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 the signalling of the T-cell receptor and acts as a negative regulator of T-cell activation. PTPN22 is essential for maintaining immune tolerance and preventing autoimmune diseases. Genetic variations in PTPN22 have been associated with an increased risk of autoimmune diseases such as rheumatoid arthritis, type 1 diabetes and systemic lupus erythematosus, highlighting its central role in immune regulation.