AHR and asthma: For example, the microbiota can metabolize tryptophan into ligands for aryl hydrocarbon receptors expressed on epithelial and immune cells, transform primary into secondary bile acids with effects on IECs and Tregs, produce monohydroxy fatty acid 12,13-diHOME linked to asthma, and the intestinal microbiota can even produce histamine in asthma patients (120–123).