Hypersensitivity: T Lymphocyte-mediated (Type IV)

Five types of hypersensitivity reactions have been described: types I, II, III and V depend on the interaction of antigen with antibody and have been termed immediate; type IV depends on the interaction of antigen with T lymphocytes and has been called delayed-type hypersensitivity, or DTH. The DTH reaction involves cellular activation of T-helper cells (CD4+) and/or cytotoxic T cells (CD8+ CTLs). Subsequent cytokine secretion gives rise to distinct pathologies. In many cases, sustained release of antigen and continued activation of sensitized T cells results in amplification of responses that provoke excessive macrophage activation. These events are the basis for development of a broad range of inflammatory pathologies which range from erythema and oedema to granuloma formation, to extended development of fibrosis and even tissue necrosis.