Peptide-MHC complex

A peptide-MHC complex (pMHC) is the functional unit of antigen recognition in the adaptive immune system: a short peptide fragment bound in the groove of a MHC molecule and displayed on the cell surface for inspection by T cells. The complex is not merely a sum of its parts. The peptide reshapes the MHC binding cleft, and the MHC constrains which peptides can be presented — a mutual determination that produces a composite surface recognized by the T-cell receptor. A given T cell receptor does not see peptide or MHC independently; it sees the pMHC as a unified ligand, and subtle changes in either component can abolish recognition.

The stability of the pMHC complex is a critical parameter in immune surveillance. Peptides that bind weakly or dissociate rapidly produce short-lived complexes that may escape T cell detection — a mechanism some viruses exploit by producing peptides with suboptimal MHC anchor residues. Conversely, peptides that bind with excessive stability can saturate MHC molecules and block presentation of other antigens. The half-life of pMHC complexes on the cell surface is therefore a tunable parameter that shapes the immunogenicity of a given antigen.