The Kyn - IDO pathway involved in the degradation of tryptophan was initially considered to have tolerance and immunomodulatory effects, moreover, there is mounting evidence that the Kyn-IDO pathway plays a crucial role in the development of atopy and allergy Kyn and its metabolites cover more than 90% of tryptophan metabolism, have been reported to activate AhR receptors with anti-inflammatory activity (184–187), thereupon then suppressing the activity of natural killer cells (NKT) (188) and APC such as dendritic cells (DC), monocytes, and macrophages (189, 190) in mice. Here, IDO1 is linked to allergic disease.