Skin testing relies on which type of hypersensitivity mechanism?

• A. Antibody-mediated hypersensitivity
• B. T cell-mediated delayed-type hypersensitivity
• C. Immune complex-mediated hypersensitivity
• D. Complement-mediated lysis

Answer: (B)

Skin testing relies on a T cell–mediated delayed-type hypersensitivity response. When the antigen is encountered again in the skin, memory T lymphocytes recognize it and release cytokines such as IFN-γ and IL-2, which activate macrophages and recruit other inflammatory cells. This creates a localized inflammatory reaction with induration and redness that peaks 24 to 72 hours after exposure. This cell-driven process is what underlies tests like the tuberculin (PPD) skin test and patch testing for contact allergens. It differs from antibody-mediated reactions, immune complex–mediated injury, or complement-mediated lysis, which are not the mechanisms typical of these skin tests.