Bacterial or virus secreted protein that induces a strong T-lymphocyte activity. When a normal antigen stimulates 0,001% of the body's T-cells, superantigens can stimulate up to 20% of body's T-cells. They simultaneously interact with the Vb domain of the T-cell receptor (TCR) and with the class II major histocompatibility complex (MHC) molecules on the surface of an antigen-presenting cell. This compels the MHC to interact with the TCR, thereby inducing T-cells activation. Most superantigens are bacterial toxins, but some are produced by viruses. Staphylococcal enterotoxins are the best known superantigens.