This commentary summarizes and highlights the recent research reports of the group headed by Professor Wenkai Ren. Their research has been focused in two important investigative areas, namely, the role of melatonin in the regulation of macrophage polarization and the functional implications of melatonin for the gastrointestinal microbiota. Both these subjects are of high interest to melatonin biologists since both have significant implications in clinical and veterinary medicine.