CD200 and neoplasm: Dampened anti-tumor cytotoxic T cell (CTL) responses are associated with the overexpression of immune checkpoints including CD200, cytotoxic T lymphocyte antigen-4 (CTLA-4) and programmed death-1 (PD-1) on tumor, immune and stromal cells within the tumor microenvironment, and the consequent immunoregulatory signaling events following binding to their respective ligands or receptors [19–21].