Type IV hypersensitivity

Type IV hypersensitivity
Other names	delayed-type hypersensitivity; DTH; cell-mediated hypersensitivity
Specialty	Immunology

Type IV hypersensitivity, in the Gell and Coombs classification of allergic reactions, often called delayed-type hypersensitivity, is a type of hypersensitivity reaction that can take a day or more to develop.^[1] Unlike the other types, it is not humoral (not antibody-mediated) but rather is a type of cell-mediated response. This response involves the interaction of T cells, monocytes, and macrophages.

This reaction is caused when CD4⁺ T_h1 cells recognize foreign antigen in a complex with the MHC class II on the surface of antigen-presenting cells. These can be macrophages that secrete IL-12, which stimulates the proliferation of further CD4⁺ T_h1 cells. CD4⁺ T cells secrete IL-2 and interferon gamma (IFNγ), inducing the further release of other T_h1 cytokines, thus mediating the immune response. Activated CD8⁺ T cells destroy target cells on contact, whereas activated macrophages produce hydrolytic enzymes and, on presentation with certain intracellular pathogens, transform into multinucleated giant cells.

The overreaction of the helper T cells and overproduction of cytokines damage tissues, cause inflammation, and cell death. Type IV hypersensitivity can usually be resolved with topical corticosteroids and trigger avoidance.^[1]