Indoleamine 2,3-dioxygenase (IDO) is an enzyme that catalyzes the initial and rate-limiting step in the catabolism of tryptophan along the kynurenine pathway, playing a critical role in immune regulation by modulating tryptophan availability and producing immunosuppressive metabolites. Its activity is associated with various physiological and pathological processes, including tumor immune evasion, chronic inflammation, and pregnancy tolerance.