IDO1 and cancer: Low IFN‐γ concentrations in stage IA NSCLC enhanced cancer cell activity via Indoleamine 2,3‐dioxygenase 1 (IDO1) upregulation, IDO1 activity regulates tryptophan consumption and is linked to cellular circuits specifically initiated by IDO1+ cells, contributing to Treg accumulation and severe impairment of T cell and NK cell function.