B cell activating factor (BAFF) is a ligand belonging to the TNF superfamily and plays an important role in regulation of lymphocytes, especially peripheral B cells. It is expressed in T cells, dendritic cells, macrophages, and monocytes and binds three receptors, TACI, BCMA, and the BAFF specific receptor (BAFF-R), which is essential for the B cell promoting activity. BAFF induces stimulation of B and T cell function, and promotes maturation and survival of B cell lineage cells and peripheral B cells. High expression of BAFF increases Tʜ1 response, and correlates with B cell malignancies and autoimmunity, such as systemic lupus erythematosus and rheumatoid arthritis, allograft reactions, and celiac disease. On the contrary, inefficient production of BAFF can lead to an impair of B cell activation and immunodeficiency. Recombinant human BAFF contains the TNF-like portion of the extracellular domain, which shares 72% amino acid sequence identity with mouse BAFF.