What is Major Histocompatibility Complex ?

 Antigens induce an immune response inside the body. Most of these antigens are large protein molecules some are polysaccharides and a few are glycoproteins or nucleoproteins.

Self antigens are the antigens which are found on the membranes of almost all the cells of human body and other vertebrate animals. These self antigens are known as major histocompatibility antigens. Major histocompatibility antigens are glycoproteins in chemical nature. major histocompatibility antigens are also known as histocompatibility antigens.

Discovery of Major Histocompatibility Antigens

These antigens were discovered when scientists were doing transplantation experiments on mice. Scientists found that sometimes the transplant tissue from the donor mouse was accepted by the recipient mouse but at other times it was rejected by the recipient Mouse.

  Now the question was why the tissues from one individual of a species were destroyed when introduced into member of the same species.
  The answer was given by an American Mouse geneticist George Snell. He gave the reason that this rejection was because the tissues of the donor and the recipient were incompatible. 

  In other words it was discovered that if the animals were not closely related the recipients rejected the grafts. So if the donor Mouse in the recipient Mouse are of the same genetic background or same inbred strain the transplanted tissue is accepted by the recipient. However if the donor and recipient Mouse are of genetically different backgrounds or different inbred mouse strains the transplanted tissue is rejected by the recipient.

A group of closely related genes was the cause of the rejection. This cluster of gene was named the Major Histocompatibility Complex or MHC. The major histocompatibility complex got its name from the fact that, The genes in this region encode proteins which determine whether a tissue transplanted between two individuals will be accepted or rejected.

Major Histocompatibility Complex Genes

It was discovered that these genes encode proteins which act as self antigens on the surface of the cells. These proteins are known as major histocompatibility antigens.
  These antigens gives the answer why rejection of the transplanted tissue happened when a tissue was transplanted from donor mouse to recipient Mouse.
  The Hajor Histocompatibility Antigens of donor Mouse were perceived as foreign or non-self by the recipient Mouse. As a result the immune system of recipient Mouse mount an immune response
and result in the destruction of the transplanted tissue.

Major Histocompatibility Complex

In the term histocompatibility "histo" means tissue and "compatibility" means getting along or agreeable. The MHC got its name from the fact that the genes in this region encode proteins that determine whether a tissue transplanted between two individuals will be accepted or rejected.

  The term complex represents that these genes are localised to a large genetic region containing multiple loci. Since the molecules encoded by these genes were found to have main effects on histocompatibility to distinguish them from other molecules encoded by genome having minor effects on histocompatibility.  
They were called major histocompatibility molecules and the genes encoding these molecules were called major histocompatibility complex.

Major histocompatibility complex is defined as - A tightly linked cluster of genes whose products play important role in intercellular recognition and in discrimination between self and non-self.

Major histocompatibility complex genes  are present on chromosome 6 in humans and they are known as human leukocyte antigens (HLA) complex. In case of mice they are present on chromosome 17 and are known as H-2 complex in mice.

MHC molecules are found on all nucleated cells in the body and they play an important role in the development of both humoral and cellular immunity. B cells react with antigen on their own but T cells recognize antigens only in peptide form and that too when combined with an MHC molecule.

The main function of MHC molecules is to bring antigen to the cell surface for recognition by T cells in humans. The genes coding for MHC molecules are found on short arm of chromosome 6.

The MHC molecules are divided into three classes namely
  MHC Class 1
  MHC Class 2 
  MHC Class 3

MHC class 1 molecules are coded at three different locations or low sigh termed a b and c. These glycoproteins are expressed on all nucleated cells.

MHC class 2 genes are situated in the D region and there are several different loci known as DR DQ and DP. These glycoproteins appear only on cells that can process non-self materials and present antigen to other cells.

The area between the class MHC 1 and MHC class 2 regions on chromosome 6 is of MHC class 3 genes. These genes code for complement proteins and cytokines which take part in immune response but these molecules are not expressed on cell surface.

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