Title 8. MHC PROTEINS.pdf ✅

PHARMD GURU Page 1 MAJOR HISTOCOMPATIBILITY COMPLEX (MHC):  HLA stands for Human Leucocyte Antigens because these antigens or genetic proteins in the body which determine one’s own tissue from non-self (histocompatibility) were first discovered on the surface of leucocytes.  Subsequently, 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 of 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.  The letter w in some of the genes (e.g. Dw3, Cw4, Bw15 etc) refers to the numbers allocated to them at international workshops. HLA system is part of immunoglobulin superfamily of CAMs. Depending upon the characteristics of MHC, they have been divided into 3 classes: 1) Class I MHC antigens have loci as HLA-A, HLA-B and HLA-C. CD8+ (i.e. T suppressor) lymphocytes carry receptors for class I MHC and these cells are used to identify class I antigen on them. 2) 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. 3) Class III MHC antigens are some components of the complement system (C2 and C4) 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-A 1, HLA-A 2, HLA-A 3 etc. MHC PROTEINS (OR) TRANSPLANTATION ANTIGENS