MHC polymorphism in the immune system

An antigen will be able to stimulate an immune response if the antigen has been presented and recognized by T-cells. Major Histocompatibility Complex (MHC) molecules are molecules that will recognize and present the peptides of the antigen. In humans, two classes of MHC are known to carry out this role, namely Class-I MHC and Class-II MHC, while Class-III MHC has a function that is not directly related to peptide binding. The genes that code for MHC molecules are so polymorphic that each individual is certain to have a different allele (heterozygous). In Class-I MHC, only the ?-chain is polymorphic while in Class-II MHC both ? and ?-chains are polymorphic. MHC polymorphism will affect (1) the ability to create immune responses including antibody production, (2) resistance or susceptibility to infectious diseases, and (3) resistance or susceptibility to autoimmune and allergic diseases. That is why MHC is so polymorphic. The immune system has to fight many different pathogens. By having many different MHC molecules, one can present widely different antigens and can spur an effective immune response.