The human major histocompatibility complex. HLA System

Abstract

The human major histocompatibility complex or HLA system, located in the short arm of chromosome 6, is the most important genetic system in the regulation of the Immune response. The HLA genes code for 3 types of antigens which can be differentiated by their molecular structure, tissue distribution and function. Class I antigens (HLA-A, B, C and E) are composed by a heavy a chain bound to B2- microglobulin and are expressed by most nucleated cells. These molecules are the restriction elements for CD8+ T Iymphocyte activation. Class II antigens (HLA-DP, DQ and DR) are dimer formed byα and β chains. These antigens are present in the membrane of a limited type of cells and are responsible for the genetic restriction in the antigen presentation to CD4+ lymphocytes. Class III antigens are plasma proteins of the complement system (C2, C4 and BF).

The HLA loci are highly polymorphic and their products are inherited in blocks known as haplotypes. The HLA system is very useful in anthropogenetic studies since the frequency of the alleles and haplotypes vary among the various ethnic groups. Some HLA antigens are present in patients with certain diseases in proportions significantly different to those found in the general population. These findings have been very important to understand the pathogenesis and the genetic resistance or susceptibility to such diseases. In tissue transplantation the HLA compatibility between donor and recipient correlates with graft survival. The HLA system also seems to playa very important role in the immunological phenomena that occur during pregnancy. Due to the theoretical and practical importance of the HLA system in genetics, Immunology and medicine, Its study will continue to be a very active field for basic and clinical research.