A humanized monoclonal antibody directed against human PD-1 (programmed cell death 1; PDCD1), with immunomodulating and antitumor activities. Pidilizumab blocks interaction between the receptor PD-1 with its ligands, PD-1 ligand 1 (PD-1L1) and PD-1 ligand 2 (PD-1L2), resulting in the attenuation of apoptotic processes in lymphocytes, primarily effector/memory T cells, and the augmentation of the anti-tumor activities of NK cells. PD-1 is an inhibitory receptor belonging to the B7-receptor family that is expressed on lymphocytes and myeloid cells; its ligands, PD-1L1 and PD-1L2, are expressed not only by hematopoietic cells but also by cells in non-lymphoid tissues.