Programmed cell death protein 1 (PD-1), also known as CD279, is a type I transmembrane protein and a member of the CD28 immunoglobulin superfamily, encoded by the _PDCD1_ gene. It is primarily expressed on the surface of various immune cells, including T cells, B cells, macrophages, dendritic cells, and natural killer cells. PD-1 plays a crucial role in regulating immune responses by binding to its ligands, PD-L1 (B7-H1) and PD-L2 (B7-DC), which are expressed on antigen-presenting cells and tumor cells. When PD-1 interacts with its ligands, it triggers inhibitory signals that downregulate T cell activity, preventing excessive immune responses and promoting self-tolerance. However, this pathway can also be exploited by cancer cells to evade immune surveillance, leading to tumor progression. As a result, PD-1 has become a key target in cancer immunotherapy, with several anti-PD-1 antibodies approved for treating various cancers, achieving significant and durable efficacy in some patients.