Second, pathways promoting allergic rhinitis: serotonin production within intestinal chromaffin cells, a process catalyzed by tryptophan hydroxylase 1 (TpH1); Third, the direct conversion of tryptophan into indole and its derivatives, as well as tryptamine, via tryptophanase through the intestinal microbiota, leading to the synthesis of multiple metabolites. Here, TPH1 is linked to allergic rhinitis.