Major Histocompatibility Complex


   The main antigen presenting cells are the B-cell, the dendritic cell, and the macrophage. These cells can present antigen to CD4-positive cells. Additionally all nucleated cells can present to CD8-positive T-cells.

    All cells of the body express class I molecules encoded within the major histo-compatibility complex, or MHC. Additionally the B-cell , the dendritic cell and the macrophage express MHC class II molecules.

These membrane molecules may contain degraded antigen peptides in a cleft on the top of the molecule, which they present to the T-cells. The molecules regulate the way that the T-cell reacts since a T-cell will only recognise foreign antigens when they are associated with the MHC molecules themselves.

There are many varieties of class I and class II molecules, although in any one individual there may be up to six of each type.

   The MHC gene locus contains three class II genes; DP , DQ and DR , each formed from two chains, beta and alpha. It also contains three class I genes; B , C , and A.

    Because each individual has two chromosomes carrying these genes, the maternal chromosome and the paternal chromosome, there are usually six different versions of the class I molecule and six different versions of the class II molecule, produced from the three different genes with the two chromosomes encoding them.

     Each of the class II molecules is formed from an alpha and a beta chain. The indent formed on the top of the molecule is where the antigenic peptide is held.

Because the MHC is dependant on the genetic make-up of the individual, it is very unlikely that two unrelated people will have the same MHC molecules.

Each of the class I genes produces a single alpha chain which folds itself up and interacts with another molecule called beta 2 microglobulin, which comes from a different gene locus.

Since each individual has different MHC molecules, the performance of everyone's immune system is different. This affects susceptibility to diseases where the immune system is involved.