Which hypersensitivity is a delayed-type reaction?

Which hypersensitivity is a delayed-type reaction?

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

Answer: (A)

Delayed-type hypersensitivity is a cell-mediated immune response driven by sensitized T lymphocytes and macrophages. It develops hours to days after exposure, typically 24–72 hours, because T cells must be activated and recruit macrophages to amplify the response. The reaction is not primarily antibody-driven; cytokines from Th1 cells activate macrophages, causing localized inflammation and sometimes granuloma formation. Classic examples include contact dermatitis from metals or plants and the tuberculin skin test. This distinguishes it from immediate hypersensitivity, which is IgE-mediated and rapid; cytotoxic antibody-mediated reactions; and immune complex–mediated reactions. So the delayed-type reaction corresponds to Type IV.

Delayed-type hypersensitivity is a cell-mediated immune response driven by sensitized T lymphocytes and macrophages. It develops hours to days after exposure, typically 24–72 hours, because T cells must be activated and recruit macrophages to amplify the response. The reaction is not primarily antibody-driven; cytokines from Th1 cells activate macrophages, causing localized inflammation and sometimes granuloma formation. Classic examples include contact dermatitis from metals or plants and the tuberculin skin test. This distinguishes it from immediate hypersensitivity, which is IgE-mediated and rapid; cytotoxic antibody-mediated reactions; and immune complex–mediated reactions. So the delayed-type reaction corresponds to Type IV.