PTPN22 belongs to a family of genes encoding protein tyrosine phosphatases (PTPs). PTPN22 affects the responsiveness of T and B cell receptors, and mutations are associated with increases or decreases in risks of autoimmune diseases. The 1858C>T polymorphism in the SNP rs2476601 within this gene is associated with significant risk for multiple autoimmune diseases and infectious diseases.
[PMID 25003765] PTPN22: the archetypal non-HLA autoimmunity gene. A 2014 article explained that "PTPN22 exemplifies a shared autoimmunity gene, affecting the pathogenesis of systemic lupus erythematosus, vasculitis and other autoimmune diseases." They explored "the role of PTPN22 in autoimmune connective tissue disease" and theorized "a number of PTPN22-dependent functional models of the pathogenesis of autoimmune diseases." (Abstract). They explained that "the PTPN22 gene is a major risk factor for autoimmunity." As well, "In addition to autoimmune diseases, PTPN22 1858C>T also affects susceptibility to infectious diseases."
See the SNP rs2476601 page for more information and references.