MHC class III

Chhandama: expansion

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'''MHC class III''' is a group of proteins belonging the class of [[major histocompatibility complex]] (MHC). Unlike other MHC types such as [[MHC class I]] and [[MHC class II]], the structure and functions of which are well defined in [[immune response]], MHC class III are poorly defined structurally and functionally. They are not involved in antigen binding (the process called antigen presentation, a classic function of MHC proteins). Only few of them are actually involved in immunity while many are [[Cell signaling|signalling molecules]] in other cell communications. They are mainly known from their genes because their gene cluster is present between those of class I and class II.<ref name="gruen01">Liquid error: wrong number of arguments (1 for 2)</ref> The gene cluster was discovered when genes (specifically those of complement components [[Complement component 2|C2]], [[Complement component 4|C4]], and [[Complement factor B|factor B]]) were found in between class I and class II genes on the short (p) arm of [[human chromosome 6]]. It was later found that it contains many genes for different signalling molecules such as [[tumour necrosis factors]] (TNFs) and [[heat shock proteins]]. More than 60 MHC class III genes are described, which is about 28% of the total MHC genes (224).<ref>Liquid error: wrong number of arguments (1 for 2)</ref> The region within MHC class III gene cluster that contains genes for TNFs is also known as MHC class VI or inflammatory region.<ref name="deakin">Liquid error: wrong number of arguments (1 for 2)</ref>

In contrast to other MHC proteins, MHC class III proteins are produced by liver cells (hepatocytes) and special white blood cells (macrophages).

==Gene structure==

MHC class III genes are located on chromosome 6 (6p21.3) in humans. It covers 700 kb and contains 61 genes. The gene cluster is the most gene-dense region of the human genome. They are basically similar with those of other animals. The functions of many genes are yet unknown.<ref>Liquid error: wrong number of arguments (1 for 2)</ref> Many retroelements such as human endogenous retrovirus (HERV) and Alu elements are located in the cluster.<ref>Liquid error: wrong number of arguments (1 for 2)</ref> The region containing genes ''G11/C4/Z/CYP21/X/Y'', varying in size from 142 to 214 kb, is known as the most complex gene cluster in the human genome.<ref>Liquid error: wrong number of arguments (1 for 2)</ref>.

===Diversity===
MHC class III genes are similar in humans, mouse, frog (''Xenopus tropicalis''), and gray short-tailed opossum. But not all genes are common. For example, human ''NCR3'', ''MIC'' and ''MCCD1'' are absent in mouse. Human ''NCR3'' and ''LST1'' are absent in opossum.<ref name="deakin"/> But birds (chicken and quail) have only a single gene, which codes for a complement component gene (C4).<ref>Liquid error: wrong number of arguments (1 for 2)</ref>

==References==


[[Category:Immune system]]
[[Category:Cell signaling]]
[[Category:Genes on human chromosome 6]]
[[Category:Cytokines]]
[[Category:Heat shock proteins]]

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