A fully human IgG1 monoclonal antibody directed against B-Lymphocyte stimulator protein (BlyS or TNFSF13B) with potential immunomodulating activity. Belimumab specifically recognizes and inhibits the biological activity of BlyS, thereby preventing the binding of BlyS to B-lymphocytes. This inhibits the maturation of B-lymphocytes and may induce apoptosis in B-lymphocytes. In addition, it may decrease B-lymphocyte proliferation and/or survival. BlyS, a member of TNF family supporting B-lymphocyte maturation and survival, has been implicated in the pathogenesis of autoimmune diseases and B-lymphocyte malignancies.