Normal gut bacteria are instrumental in inducing an immune response in the presence of invading pathogens. However, exactly how commensal bacteria cause CX3CR1+ macrophages in the intestine to protrude their tentacle-like dendrites to capture antigens, triggering the immune response, was unclear. Now, a research team has shown that common bacterial metabolites pyruvate and lactate interact with the GPR31 receptor on CX3CR1+ cells, enhancing the immune response and protecting against gut pathogens.