B-cell activating factor (BAFF), also known as B Lymphocyte Stimulator (BLyS) or tumor necrosis factor ligand superfamily member 13B (TNFSF13B), is expressed in B cell lineage cells, and acts as a potent B cell activator. It has been also shown to play an important role in the proliferation and differentiation of B cells. As an immunostimulant, BAFF is necessary for maintaining normal immunity. Inadequate level of BAFF will fail to activate B cells to produce enough immunoglobulin and will lead to immunodeficiency. Excessive level of BAFF causes abnormally high antibody production, results in systemic lupus erythmatosis, rheumatoid arthritis, and many other autoimmune diseases.