<P> In a cell, protein molecules of the host's own phenotype or of other biologic entities are continually synthesized and degraded . Each MHC molecule on the cell surface displays a molecular fraction of a protein, called an epitope . The presented antigen can be either self or non-self, thus preventing an organism's immune system targeting its own cells . In its entirety, the MHC population is like a meter indicating the balance of proteins within the cell . </P> <P> The MHC gene family is divided into three subgroups: class I, class II, and class III . Class I MHC molecules have β2 subunits so can only be recognised by CD8 co-receptors . Class II MHC molecules have β1 and β2 subunits and can be recognised by CD4 co-receptors . In this way MHC molecules chaperone which type of lymphocytes may bind to the given antigen with high affinity, since different lymphocytes express different T - Cell Receptor (TCR) co-receptors . </P> <P> Diversity of antigen presentation, mediated by MHC classes I and II, is attained in at least three ways: (1) an organism's MHC repertoire is polygenic (via multiple, interacting genes); (2) MHC expression is codominant (from both sets of inherited alleles); (3) MHC gene variants are highly polymorphic (diversely varying from organism to organism within a species). Major histocompatibility complex and sexual selection has been observed in male mice making mate choices of females with different MHCs and thus demonstrating sexual selection . Also, at least for MHC I presentation, there has been evidence of antigenic peptide splicing which can combine peptides from different proteins, vastly increasing antigen diversity . </P> <P> The first descriptions of the MHC were made by British immunologist Peter Gorer in 1936 . MHC genes were first identified in inbred mice strains . Clarence Little transplanted tumors across differing strains and found rejection of transplanted tumors according to strains of host versus donor . George Snell selectively bred two mouse strains, attained a new strain nearly identical to one of the progenitor strains, but differing crucially in histocompatibility--that is, tissue compatibility upon transplantation--and thereupon identified an MHC locus . For this work, Snell was awarded the 1980 Nobel Prize in Physiology or Medicine, together with Baruj Benacerraf and Jean Dausset . </P>

Why do mhc i and mhc ii molecules generally bind different peptides
find me the text answering this question