Which hypersensitivity is primarily a T-cell–mediated delayed-type reaction?

Which hypersensitivity is primarily a T-cell–mediated delayed-type reaction?

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

Answer: (D)

Delayed-type hypersensitivity is driven by T lymphocytes rather than antibodies, and the inflammatory response develops days after exposure. After sensitization, antigen-presenting cells display the culprit to T cells, which then activate and release cytokines that recruit and activate macrophages. This cellular army causes tissue inflammation and damage, often peaking about 48–72 hours later. Helper T cells (especially Th1) orchestrate the response by producing IFN-γ and other signals that boost macrophage activity; cytotoxic T cells can also directly attack antigen-bearing cells. That delayed, cell-mediated, antigen-specific reaction is the hallmark of this hypersensitivity type. Classic examples include the tuberculin skin test and contact dermatitis from certain chemicals, like poison ivy. In contrast, rapid responses in Type I are antibody- and IgE-mediated; Type II involves antibodies targeting cell or matrix antigens with cytotoxic effects; Type III is driven by immune complex deposition and complement activation.

Delayed-type hypersensitivity is driven by T lymphocytes rather than antibodies, and the inflammatory response develops days after exposure. After sensitization, antigen-presenting cells display the culprit to T cells, which then activate and release cytokines that recruit and activate macrophages. This cellular army causes tissue inflammation and damage, often peaking about 48–72 hours later. Helper T cells (especially Th1) orchestrate the response by producing IFN-γ and other signals that boost macrophage activity; cytotoxic T cells can also directly attack antigen-bearing cells. That delayed, cell-mediated, antigen-specific reaction is the hallmark of this hypersensitivity type. Classic examples include the tuberculin skin test and contact dermatitis from certain chemicals, like poison ivy. In contrast, rapid responses in Type I are antibody- and IgE-mediated; Type II involves antibodies targeting cell or matrix antigens with cytotoxic effects; Type III is driven by immune complex deposition and complement activation.