MHC encoded by HLA genes. Present antigen fragments to T cells and bind T-cell receptors (TCRs).

MHC I:- HLA-A, HLA-B, HLA-C- Binding: TCR and CD8- 1 long chain, 1 short chain, β₂-microglobulin- Expression: All nucleated cells, APCs, platelets. Not on RBCs.- Present endogenously synthesized antigens (eg, viral or cytosolic proteins) to CD8+ cells- Antigen peptides loaded onto MHC I in RER after delivery via TAP (transporter associated with antigen processing).

MHC II:- HLA-DP, HLA-DQ, HLA-DR- Binding: TCR and CD4- 2 equal-length chains, invariant chain- Expression: APCs- Present exogenously synthesized antigens (eg, bacterial proteins) to CD4+ cells- Antigen loaded following release of invariant chain in an acidified endosome