Mechanism of superantigens?

• A. Blocks antibody-antigen binding
• B. Inhibits T cell activation
• C. Cross-links beta-chain of TCR and MHC receptor
• D. Neutralizes bacterial toxins

Answer: (C)

Superantigens trigger a massive, non-specific T cell response by directly binding to the T cell receptor’s beta-chain variable region and to MHC class II molecules on antigen-presenting cells. This bridges T cells and APCs without the need for specific peptide antigens, activating a large fraction of T cells and unleashing a storm of cytokines (like IL-2, IFN-γ, TNF-α). The result is fever, rash, and potentially shock, seen in toxic shock syndromes. This mechanism is distinct from normal antigen presentation, which requires processing and presentation of a specific peptide in the MHC groove and activates only T cells with matching TCR specificity. Therefore, the mechanism is cross-linking the TCR beta-chain with MHC II on APCs.