PD-1 is expressed on the surface of activated T cells, B cells, and myeloid cells. When PD-1 binds to its ligands, PD-L1 or PD-L2, it delivers an inhibitory signal that reduces the proliferation of T cells, decreases cytokine production, and inhibits cytotoxic activity. This interaction ensures that the immune response is not overactive, which is beneficial in preventing autoimmune diseases but problematic in the context of tumor immunity.
