Editing Histocompatibility (section)

== Major histocompatibility complex (MHC) ==
{{Main|Major histocompatibility complex}}

HLA, the human form of the [[major histocompatibility complex]] (MHC), is located on [[Chromosome 6 (human)|chromosome 6]] at 6p21.3.<ref>{{cite book|title=Major Histocompatibility Complex: Evolution, Structure, and Function|last=Kasahara|first=M. | name-list-style = vanc |publisher=Springer|year=2000|isbn=978-4-431-70276-4|location=New York}}</ref> Individuals inherit two different HLA [[haplotype]]s, one from each parent, each containing more than 200 genes relevant to helping the immune system recognize foreign invaders. These genes include [[MHC class I]] and [[MHC class II|class II]] cell-surface proteins.<ref name="Merck_Manual">{{cite web | url = http://www.merckmanuals.com/professional/immunology-allergic-disorders/biology-of-the-immune-system/human-leukocyte-antigen-hla-system | title = Human Leukocyte Antigen (HLA) System | last = Delves | first = Peter J. | name-list-style = vanc |date=January 2017|website=Merck Manual }}</ref> MHC Class I [[molecule]]s—[[HLA-A]], [[HLA-B]], and [[HLA-C]]—are present on all nucleated cells and are responsible for signaling to an immune cell that an [[antigen]] is inside the cell.<ref name="GHR_HLA" /> MHC Class II molecules—[[HLA-DR]], and [[HLA-DQ]] and [[HLA-DP]]—are only present on antigen presenting cells and are responsible for presenting molecules from invading organisms to cells of the immune system.<ref>{{cite journal | vauthors = Schwartz RH | title = T-lymphocyte recognition of antigen in association with gene products of the major histocompatibility complex | journal = Annual Review of Immunology | volume = 3 | issue = 1 | pages = 237–61 | date = 1985-01-01 | pmid = 2415139 | doi = 10.1146/annurev.iy.03.040185.001321 }}</ref>

The MHC genes are highly [[Genetic polymorphism|polymorphic]], with thousands of versions of the MHC receptors in the population, though any one individual can have no more than two versions for any one locus.<ref name="Ayala_García_2012">{{cite journal | vauthors = Ayala García MA, González Yebra B, López Flores AL, Guaní Guerra E | title = The major histocompatibility complex in transplantation | journal = Journal of Transplantation | volume = 2012 | pages = 842141 | date = 2012 | pmid = 22778908 | doi = 10.1155/2012/842141 | pmc=3388305| doi-access = free }}</ref> MHC receptors are codominantly expressed, meaning all inherited alleles are expressed by the individual.<ref>{{cite book | title = Immunobiology: The Immune System in Health and Disease | edition = 5th | year = 2001 | vauthors = Janeway CA, Travers P, Walport M, Shlomchik M | isbn = 978-0-8153-3642-6 | chapter = The Major Histocompatibility Complex and Its Functions | publisher = Elsevier España | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK27156 | url-access = registration | url = https://archive.org/details/immunobiology00char }}</ref> The wide variety of potential alleles and multiple loci in the HLA allow for many unique combinations in individuals.