IDO1 and neoplasm: The production of soluble factors such as IL‐10 and TGF‐β, and metabolic mediators prostaglandin E2 (PGE2), indoleamine 2,3‐dioxygenase (IDO) and arginase can impair activation and cytotoxicity or push responses towards tumour‐promoting TH2 and TH17 responses [280, 281, 282, 283].