In addition, in the gut, tryptophan metabolites of the microbiota, e.g., indole-3-aldehyde, kynurenine, indole-3-acetic acid, and tryptamine, can act as ligands for aryl hydrocarbon receptor (Ahr), a transcription factor that regulates IL-22 gene expression, controls autoimmunity processes, and promotes rapid recovery from colitis [15]. This evidence concerns the gene AHR and colitis.