Mucosal-associated invariant T (MAIT) cells represent the largest innate T-cell subset that recognizes bacteria in humans. MAIT cells tend to accumulate at the site of bacterial infections, where they can recognize antigens derived from microorganisms possessing the riboflavin synthesis pathway presented by the nonpolymorphic MHC class I related molecule MR1 to the semi-invariant T-cell receptor, Vα7.2-Jα33.1 This transcript is enriched in the gut, indicating “mucosal assignment” of these cells.