MHC class III

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, of which their structure and functions in immune response are well defined, 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 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.^[1] The gene cluster was discovered when genes (specifically those of complement components C2, C4, and 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 signaling 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).^[2] The region previously considered within MHC class III gene cluster that contains genes for TNFs is now known as MHC class IV^[3] or inflammatory region.^[4]

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