Programmed cell death protein 1 (PD-1), also known as CD279, is a type I transmembrane protein belonging to the CD28 immunoglobulin superfamily. It is encoded by the Pdcd1 gene and consists of 288 amino acids, including an extracellular IgV-like domain, a transmembrane region, and a cytoplasmic tail. PD-1 is expressed on activated T cells, B cells, macrophages, dendritic cells, and natural killer cells. Its cytoplasmic domain contains two tyrosine residues that form an immunoreceptor tyrosine-based inhibitory motif (ITIM) and an immunoreceptor tyrosine-based switch motif (ITSM), which play key roles in inhibiting T cell receptor signaling. PD-1 interacts with its ligands PD-L1 and PD-L2 to negatively regulate immune responses, contributing to peripheral tolerance and preventing autoimmune diseases. However, this inhibitory function can also suppress anti-tumor immunity, making PD-1 a promising target for cancer immunotherapy.