PDCD1 and neoplasm: PD-1 (CD279) is expressed by various immune cells, including activated T cells, natural killer cells, B cells, macrophages, monocytes, and dendritic cells (DCs), and its ligand PD-L1/2 is predominantly expressed on tumor cells and can also be found on the surface of activated T cells, B cells, epithelial cells, and DCs (10–13).