Before further discussion of the development or function of T cells can occur, we must first understand the MHC system because both T Cell development and function depend on it. MHC stands for Major Histocompatibility Complex. These molecules were first discovered as the molecules our immune systems recognized as foreign on transplant tissues. This gives us a clue that each person's MHC molecules are slightly different from each other. In fact, there are several groups of MHC molecules and each group has multiple alleles. So the whole HLA (human leukocyte antigen complex), which codes for the MHC molecules, is highly polymorphic. For instance. we all have MHC I antigens on the A, B, C, E, F, and H loci, but the HLA-A has 1939 allotypes (alleles) alone! The MHC II antigens have thirteen loci or groups, each having multiple alleles. When we combine all of these possibilities, it is evident that no two people except identical twins have the same haplotypes - the cluster of genes coding for the HLA complex - MHC molecules. Although this obviously represents a challenge when attempting to match tissues for transplantation, it evolved as a safety net in that if a very pathogenic organism becomes epidemic, there will always be some people who have the right MHC molecules to deal with it, and so the human population will not all die out.
This brings us to how antigens are processed, combined with an MHC molecule, and then presented on the cells' surface. This will be discussed in the second part of the MHC discussion.
