PD-L1, also known as CD274 or B7-H1, is a type I transmembrane glycoprotein and a member of the B7 family. It was first identified in 1999. PD-L1 plays a crucial role in immune regulation by binding to PD-1 on T cells, inhibiting their activation and proliferation through multiple pathways, thus dampening tumor-reactive T cell responses. This protein is widely expressed in various cells, including antigen-presenting cells, activated T cells, B cells, and some non-hematopoietic cells. Its expression is often upregulated in tumor cells, contributing to immune evasion. Targeting PD-L1 has become a major strategy in cancer immunotherapy, with several antibodies developed to block its interaction with PD-1, enhancing anti-tumor immunity.