PD-L1 (Programmed Death-Ligand 1), encoded by the CD274 gene, is a crucial immune checkpoint transmembrane protein. It is primarily expressed on the surfaces of antigen-presenting cells, epithelial cells, and tumor cells. By binding to the PD-1 receptor on T cells, it transmits inhibitory signals, negatively regulating T cell activation and function, thereby maintaining immune tolerance and preventing excessive immune responses. In the tumor microenvironment, cancer cells often aberrantly overexpress PD-L1 to "hijack" this pathway, suppressing the anti-tumor immune response of T cells and facilitating immune escape. This mechanism establishes PD-L1 as a core target for cancer immunotherapy. Inhibitors targeting the PD-1/PD-L1 pathway, such as monoclonal antibodies, can block this signaling axis, restoring the tumor-killing function of T cells. These inhibitors have achieved revolutionary breakthroughs in the treatment of various cancers and have led to the development of companion diagnostic strategies based on PD-L1 expression.