Major histocompatibility complex class I proteins form a functional receptor on most nucleated cells of the body. There are 3 major and 3 minor major histocompatibility complex (MHC) class I genes in HLA. Major genes are HLA-A, HLA-B and HLA-C. Minor genes are HLA-E, HLA-F and HLA-G. β2-microglobulin binds with major and minor gene subunits to produce heterodimer. There are 3 major and 2 minor major histocompatibility complex (MHC) class II proteins encoded by the HLA. The genes of the class II combine to form heterodimeric (αβ) protein receptors that are typically expressed on the surface of antigen-presenting cells. Major-major histocompatibility complex (MHC) class II HLA-DP:α-chain encoded by HLA-DPA1 locus and β-chain encoded by HLA-DPB1 locus. HLA-DQ:α-chain encoded by HLA-DQA1 locus and β-chains encoded by HLA-DQB1 locus. The last one is HLA-DR: α-chain encoded by HLA-DRA locus and four β-chains (only 3 possible per person), encoded by HLA-DRB1, DRB3, DRB4 and DRB5 loci. The other major histocompatibility (MHC) protein class II proteins, DM and DO, are used in the internal processing of antigens, loading the antigenic peptides generated from pathogens onto the HLA molecules of antigen-presenting cell.