The immune system is the body's natural defense against disease. The immune system sends a type of cells called T cells throughout the body to detect and fight infections and diseases—including cancers. One way the immune system controls the activity of T cells is through the PD-1 (programmed cell death protein-1) pathway. However, some cancer cells hide from T-cell attack by taking control of the PD-1 pathway and this stops T cells from attacking cancer cells.