The most severe cases of lupus are treated with immunosuppressants. The drugs inhibit proteins essential to immune cell signaling pathways, thus blocking heightened immune responses. Azathioprine, mycophenolate, leflunomide and methotrexate are the most commonly prescribed immunosuppressants. In 2011, Belimumab became the first new FDA-approved lupus treatment drug in 50 years of research. The drug is used to treat patients with active, autoantibody-positive lupus who are receiving standard therapies listed above. It interferes with B-lymphocyte stimulator protein and reduces the number of dysfunctional B-cells in lupus patients.