A child is brought to the emergency department with severe bacterial sepsis. The resident notes that the pathogen produces a toxin that acts as a superantigen. Which of the following BEST describes the mechanism of action of superantigens?

• A They bind within the conventional antigen-binding groove of MHC class II
• B They activate cytotoxic T cells by binding to MHC class I
• C They stimulate B cells directly without T-cell involvement
• D They cross-link the outside of MHC class II with the V-beta region of T-cell receptor, bypassing normal antigen processing ✓

Explanation

Superantigens (e.g., staphylococcal toxic shock syndrome toxin-1) bypass normal antigen processing by simultaneously binding to the outside of MHC class II molecules and the V-beta chain of the T-cell receptor, activating up to 20–30% of all T cells. This massive polyclonal T-cell activation causes a cytokine storm. They do NOT require intracellular antigen processing or presentation via the conventional peptide groove.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.