Question
Super antigens
Answer
A superantigen is a microbial toxin/protein that activates T cells in a manner that bypasses conventional, specific antigen processing and presentation — instead of binding within the MHC peptide-binding groove and being recognized by a specific TCR, a superantigen binds directly and simultaneously to the outer (non-polymorphic) surface of an MHC class II molecule on an antigen-presenting cell and to the Vβ region of the T-cell receptor, outside its normal antigen-binding site.
Consequence: this non-specific bridging activates a very large proportion of T cells (up to 5–20% of the total T-cell pool, versus <0.01% activated in a conventional response), triggering massive, uncontrolled cytokine release (“cytokine storm” — IL-1, IL-2, TNF-α, IFN-γ), which produces systemic toxicity, capillary leak, hypotension, and shock rather than a normal, regulated immune response.
Examples: Toxic Shock Syndrome Toxin-1 (TSST-1) produced by certain Staphylococcus aureus strains (causes toxic shock syndrome), staphylococcal enterotoxins (food poisoning), and streptococcal pyrogenic exotoxins produced by Streptococcus pyogenes (scarlet fever, streptococcal toxic shock-like syndrome).