 Significant differences exist in the sequences of MHC Class II alleles and TCR Vbeta elements expressed by different species, and these differences have important effects on the interaction of PTSAgs and with MCH class II and TCR molecules.TSST-1 binds primarily to the alpha-chain of class II MHC exclusively through a low-affinity (or generic) binding site on the SAG N-terminal domain.This is opposed to other super antigens (SAGs) such as DEA and SEE, that bind to class II MHC through the low-affinity site, and to the beta-chain through a high-affinity site.