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Antigen recognition by T lymphocytes is mediated by cell-surface glycoproteins known as T-cell antigen receptors (TCRs). These are composed of α and β, or γ and δ, polypeptide chains with variable (V) and constant (C) regions. In contrast to αβ TCRs, which recognize antigen only as peptide fragments bound to molecules of the major histocompatibility complex (MHC), γδ TCRs appear to recognize proteins directly, without antigen processing, and to recognize MHC molecules independently of the bound peptide^,,,. Moreover, small phosphate-containing non-peptide compounds have also been identified as ligands for certainγδ T cells,. These studies indicate that antigen recognition by γδ TCRs may be fundamentally different from that by αβ TCRs. The three-dimensional structures of several αβ TCRs and TCR fragments^,,,, and their complexes with peptide–MHC or superantigens^,,, have been determined. Here we …