Programmed Cell Death Protein 1 (PD-1; CD279), an inhibitory immune checkpoint receptor predominantly expressed on activated T cells, B cells, and myeloid cells. In the tumor microenvironment, PD-1 engagement by its ligands (PD-L1/PD-L2, often overexpressed on tumor cells or antigen-presenting cells) delivers a potent signal that suppresses T cell receptor (TCR) signaling. This leads to T cell exhaustion—characterized by reduced cytokine production, impaired proliferation, and diminished cytotoxic function—which allows tumors to evade immune destruction.