HLA is a highly polymorphic alloantigen, chemically a type of glycoprotein consisting of an α heavy chain (glycosylated) and a β light chain non-covalently bound. The peptide chain's amino terminus faces outward (about 3/4 of the entire molecule), with the carboxyl terminus penetrating into the cytoplasm, and the hydrophobic portion located within the cell membrane. HLA is classified into Class I and Class II antigens based on distribution and function.
HLA encoding system (MHC): a highly polymorphic genetic complex composed of a series of tightly linked gene loci within a 3600kb range, containing at least 224 gene loci, each with multiple alleles.
The high polymorphism of HLA genes is manifested in several aspects:
1) Multiple gene loci: a diverse array of functionally distinct gene loci contribute to the composition and expression of HLA genes and their products.
2) Multiple alleles: the HLA encoding gene group exhibits rich allelic diversity that reflects individual differences and population structure.
3) Co-dominant expression: alleles express corresponding products, significantly increasing the diversity of HLA phenotypes in the population.
4) Gene transcription and regulation: regulatory elements upstream of the HLA gene transcription start site vary due to different alleles, thereby regulating gene transcription.
The significance of the polymorphism of the HLA encoding system lies in enabling populations to generate appropriate immune responses against various pathogens to ensure the continuity and stability of the population.
