Mucosal associated invariant T (MAIT) cells are restricted by a nonpolymorphic MHC-related molecule-1 (MR1), and express an invariant TCRα chain: Vα7.2-Jα33 in humans and Vα19-Jα33 in mice. MAIT cells are selected in the thymus, but, interestingly, MAIT cells require B cells as well as commensal flora for their peripheral expansion. Bourhis et al demonstrated that MAIT cells display antimicrobial capacity. Both human and mouse MAIT cells have been shown to be activated by Escherichia coli-infected antigen presenting cells in an MR1-dependent manner. MAIT cells show a protective role against Mycobacteriu abscessus or E. coli infections in mice. Human MAIT cells are capable of producing IFNγ and IL-17 and are found in Mycobacterium tuberculosis-infected lung tissues. Thus, MAIT cells play an antimicrobial function under these infectious conditions. MAIT cells play a protective role against autoimmune encephalomyelitis (EAE), an animal model of human multiple sclerosis (MS), whereas they play a pathogenic role in murine models of arthritis. In patients with autoimmune diseases, the frequency of MAIT cells in peripheral blood was significantly reduced. The frequency of MAIT cells reflected the disease activity in MS patients, suggesting the involvement of MAIT cells in the regulation of autoimmune diseases.