What characterizes Type IV hypersensitivity?

• A. A T-cell response that occurs 24-72 hours after exposure to an antigen
• B. Immediate antibody-mediated reaction
• C. Deposition of immune complexes in vessels
• D. Autoantibody destruction of tissue on exposure

Answer: (A)

Type IV hypersensitivity is a delayed-type, cell-mediated immune response driven by T lymphocytes rather than antibodies. After exposure to the antigen, the reaction develops over 24–72 hours as sensitized T cells release cytokines that recruit and activate macrophages, leading to localized inflammation and tissue damage. This delayed timing and reliance on T cells distinguish it from antibody-mediated hypersensitivities.

So the description that a T-cell–driven response occurs 24–72 hours after exposure fits Type IV hypersensitivity best. By contrast, immediate antibody-mediated reactions describe Type I; deposition of immune complexes describes Type III; and autoantibody destruction of tissue on exposure describes Type II.