PD-1, or Programmed Cell Death Protein 1, is a protein found on the surface of immune cells, particularly T cells. It plays a crucial role in regulating the immune response by inhibiting T cell activity. When PD-1 binds to its ligands, PD-L1 and PD-L2, it helps prevent the immune system from attacking normal cells, thus maintaining balance and preventing autoimmunity. In cancer, some tumors exploit the PD-1 pathway to evade immune detection by expressing PD-L1. This interaction can suppress the immune response against the tumor, allowing it to grow unchecked. Therapies that block PD-1 or its ligands are used to enhance the immune response against cancer cells.