Major histocompatibility complex

Major histocompatibility complex (MHC), or human leukocyte antigen (HLA), proteins serve two general roles.

MHC proteins function as carriers to present antigens on cell surfaces. MHC class I proteins are essential for presenting viral antigens and are expressed by nearly all cell types, except red blood cells. Any cell infected by a virus has the ability to signal the problem through MHC class I proteins. In response, CD8+ T cells (also called CTLs) will recognize and kill infected cells. MHC class II proteins are generally only expressed by antigen-presenting cells like dendritic cells and macrophages. MHC class II proteins are important for presenting antigens to CD4+ T cells. MHC class II antigens are varied and include both pathogen- and host-derived molecules.

MHC proteins also signal whether a cell is a host cell or a foreign cell. They are very diverse, and every person has a unique set of MHC proteins inherited from his or her parents. As such, there are similarities in MHC proteins between family members. Immune cells use MHC to determine whether or not a cell is friendly. In organ transplantation, the MHC or HLA proteins of donors and recipients are matched to lower the risk of transplant rejection, which occurs when the recipient's immune system attacks the donor tissue or organ. In stem cell or bone marrow transplantation, improper MHC or HLA matching can result in graft-versus-host disease, which occurs when the donor cells attack the recipient’s body.

Source: NIAID (NIH)1

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Major Histocompatibility Complex: The genetic region which contains the loci of genes which determine the structure of the serologically defined (SD) and lymphocyte-defined (LD) Transplantation Antigens, genes which control the structure of the Immune Response-Associated Antigens, Human; the Immune Response Genes which control the ability of an animal to respond immunologically to antigenic stimuli, and genes which determine the structure and/or level of the first four components of complement.2

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  1. Source: NIAID (NIH): topics/ immuneSystem/ Pages/ immuneCells.aspx
  2. Source: MeSH (U.S. National Library of Medicine)
  3. Source: NIH News in Health (NIH): issue/ mar2011/ feature1
  4. Source: NIDDK (NIH): health-information/ health-topics/ liver-disease/ liver-transplant/ Pages/ facts.aspx
  5. Source: EMA (EU): ema/ index.jsp? curl=pages% 2Fmedicines% 2Fhuman% 2Forphans% 2F2009% 2F11% 2Fhuman_orphan_000044.jsp& mid=WC0b01ac058001d12b

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