In contrast to the existing MR1 tetramer loaded with a single ligand, 5-(2-oxopropylideneamino)-6-d-ribityluracil (MR1/5-OP-RU) (10), our approach produces tetramers composed of hpMR1 monomers loaded with a heterogeneous population of medium-derived, host cell–derived, and bacterially derived ligands generated in the context of infection. The gene discussed is MR1; the disease is infection.