Programmed cell death 1 ligand 1 (PDL1) is one of the ligands for programmed cell death protein 1 (PD1). It belongs to the B7 family and immunoglobulin superfamily, along with B7-1 and B7-2. The PD1-mediated inhibitory pathway is exploited by tumors to attenuate anti-tumor immunity and evade destruction by the immune system, thereby facilitating tumor survival. PDL1 modulates the activation threshold of T-cells and restricts their effector response. Its interaction with PD1 initiates a negative signaling cascade in T cells, leading to inhibition of T cell activation. PDL1 plays a pivotal role in inducing and maintaining immune tolerance to self. It provides a molecular stop signal to the adaptive immune system, aiding in distinguishing between self and foreign antigens. Additionally, PDL1 contributes to the development of immune tolerance by promoting T cell anergy and enhancing regulatory T cell development. Increased expression of PDL1 has been associated with suppression of antitumor immune response in cancer as well as upregulation of IL10 production in HIV-infection, resulting in heightened susceptibility of antigen-specific T cells to apoptosis. It may inhibit ongoing T-cell responses through induction of apoptosis and arrestment of cell-cycle progression. Numerous cancers exhibit elevated levels of PDL1 protein expression, which have been correlated with increased tumor aggressiveness and poor prognosis. Consequently, it is considered a promising therapeutic target for human autoimmune diseases and malignant cancers.