Compelling evidence has accumulated supporting a role for the AHR in regulating adaptive immune responses relevant to the pathogenesis of diseases, such as inflammatory bowel disease (IBD), multiple sclerosis (MS), rheumatoid arthritis (RA), cancer, and obesity.7,41–44 These observations have led to the hypothesis that the AHR participates in innate immune responses to microbial invasion of barrier tissues. Here, AHR is linked to obesity due to melanocortin 4 receptor deficiency.