Wikipedia
HLA-A is a group of human leukocyte antigens (HLA) that are coded for by the HLA-A locus, which is located at human chromosome 6p21.3. HLA is a major histocompatibility complex (MHC) antigen specific to humans. HLA-A is one of three major types of human MHC class I cell surface receptors. The others are HLA-B and HLA-C. The receptor is a heterodimer, and is composed of a heavy α chain and smaller β chain. The α chain is encoded by a variant HLA-A gene, and the β chain (β-microglobulin) is an invariant β microglobulin molecule. The β microglobulin protein is coded for by a separate region of the human genome.
MHC Class I molecules such as HLA-A are part of a process that presents short polypeptides to the immune system. These polypeptides are typically 7-11 amino acids in length and originate from proteins being expressed by the cell. There are two classes of polypeptide that can be presented by an HLA protein: those that are supposed to be expressed by the cell (self) and those of foreign derivation (non-self). Under normal conditions cytotoxic T cells, which normally patrol the body in the blood, "read" the peptide presented by the complex. T cells, if functioning properly, only bind to non-self peptides. If binding occurs, a series of events is initiated culminating in cell death via apoptosis. In this manner, the human body eliminates any cells infected by a virus or expressing proteins they shouldn't be (e.g. cancerous cells).
For humans, as in most mammalian populations, MHC Class I molecules are extremely variable in their primary structure, and HLA-A is ranked among the genes in humans with the fastest-evolving coding sequence. As of December 2013, there are 2432 known HLA-A alleles coding for 1740 active proteins and 117 null proteins. This level of variation on MHC Class I is the primary cause of transplant rejection, as random transplantation between donor and host is unlikely to result in a matching of HLA-A, B or C antigens. Evolutionary biologists also believe that the wide variation in HLAs is a result of a balancing act between conflicting pathogenic pressures. Greater variety of HLAs decreases the probability that the entire population will be wiped out by a single pathogen as certain individuals will be highly resistant to each pathogen. The effect of HLA-A variation on HIV/AIDS progression is discussed below.