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Certain sites of the human body have immune privilege, meaning they are able to tolerate the introduction of antigens without eliciting an inflammatory immune response. Tissue grafts are normally recognised as foreign antigen by the body and attacked by the immune system. However in immune privileged sites, tissue grafts can survive for extended periods of time without rejection occurring. Immunologically privileged sites were thought to include: * the brain, but this is now known to be incorrect and indeed immune cells of the CNS contribute to the maintenance of neurogenesis and spatial learning abilities in adulthood 〔Ziv, Y.et al (2006). Nature Neuroscience, Immune cells contribute to the maintenance of neurogenesis and spatial learning abilities in adulthood 9, 268 - 275.〕 * the eyes * the placenta and the fetus * the testicles Immune privilege is also believed to occur to some extent, or able to be induced, in articular cartilage. Immune privilege is thought to be an evolutionary adaptation to protect vital structures from the potentially damaging effects of an inflammatory immune response. Inflammation in the brain or eye can lead to loss of organ function, while immune responses directed against a fetus can lead to the loss of the fetus. Medically, a cornea transplant takes advantage of this, as does knee meniscal transplantation. ==Mechanisms== Antigens from immune privileged regions have been found to interact with T cells in an unusual way inducing tolerance as previously opposed to a destructive response.〔Janeway, C. A.Jr., Travers, P., Walport, M., Shlomchik. M.J. (2005). ImmunoBiology, the immune system in health and disease 6th Edition. Garland Science.〕 Immune privilege has emerged as an active rather than a passive process. Physical structures surrounding privileged sites cause a lack of lymphatic drainage, limiting the immune system's ability to enter the site. Other factors that contribute to the maintenance of immune privilege include: * low expression of classical MHC class Ia molecules * expression of immunoregulatory nonclassical, low polymorphic class Ib MHC molecules * increased expression of surface molecules that inhibit complement activation * local production of immunosuppressive cytokines such as TGF-β〔(【引用サイトリンク】title=Autoimmunity )〕 * presence of neuropeptides * constitutive expression of Fas ligand that controls the entry of Fas-expressing lymphoid cells.〔 The nature of isolation of immunologically privileged sites from the rest of the body's immune system can cause them to become targets of autoimmune diseases or conditions, including sympathetic ophthalmia in the eye. 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「Immune privilege」の詳細全文を読む スポンサード リンク
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