AHR and intestinal disorder: Indole derivatives are aromatic hydrocarbon receptors (AHR) ligands to activate the production of IL-22 cytokines, that is, tryptophan catabolic metabolites produced mainly by the metabolism of microbial in the gastrointestinal tract activate the immune regulation of the body through AHR, and then maintain the intestinal homeostasis to promote the recovery of intestinal diseases [37, 38].