Programmed death ligand 1 (PD-L1) belongs to the B7 series and is a 33-kDa type 1 transmembrane glycoprotein that contains 290 amino acids with Ig-V and IgC domains in its extracellular region. PD-L1 expression can be detected on hematopoietic cells including T cells, B cells, macrophages, dendritic cells (DCs), and mast cells, and non-hematopoietic healthy tissue cells including vascular endothelial cells, keratinocytes, pancreatic islet cells, astrocytes, placenta syncytiotrophoblast cells, and corneal epithelial and endothelial cells. PD-L1 is an essential immune checkpoint protein that binds to programmed death 1 (PD-1) on T-lymphocytes. Engagement of PD-1 by PD-L1 alters the activity of T cells in many ways, inhibiting T cell proliferation, survival, cytokine production, and other effector functions. T cell plays a critical role in killing cancer cells while the cancer cell exhibits immune escape by the expression of PD-L1. The binding of PD-L1 to PD-1 inhibits T cell proliferation and activity, leading to tumor immunosuppression.