CTLA4 and neoplasm: The possible causes of extrinsic mechanisms of immune evasion in tumors are: 1) The presence of immunosuppressive cells, such as Tregs, MDSCs, and M2 macrophages, which actively suppress anti-tumor immune responses; 2) Co-inhibitory signals on lymphocytes, such as PD-1 and CTLA-4, which are upregulated in the TME and suppress T cell activation and function; 3) Cytokines such as TGF-β and IL-10, along with other soluble factors and signaling pathways, dampen immune cell activity and foster tumor growth.