Major Histocompatibility Complex

MAJER HISTOCOMPATIBILITY COMPLEX:
Definition: Major histocompatibility complex (MHC), group of genes that
code for proteins found on the surfaces of cells that help the immune
system recognize foreign substances. MHC proteins are found in all higher
vertebrates. In human beings the complex is also called the human leukocyte
antigen (HLA) system.
Types of MHC:
1- MHC class I.
2- MHC class II.
3- MHC class III.
Structure of MHC:
The MHC complex resides in the short arm of chromosome 6 and overall
size of the MHC is approximately 3.5 million base pairs. The complete three dimensional
structure for both class I and class II MHC molecules has been
determined by x-ray crystallography. The class I gene complex contains three
loci A, B and C, each of which codes of ? chain polypeptides. The class II gene
complex also contains at least three loci, DP, DQ and DR; each of these loci
codes for one ? and a variable number of ? chain polypeptides. Class III region
is not actually a part of the HLA complex, but is located within the HLA
region, because its components are either related to the functions of HLA
antigens or are under similar control mechanisms to the HLA genes. Class III
antigens are associated with proteins in serum and other body fluids and have
no role in graft rejection.
Expression of MHC:
Class I antigens are expressed on all nucleated cells (except those of the
central nervous system) and platelets. The class II antigens are expressed on
antigen presenting cells such as B lymphocytes, dendritic cells, macrophages,
monocytes, Langerhans cells, endothelial cells and thymic epithelial cells.
Cytokines, especially interferon gamma (IFN-?), increase the level of
2
expression of class I and class II MHC molecules.