 Residues in the beta claw motif of TSST-1 are known to interact primarily with the invariant region of the Alpha chain of this MHC class II molecule.Residues forming minor contacts with TSST-1 were also identified in the HLA-DR1 β-chain, as well as the antigenic peptide, located in the interchain groove.The arrangement of TSST-1 with respect to the MHC class II molecule imposes steric restriction on the three component complex composed of TSST-1, MHC class II, and the TCR.