine and other mediators. This leads to the quick onset of symptoms such as sneezing, itching, runny nose, urticaria, bronchoconstriction, or, in severe cases, anaphylaxis. Classic examples include allergic rhinitis and anaphylaxis. The primary mediator here is histamine released from mast cells and basophils. Other hypersensitivity types involve different antibodies or T-cell–driven mechanisms (for instance, Type II with IgG/IgM causing cytotoxicity, Type III with immune complexes and complement, and Type IV as a delayed T-cell–mediated reaction), which is why this description best fits Type I.