HLA SYSTEM AND MAJOR HISTOCOMPATIBILITY COMPLEX

Edited by, SRIRAM

HLA stands for Human Leucocyte Antigens although not a component of immune system, it plays a major role in regulating the immune system. These antigens or genetic proteins in the body determine one's own tissue from non self were first discovered on the surface of Leucocytes. It was found that HLA are actually gene complexes of proteins on the surface of all nucleated cells of the body and platelets. Since these complexes are if immense importance in matching donor and recipient for organ transplant, they are called Major Histocompatibility Complex (MHC) or HLA Complex.

Out of various genes for Histocompatibility, most of the transplantation antigens  or MHC are located on a portion of chromosome 6 of all nucleated cells of the body and platelets. These genes occupy four regions or loci - A, B, C and D, on the short(p) arm of chromosome 6 and exhibit marked variation in allelic genes at each locus. Therefore the product  of HLA antigens is highly polymorphic.

Depending upon the characteristics of MHC, they have been divided into 3 classes :

Class I MHC antigens have loci as HLA-A, HLA-B and HLA-C.T Suppressor cells i.e. CD8+ lymphocytes carry receptors for class I MHC and these cells are used to identify class I antigen on them

Class II MHC antigens have single locus as HLA-D. These antigens have further 3 loci : DR, DQ and DP. Class II MHC is identified by B cells and CD4+ i.e. T helper cells.

CLASS III MHC antigens are some components of the complement system coded on HLA Complex but are not associated with HLA expression and are not used in antigen identification.

In view of high polymorphism of class I and class II genes, they have a number of alleles on loci numbered serially like HLA-A1, HLA-A2, HLA-A3 etc.

MHC antigens present on the cell surface help the macrophage in its function of bacterial  antigen recognition i.e., they help to identify self from foreign, and accordingly present the foreign antigen to T cells or to B cells.