T lymphocytes express clonal receptors, called T cell receptors (TCRs), which specifically recognize antigens presented in combination with major histocompatibility molecules (MHC). To date, T cell antigens can be broadly categorized into two classes: peptides and lipids. A recent paper published in Nature by Kjer-Nielsen and colleagues reveals that a unique population of T lymphocytes expresses TCRs that recognize a completely new and unexpected class of antigens, vitamin metabolites.