Fig. 3. Schematic presentation of HLA class I and II gene organization and protein structure. In humans, separate clusters of HLA class I and II genes are located on chromosome 6. Three main class I genes are HLA-A, HLA-B, and HLA-C. HLA class I molecules consist of one α chain and β2microglobulin (β2m). The class II region includes the α and β chains for HLA-DR, -DP, and -DQ molecules. The most polymorphic regions are considered to reside in peptide-binding regions, which are exons 2 and 3 in HLA class I genes and exon 2 in HLA class II genes. HLA, human leukocyte antigen.