There is substantial evidence suggesting a correlation between the modulation of immunological tolerance and the pathogenesis of various disorders, including depression and neurodegenerative conditions, through the involvement of kynurenine (Kyn) and other metabolites of Trp.11 Elevated levels of Kyn and increased activity of the enzyme indoleamine 2,3-dioxygenase 1 (IDO1) have been observed not only in inflammatory bowel diseases (IBD) and irritable bowel syndrome (IBS) but also in neurodegenerative diseases such as AD and PD. Here, IDO1 is linked to major depressive disorder.