IDO1 and hepatocellular carcinoma: The efficacy of CD8+ T cell‐mediated anti‐tumour immune response in HCC is hindered by several factors: the high expression of immunomodulatory molecules (e.g. PD‐1, PD‐L1, TIM‐3, LAG‐3, IDO1) in white blood cells or malignant cells, physicochemical imbalances in the microenvironment (like hypoxia and acidic pH), metabolic competition with malignant cells, and the lack of support from CD4+ T cells.126, 127