PTPN22 (Protein Tyrosine Phosphatase, Non-Receptor Type 22): PTPN22 is an enzyme that reflects the regulation of immune responses in the body. It is a lymphoid-specific phosphatase that modulates T-cell receptor signaling, acting as a negative regulator of T-cell activation. PTPN22 is essential for maintaining immune tolerance and preventing autoimmunity. Genetic variations in PTPN22 have been linked to an increased risk of autoimmune diseases such as rheumatoid arthritis, type 1 diabetes, and systemic lupus erythematosus, highlighting its key role in immune regulation.