The histocompatibility antigens are cell surface glycoproteins expressed on nucleated cells whose major function is to bind peptides within the cell or from outside and present them at the cell surface for inspection by T cells of the immune system. These antigens are a part of the major histocompatibility complex (MHC), which exists in most vertebrate species and whose products were originally defined as the most crucial elements controlling graft rejection. Human MHC research has revealed human leucocyte antigen (HLA) to be the most polymorphic genetic system in the mammalian genome, and this unique diversity parallels the need for diverse peptide sites within the MHC to combat a fast-evolving range of pathogens. Clinical screening of HLA polymorphism is now an important prophylactic biomarker system for various diseases and a useful tool for development of peptide-based vaccination approaches.

Key Concepts:

• HLA is the most polymorphic component of the human genome. It is a part of the major histocompatibility complex that exists in many animal species.
  
• The major function of HLA molecules is antigen presentation.
  
• High degree of molecular diversity of classical HLA class I and class II molecules is a reflection of their direct role as antigen presenting molecules against a wide range of pathogens
  
• MHC region is characterised by strong linkage disequilibrium between its loci such that conserved ancestral haplotypes can be identified.
  
• There are population-based differences in allelic distribution of MHC molecules primarily due to historical human migration patterns and varying microbial pressures in different geographical regions
  
• An individual heterozygous at a particular HLA locus has an added advantage than homozygous as the two different HLA molecules can bind and present large number of peptides.
  
• The function of the ubiquitous HLA class I gene products (HLA-A, HLA-B and HLA-C) is to present endogenous peptides to responding CD8+ T cells, whereas the class II coded molecules (HLA-DR, HLA-DQ and HLA-DP) have restricted tissue distribution and process exogenous peptides for presentation to CD4+ helper T cells
  
• The antigen-binding cleft of HLA class I molecules can bind peptides that are usually nonamers, whereas the HLA class II antigen-binding cleft can bind longer peptides of 11–18 mers.
  
• Polymorphisms in the MHC have been associated with disease susceptibility and varying rates of disease progression.
  
• MHC matching is crucial in donor selection for haematopoietic stem cell and organ transplantation.