HLA polymorphism

JANEWAY'S VIEW

MHC variability

A gene is referred to as polymorphic when more than one allele is expressed in a population. The HLA genes have the highest level of polymorphism of the human genome. The variability of HLA molecules is mainly located in the peptide binding domain (see the Janeway's view). Coevolution of pathogens and responder immune system exerted a selection pressure that favored the variability of HLA molecules in the population. Increasing the potential to bind different viral peptides maximize the capacity to respond to pathogens.

Gene

Number of alleles

HLA Class I	A	429
	B	752
	C	219

HLA Class II	DRA	3
	DRB	511
	DQA	32
	DQB	69
	DPA	23
	DPB	121

HLA genes are extremely polymorphic but the same alleles are frequently associated in individual. This is explained by the fact that the HLA genes are inherited in a strong linkage disequilibrium with each other. This phenomenon increases the chance to find a compatible donor significantly.

Furthermore, although a very large number of HLA alleles can be found in the human population (in total more than 2400), diversity in a given ethnic group or given population group is more restricted.

The complexity of the HLA system demands a extremely systematic nomenclature.