TCR recognition of peptides, lipids, vitamin B Ags and PAgs. MHC class I and II molecules present peptides to CD8 and CD4 T cells, respectively; MHC class I-like molecules CD1 present lipids to CD4, CD8 and NKT cells; the MHC class I-related protein MR1 presents vitamin B metabolites to MAIT cells and BTN3A1 presents PAgs to Vγ9Vδ2 T cells. Ag, antigen; BTN, butyrophilin; MHC, major histocompatibility complex; NKT, natural killer T; PAg, phosphoantigen; TCR, T-cell receptor.