IL4 and inflammatory bowel disease: Interestingly, previous studies have reported an elevated presence of Kyn or higher Kyn/Trp ratios in IBD patients, indicating enhanced tryptophan metabolism through the Kyn pathway during active IBD [128].Additionally, there is an enrichment of Th17 cells and a reduction in Treg cells which can produce anti-inflammatory IL-4 and IL-10, attributed to the interaction of indole metabolites and kynurenine with the AhR on immune cells [77,129].