Indoleamine 2,3-dioxygenase (IDO) is a heme-containing intracellular dioxygenase catalyzing the degradation of the essential amino acid L-tryptophan to N-formyl-kynurenine. IDO is widely expressed in dendritic cells, macrophages, microglia, eosinophils, fibroblasts, endothelial cells, and most tumor cells. In immune cells, its expression is mainly induced by cytokines such as IFN gamma, IFN alpha, IFN beta, and IL-10. IDO has an antimicrobial function and induces immunosuppression during infection, pregnancy, transplantation, autoimmunity, and neoplasia.