FIGURE 16-2 Polyclonal activation of T cells by bacterial superantigens. A, Conventional microbial T cell antigens, composed of a peptide bound to the peptide-binding groove of an MHC molecule, are recognized by a very small fraction of T cells in any one individual, and only these T cells are activated to become effector T cells that protect against the microbe. B, In contrast, a superantigen binds to class II MHC molecules outside the peptide-binding groove and simultaneously binds to the variable region of different TCR β chains, regardless of the peptide specificity of the TCR. Different superantigens bind to TCRs of different Vβ families. Because many T cells express a TCR β chain from a particular Vβ family, superantigens can activate a large number of T cells. In the example shown, the superantigen staphylococcal enterotoxin B (SEB) binds to HLA-DR and the V regions of TCRs belonging to the Vβ3 family. APC, antigen-presenting cell.