How do the MHC molecules control what the T cells see?
They do this in two ways. First, the sequence of amino acids in an HLA molecule, which is determined by an individual’s genetic polymorphisms, determines which antigenic peptide fragments can bind to that molecule. Only those “selected” antigenic peptides that can bind to one of an individual’s HLA molecules have the potential to be specifically recognized. Second, not all T cells can see all the HLA molecules. The peptides presented in the context of MHC class I molecules can only be seen by T cells that have CD8 molecules associated with their TCR, whereas the peptides presented in the context of MHC HLA class II molecules can only be seen by T cells that have CD4 molecules associated with their TCR.