While MR1Ts are diverse with regard their phenotype and TCR usage in the human thymus, canonical MAIT cells emerge rapidly following birth. We first analyzed MR1:5-OP-RU tetramer positive cells from human cord blood, and found them to be diverse with regard to effector function, their ability to recognize both riboflavin producing and non-riboflavin producing bacteria, and following crystallization of one of these TCRs find it to be altered in its ability to discern the MR1-5-0P-RU TCR complex. Following infant vaccination with BCG, we note the persistence of a population MAIT cells responsive to IFN-a, as well as expansion of a distinct population of MAIT cells with TCRs that could be clustered by the DIST algorithm. This TCR preferentially recognizes mycobacterial ligands. Finally, we find that mycobacteria deficient in riboflavin biosynthesis are incapable of MR1 T cell stimulation, but non-the-less find diverse array of ligands that we postulate are dependent of 5-AR-U.