SUPERANTIGEN

Superantigens (SAgs) are secreted proteins (exotoxins) that exhibit highly potent lymphocyte-transforming (mitogenic) activity directed towards T lymphocytes [2,4,6]. Compared to a normal antigen-induced T-cell response where .001-.0001% of the body’s T-cells are activated, SAgs are capable of activating up to 20% of the body’s T-cells [23]. This causes a massive immune response that is not specific to any particular epitope on the SAg. Since one of the fundamental strengths of the adaptive immune system is its ability to target antigens with high specificity, SAgs produce an immune response that is effectively useless. Microbes (including viruses, mycoplasma, and bacteria [2]) produce SAgs as a defense mechanism to aid them in evading the immune system [4].
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Superantigen /su·per·an·ti·gen/ any of a group of powerful antigens occurring in various bacteria and viruses that binds outside of the normal T [more]Superantigen - Community and Resources

An antigen of a virus or bacteria which bind to non-polymorphic parts (outside of the antigen-binding cleft) of MHC class II molecules and interact with the domain of the T-cell receptor.
This way, they activate an entire subgroup of T cells (rather than a specific clone) expressing the appropriate and this is followed by deletion of the activated T cells upon exposure to the
superantigen. One superantigen can activate up to 20% of the helper T cell repertoire.The prototype bacterial superantigen is the staphylococcal enterotoxins.