CD8A and neoplasm: Chemokines such as colony-stimulating factor 1 (CSF1) and C-C motif chemokine ligand 2 (CCL2) secreted by tumor cells can recruit monocytes in peripheral circulation to the tumor microenvironment, and then monocytes differentiate into macrophages that are often classified as M1 and M2 types, with the former capable of killing tumor cells, while the latter favors tumor growth, metastasis, and weakening the effect of CD8+ T cells [42].