PDCD1 and allergic disease: Signaling pathways triggered by the binding of PD-1 to its ligands, PD-L1 (B7-H1) and PD-L2 (B7-DC), are essential for regulating the balance between T-cell activation and tolerance, thereby contributing to the pathogenesis of various diseases such as autoimmunity, tumors, infectious diseases, and allergies (17, 106, 107).