**4.5 PDL-1-PD1**

PD-1 (programmed cell death protein 1) and its ligand PD-L1 (programmed cell death ligand 1) are proteins that are involved in regulating the immune response. They are known as immune checkpoint molecules because they prevent the immune system from overreacting and attacking healthy tissues. Traditionally, PD-1/PD-L1 has been viewed as a T cell immune checkpoint, where PD-1 on the surface of T cells interacts with PD-L1 on the surface of other cells, including cancer cells and antigen-presenting cells. This interaction leads to the inhibition of T cell activity, which prevents the immune system from attacking healthy tissues and can allow cancer cells to evade the immune system. However, recent research has also shown that PD-1/PD-L1 is involved in regulating phagocytosis. PD-L1 can be expressed on the surface of tumor cells and other cells, and when it interacts with PD-1 on the surface of phagocytes, it inhibits their ability to perform phagocytosis. This means that by blocking the interaction between PD-1 and PD-L1, it may be possible to enhance the ability of phagocytes to remove foreign particles and to enhance the immune response against cancer cells. As a result, there is growing interest in the development of drugs that target PD-1/PD-L1 for the treatment of cancers [18, 69].
