targeted for T cell recognition, individuals may vary in their ability to mount immune responses to pathogens depending on the MHC molecules that they express (11).
The genes for the HLA-A, -B, -C, -DR, -DQ, and -DP molecules are found next to one another within the MHC on chromosome 6 (68). The alpha (or heavy) chains of the class I molecules are encoded in the MHC; the second class I polypeptide chain, beta-2 microglobulin, is encoded on another chromosome. The genetic information needed to make a class II molecule is found in two different genes, a class II A (alpha) and a class II B (beta) gene. For example, a DQA1 gene and a DQB1 gene together provide the information needed to make a DQ molecule. Other class II A and B gene pairs include: DPA1 and DPB1, and DRA and DRB1. Some versions of chromosome 6 carry a second DRB gene, DRB3, DRB4, or DRB5. Its product can also associate with DRA to form a second DR molecule. Thus some individuals carry a copy of chromosome 6 that encodes two different DR molecules.