Several features of MHC genes and proteins are important for the normal function of these molecules

- MHC genes are highly polymorphic, meaning that many different alleles (variants) are present among the different individuals in the population. The total number of different HLA proteins in the population is estimated to be more than 14,000, with about 10,500 class I and 3500 class II molecules, making MHC molecules the most polymorphic of all proteins in mammals. The polymorphism of MHC proteins is so great that any two individuals in an outbred population are extremely unlikely to have exactly the same MHC molecules. These different polymorphic variants are inherited and not generated de novo in individuals by somatic gene recombination, as are antigen receptors . Any one individual inherits and expresses only two alleles of each MHC gene (one from each parent), which represent very few of the many variants in the population. Because the polymorphic residues determine which peptides are presented by specific MHC molecules, the existence of multiple alleles ensures that there are always some members of the population who will be able to present some peptide from any particular microbial protein antigen. There- fore, MHC polymorphism ensures that a population will be able to deal with the diversity of microbes, and at least some individuals will be able to mount effective  immune responses to the peptide antigens of these microbes. Thus, everyone will not succumb to a newly encountered or mutated microbe.

 - MHC genes are codominantly expressed, meaning that the alleles inherited from both parents are expressed equally. Codominant expression maximizes the number of HLA proteins expressed by each individual and thus enables each individual to display a large number of peptides.

- Class I molecules are expressed on all nucleated cells, but class II molecules are expressed mainly on dendritic cells, macrophages, and B lymphocytes. The physiologic significance of this strikingly different expression pattern is described later. Class II molecules also are expressed on thymic epithelial cells and endothelial cells and can be induced on other cell types by the cytokine interferon-γ.