type IV

a reaction of cell-mediated immunity, the immune response being initiated by antigen-specific T lymphocytes; in contrast to the reactions of immediate hypersensitivity, reactions take one or more days to develop and can be transferred by lymphocytes but not by serum. Reactions are mediated by T lymphocytes both through release of cytokines and through direct cytolysis. In the former mechanism, release of vasoactive and chemotactic cytokines is triggered by contact between the T cells and specific antigens on antigen-presenting cells; the cytokines attract and activate non–antigen-specific monocytes and macrophages, resulting in local erythema and induration, and leading to granuloma formation and necrosis if the eliciting stimulus cannot be eliminated. A common example is the tuberculin reaction elicited in skin testing for tuberculosis. In direct cytolysis, sometimes called cell-mediated cytotoxicity, cytotoxic T lymphocytes interact with foreign antigens presented by class I MHC molecules on cell surfaces, causing lysis of these foreign cells, as in allograft rejection. Type IV reactions can be induced by intracellular parasites, such as certain viruses, mycobacteria, and fungi, foreign tissue, tumor cells, soluble proteins, and haptens. The term is often equated with delayed hypersensitivity reaction, although the latter term is sometimes restricted to cytokine-mediated reactions (as contrasted with direct cytolysis). Called also cell-mediated or T cell–mediated hypersensitivity r.