<P> Of the three MHC classes identified, attention commonly focuses on classes I and II . By interacting with CD4 molecules on surfaces of helper T cells, MHC class II mediates establishment of specific immunity (also called acquired immunity or adaptive immunity). By interacting with CD8 molecules on surfaces of cytotoxic T cells, MHC class I mediates destruction of infected or malignant host cells, the aspect of specific immunity termed cellular immunity . (The other arm of specific immunity is humoral immunity, whose relation to MHC is more indirect .) </P> <P> MHC is the tissue - antigen that allows the immune system (more specifically T cells) to bind to, recognize, and tolerate itself (autorecognition). MHC is also the chaperone for intracellular peptides that are complexed with MHCs and presented to T cell receptors (TCRs) as potential foreign antigens . MHC interacts with TCR and its co-receptors to optimize binding conditions for the TCR - antigen interaction, in terms of antigen binding affinity and specificity, and signal transduction effectiveness . </P> <P> Essentially, the MHC - peptide complex is a complex of autoantigen / alloantigen . Upon binding, T cells should in principle tolerate the auto - antigen, but activate when exposed to the allo - antigen . Disease states occur when this principle is disrupted . </P> <P> Antigen presentation: MHC molecules bind to both T cell receptor and CD4 / CD8 co-receptors on T lymphocytes, and the antigen epitope held in the peptide - binding groove of the MHC molecule interacts with the variable Ig - Like domain of the TCR to trigger T - cell activation </P>

What are the roles of mhc i and ii molecules