In addition to regulate metabolism, leptin also has certain effects in immune system, particularly in the T-cell proliferation and differentiation,[47] and the regulation of immune function.[48] Decreased T lymphocytes and immune system function were found in leptin receptor lacking mouse model.[49] Leptin levels in serum and synovial fluid were increased significantly in patients with OA.[50] Inflammation was weakened in leptin-lacking mouse model where symptoms of RA were also relieved. Here, LEP is linked to rheumatoid arthritis.