B cell activating factor (BAFF), also known as B lymphocyte stimulating factor (BLys) or tumor necrosis factor ligand superfamily member 13B(TNFSF13B), is expressed in B cell line cells and plays an important role in the proliferation and differentiation of B cells as a potent B cell activator. BAFF acts as an immune stimulant and is necessary to maintain normal immunity. Underexpression of BAFF will not activate B cells to produce sufficient immunoglobulins and lead to immune deficiency. Overexpression of BAFF can lead to oversecretion of antibodies, leading to systemic lupus erythematosus, rheumatoid arthritis and other autoimmune diseases.