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The T-cell antigen receptor (TCR) is a heterodimeric structure composed of highly variable a and^ chains. Foreign antigens are presented to the TCR in the form of short peptides bound to self tnajor histocotnpatibility complex (MHC) molecules. TCR on CD4"^(helper) T cells recognize peptides bound to self MHC class II molecules whereas TCR on CDS"^(cytotoxic) T cells react with peptideiMHC class I complexes (Janeway 1992). In general, most (or all) of the variable domains of the TCR (ie Va, V^, Ja, J^ and D^ are believed to be involved in TCR: MHC: peptide interactions (Jorgensen et al. 1992). In contrast to conventional peptide antigens, superantigens (SAgs) react in a much more restricted way with the TCR: MHC complex (Herman et al. 1991, Herrmann & MacDonald 1991). In particular SAgs bind to the Vjff domain with relatively little involvement of other variable TCR components (but see Vacchio et al …