Normally, humans have 46 chromosomes arranged into 23 pairs of chromosomes —22 of these pairs are autosomal, and the 23rd is the sex chromosomes (see image to the top right).
The HLA genes are located on chromosome 6 on the short arm (p) at band 21. This region contains highly diverse genes that encode about 35 to 40 genes grouped into three "regions" on the chromosome that encodes for the major and minor histocompatibility (MHC) proteins and complement.
The classical HLA class I loci are HLA-A, HLA-B, and HLA-C. There are non-classical HLA class I loci as well that include E, F, and G. These HLA genes are not as well characterized. HLA class I glycoprotein structure consists of three heavy chain alpha proteins and one beta light chain. The beta light chain in B2 microglobulin is encoded by a gene from chromosome 15 (not pictured) and incorporated into HLA class I proteins only.
HLA class II encodes for a heterodimer complex of two alpha and beta chains of HLA-DR, HLA-DP, and HLA-DQ. HLA class II heavy and light chains are both made by class II genes found on chromosome 6, unlike class I.
The class III genes encode for non-HLA molecules, like complement C4, C2, 21-hydroxylase, and factor B. Class III genes are not depicted in the diagram but spatially are located between class I and class II genes on band 21 of chromosome 6.
