PDCD1 and neoplasm: Monocytes can differentiate into macrophages within the tumor microenvironment, where they secrete immunosuppressive factors, such as interleukin-10 and tumor growth factor-beta, inhibiting anti-tumor immune responses and promoting immune escape.29,30 Furthermore, during tumor progression, monocytes can express immune checkpoint molecules, such as programmed cell death protein 1 and cytotoxic T-lymphocyte protein 4, facilitating the development of immune tolerance and supporting tumor cell survival.