IDO2 and obesity due to melanocortin 4 receptor deficiency: Tryptamine, a byproduct of tryptophan catabolism, is a neurotransmitter that interacts with both indoleamine 2,3-dioxygenase and the aryl hydrocarbon receptor to heighten immune surveillance, and dampen the expression of pro-inflammatory cytokines, which have been implicated in the pathology of IBD and obesity [89,90,91,92].