PTPN22 (Protein Tyrosine Phosphatase, Non-Receptor Type 22): PTPN22 is a lymphoid-specific phosphatase that plays a critical role in regulating immune responses by modulating T-cell receptor signaling. It acts as a negative regulator of T-cell activation, maintaining immune tolerance and preventing autoimmunity. Genetic variations in PTPN22 have been associated with an increased risk of several autoimmune diseases, including rheumatoid arthritis, type 1 diabetes, and systemic lupus erythematosus, highlighting its importance in immune regulation and autoimmunity....