What is the maximum number of MHC class I and class II molecules that a heterozygous individual could theoretically express?

Question

Explain your answer. (Ignore the possibility of MHC class II molecules composed of chains from different isotypes.)

Answer 1

There are three MHC class I isotypes in humans (HLA-A, HLA-B, and HLA-C) and they are expressed from both chromosomes. Assuming that each gene is heterozygous, the maximum number of different MHC class I α chains that could be expressed is 6. Because β-microglobulin is invariant, this means that six different MHC class I molecules could be produced. For MHC class II molecules, assuming complete heterozygosity and the presence of two functional DRB genes (DRB1 and DRB3, 4, or 5) on both chromosomes, the maximum number of MHC class II molecules that could be expressed is 16 (Figure A5–89). Therefore, the total number of different MHC class I and MHC class II molecules that can be expressed is 22.