What is Major histocompatibility complex?
A '''major histocompatibility complex''', or MHC, is a [[molecule]] on the outside of [[immune cell]]s such as [[white blood cells]]. It is [[code]]d for by a large [[Gene duplication|gene family]] in all [[vertebrate]]s.<ref name=Janeway>Janeway, Charles A. ''et al'' 2001. ''Immunobiology: the immune system in health and disease''. 5th ed, Garland, 167{{ndash}}184. {{ISBN|0-4430-7098-9}}</ref> The function of is MHC molecules to sense foreign [[antigen]]s, and hence the presence of 'foreign' proteins. They bind [[peptide]] fragments from [[pathogen]]s onto their [[cell membrane|cell surface]]. Once there, the fragments are recognized by [[T-cell]]s. [[Adaptive immunity]] depends on this reaction. == Effectiveness == Two properties of MHC molecules make it difficult for pathogens to dodge the immune response.<ref name=Janeway/> 1. MHC is polygenic.<ref>polygenic = many genes</ref> There are several different genes, so that each person or animal has a set of MHC molecules which bind somewhat different ranges of peptides. 2. The MHC genes are highly [[polymorphism|polymorphic]]. This means there are multiple variants [[alleles]] of each gene in the [[population]]. The polymorphism is so high that in a mixed population there are no two individuals with exactly the same set of MHC genes and molecules, with the exception of identical twins. Inside a population, the presence of many different alleles means there will almost always be an individual with a specific MHC molecule able to load the correct [[peptide]] to recognize a specific [[microbe]]. The evolution of the MHC polymorphism means that a population will not succumb to a new pathogen or a mutated one, because at least some individuals will be able to develop an adequate immune response to defeat the pathogen. The variations in the MHC molecules are the result of the [[inheritance]] of different MHC molecules. === Transplants === [[Transplant]]s are limited to those which get the least reaction from the MHC system in tests of tissue matches. === Autoimm