Superantigens (SAgs) are a class of antigens which cause non-specific activation of T-cells resulting in polyclonal T cell activation and massive cytokine release. SAgs can be produced by pathogenic microbes (including viruses, mycoplasma, and bacteria) as a defense mechanism against the immune system. Compared to a normal antigen-induced T-cell response where .001-.0001% of the body’s T-cells are activated, these SAgs are capable of activating up to 83% of the body’s T-cells. Furthermore, Anti-CD3 and Anti-CD28 Antibodies (CD28-SuperMAB) have also shown to be highly potent superantigens (and can activate up to 100% of T cells).
The large number of activated T-cells generates a massive immune response which is not specific to any particular epitope on the SAg thus undermining one of the fundamental strengths of the adaptive immune system, that is, its ability to target antigens with high specificity. More importantly, the large number of activated T-cells secrete large amounts of cytokines, the most important of which is Interferon gamma. This excess amount of IFN-gamma in turn activates the macrophages. The activated macrophages, in turn, over-produce proinflammatory cytokines such as IL-1, IL-6 and TNF-alpha. TNF-alpha is particularly important as a part of the body's inflammatory response. In normal circumstances it is released locally in low levels and helps the immune system defeat pathogens. However when it is systemically released in the blood and in high levels (due to mass T-cell activation resulting from the SAg binding), it can cause severe and life-threatening symptoms, including shock and multiple organ failure.
Read more about Superantigen: Structure, Binding, T-cell Signaling, Direct Effects, Superantigenicity Independent (indirect) Effects, Diseases Associated With Superantigen Production, Treatment, Evolution of Superantigen Production, Endogenous SAgs
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... Similar endogenous SAg-dependent selection has yet to be identified in the human genome, but endogenous SAgs have been discovered and are suspected of playing an integral role in viral infection ... Infection by the Epstein-Barr virus, for example, is known to cause production of a SAg in infected cells, yet no gene for the toxin has been found on the genome of the virus ...