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Alloimmunity is an immune response to foreign antigens (alloantigens) from members of the same species. The body attacks mainly transplanted tissue and even the fetus in some cases. Alloimmune response results in graft rejection, which is manifested as deterioration or complete loss of graft function. In contrast, autoimmunity is an immune response to the self's own proteins. (The ''allo-'' prefix means "other", whereas the ''auto-'' prefix means "self".) Alloimmunity is caused by the difference between products of highly polymorphic genes, primarily genes of MHC complex, of the donor and graft recipient. These products are recognized by T-lymphocytes and other mononuclear leukocytes which infiltrate the graft and damage it. ==Types of the rejection== * Acute rejection is caused by antigen-specific Th1 and cytotoxic T-lymphocytes. They recognize transplanted tissue because of expression of alloantigens. A transplant is rejected during first several days or weeks after transplantation.〔Sánchez-Fueyo A, Strom TB (2011), Immunologic basis of graft rejection and tolerance following transplantation of liver or other solid organs. Gastroenterology 140(1):51-64〕 * Hyperacute and accelerated rejection is antibody-mediated immune response to the allograft. Recipient’s blood already contains circulating antibodies before the transplantation 〔 – either IgM or antibodies incurred by previous immunization (e.g. by repeated blood transfusion). In case of hyperacute rejection, antibodies activate complement; moreover the reaction can be enhanced by neutrophils. This type of rejection is very fast, the graft is rejected in a few minutes or hours after the transplantation. Accelerated rejection leads to phagocyte and NK cell activation (not of the complement) through their Fc receptors that bind Fc parts of antibodies. Graft rejection occurs within 3 to 5 days. This type of rejection is typical response to xenotransplantations. * Chronic rejection mechanism is not fully understood so far but it is known that it is associated with alloantibody and cytokine production. Endothelium of the blood vessels is being damaged, therefore the graft is not sufficiently supplied with blood and is replaced with fibrous tissue (fibrosis).〔Seetharam A, Tiriveedhi V, Mohanakumar T (2010), Alloimmunity and autoimmunity in chronic rejection. Curr Opin Organ Transplant 15(4):531-536〕 It takes two months at least to reject the graft in this way. 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「alloimmunity」の詳細全文を読む スポンサード リンク
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