NBME Immunology Practice Test

Delayed-type (Type IV) hypersensitivity is driven by a T cell–mediated response, specifically that the T helper 1 (Th1) subset steers this process. Th1 cells differentiate and release cytokines like IFN-γ that activate macrophages, leading to the macrophage-dominated inflammation seen in DTH reactions. The response takes time to develop because it requires antigen presentation, T cell activation, and macrophage activation, typically peaking after 24–72 hours. This is why DTH reactions occur later after exposure, as in the tuberculin skin test or contact dermatitis. Other helper T cell subsets have different roles: Th2 cells promote antibody-mediated responses and allergic reactions (not the classic DTH), Th17 cells recruit neutrophils and are involved in certain mucosal defenses and autoimmune processes, and regulatory T cells suppress immune responses. Therefore, Th1 cells are the best fit for delayed-type hypersensitivity.