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Type 4 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. CD4+ helper T cells recognize antigen in a complex with Class II major histocompatibility complex. The antigen-presenting cells in this case are macrophages that secrete IL-12, which stimulates the proliferation of further CD4+ Th1 cells. CD4+ T cells secrete IL-2 and interferon gamma, further inducing the release of other Th1 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. ==Examples== An example of a TB infection that came under control: ''M. tuberculosis'' are engulfed by macrophages after being identified as foreign, but due to an immuno-escape mechanism peculiar to mycobacteria, TB bacteria are able to block the fusion of their enclosing phagosome with lysosomes which would destroy the bacteria. Thereby TB can continue to replicate within macrophages. After several weeks, the immune system somehow (as yet unexplained ) ramps up and, on stimulation with IFN-gamma, the macrophages become capable of killing ''M. tuberculosis'' by forming phagolysosomes and nitric oxide radicals. However the hyper-activated macrophages secrete TNF which recruits multiple monocytes into the battle. These cells differentiate into epithelioid histiocytes which wall off the infected cells, but at the cost of significant inflammation and local damage. Some other clinical examples: *Temporal arteritis *Hashimoto's thyroiditis *Symptoms of leprosy *Symptoms of tuberculosis *Coeliac disease *Graft-versus-host disease〔(【引用サイトリンク】title=eMedicine - Hypersensitivity Reactions, Delayed : Article by Walter Duane Hinshaw )〕 *Chronic transplant rejection 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Type IV hypersensitivity」の詳細全文を読む スポンサード リンク
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