Mucosal-associated invariant T (MAIT) cells are a unique T cell subset in mammals. They are present at high frequencies at mucosal tissue sites and have an intrinsic capacity to respond to microbial infections. The semi-invariant antigen recognition receptor of MAIT cells detects the non-polymorphic antigen-presenting molecule major histocompatibility complex class I-related protein 1 (MR1), which can bind microorganism-derived riboflavin metabolites. The striking evolutionary conservation in both the MR1 molecule and the MAIT T cell receptor suggests that strong selective pressures maintain this T cell pattern recognition system which detects microbial infection.