Question
Super antigen with an example
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, since it does not depend on the rare T cells with the exact matching antigen specificity — compare with <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.
Example: Toxic Shock Syndrome Toxin-1 (TSST-1), produced by certain strains of Staphylococcus aureus, causes toxic shock syndrome via this superantigen mechanism. Other examples include staphylococcal enterotoxins (food poisoning) and streptococcal pyrogenic exotoxins (streptococcal toxic shock-like syndrome, scarlet fever).