IDO1 and Allergy: 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.