The major histocompatibility complex (MHC) is a series of closely linked and highly polymorphic genes found in humans on short arm of chromosome number 6. MHC spans almost 4 megabases of chromosome 6 and includes more than 200 known genes(128 genes as well as 96 pseudogenes).

The most intensely studied HLA genes are the nine classical Mhc genes: HLA-A, HLA-B, HLA-C, HLA-DPA1, HLA-DPB1, HLA-DQA1, HLA-DQB1, HLA-DRA, and HLA-DRB1. The MHC is divided into three regions: Class I, II, and III. The A, B, and C genes belong to MHC class I while the six D genes belong to class II.

MHC shows a high degree of allelic diversity especially among the nine classical genes.The most diverse loci HLA-A, HLA-B, and HLA-DRB1, have roughly 250, 500, and 300 known alleles respectively.