CD200 and neoplasm: Similar to other immune checkpoint proteins, such as cytotoxic T lymphocyte antigen 4 (CTLA-4) and program death-1 (PD-1), CD200 is thought to play a pro-tumorigenic role, via engagement of CD200R, in many tumor types primarily by suppressing anti-tumor T-cell and natural killer cell responses [46–49] (Figure 1A).