Type IV hypersensitivity

Type IV hypersensitivity is often called delayed type hypersensitivity as the reaction takes two to three days to develop. Unlike the other types, it is not antibody mediated but rather is a type of cell-mediated response.

CD8+ cytotoxic T cells and CD4+ helper T cells recognize antigen in a complex with either type 1 or 2 major histocompatibility complex. The antigen-presenting cells in this case are macrophages which secrete IL-12, which stimulates the proliferation of further CD4+ T cells. CD4+ T cells secrete IL-2 and interferon gamma, further inducing the release of other Type 1 cytokines, thus mediating the immune response. Activated CD8+ T cells destroy target cells on contact while activated macrophages produce hydrolytic enzymes and, on presentation with certain intracellular pathogens, transform into multinucleated giant cells.

Examples
Some clinical examples:
 * Contact dermatitis (poison ivy rash, for example)
 * Temporal arteritis
 * Hashimoto's thyroiditis
 * Symptoms of leprosy
 * Symptoms of tuberculosis
 * Mantoux test
 * Coeliac disease
 * Graft-versus-host disease
 * Chronic transplant rejection