PD-1 (also known as CD279) is mainly expressed on the activated CD8+ T-cells (13), although it is also present on the surface of macrophages, dendritic cells, B-cells, and while it is also similar to CD28, it binds to its specific ligands: PD-L1 (expressed on immune cells, hepatocytes, pancreatic islet cells, endothelial cells, myocytes, thyroid cells and many other cells, including various tumor cells) and PD-L2 (only expressed on macrophages and dendritic cells) (12). Here, PDCD1 is linked to neoplasm.