B-cell activating factor (BAFF) is the tumor necrosis factor ligand superfamily member 13B (TNFLSF13B), also known as B Lymphocyte Stimulator (BLyS) and TNF- and APOL-related leukocyte expressed ligand (TALL-1) and the Dendritic cell-derived TNF-like molecule (CD257 antigen; cluster of differentiation 257). BAFF is a 285-amino acid long peptide glycoprotein which undergoes glycosylation at residue 124. It is expressed as transmembrane protein on various cell types including monocytes, dendritic cells and bone marrow stromal cells. The transmembrane form can be cleaved from the membrane, generating a soluble protein fragment. BAFF is the natural ligand of three unusual tumor necrosis factor receptors named BAFF-R, TACI, and BCMA, all of which have differing binding affinities for it. These receptors are expressed mainly on mature B lymphocytes (TACI is also found on a subset of T-cells and BCMA on plasma cells). TACI binds worst since its affinity is higher for a protein similar to BAFF, called A Proliferation Inducing Ligand (APRIL). BCMA displays an intermediate binding phenotype and will work with either BAFF or APRIL to varying degrees. Signaling trough BAFF-R and BCMA stimulates B lymphocytes to undergo proliferation and to counter apoptosis. All these ligands act as heterotrimers (i.e. three of the same molecule) interacting with heterotrimeric receptors.

BAFF is a master regulator of peripheral B cell survival, and also acts in processes such as immunoglobulin isotype switch and B cell co-stimulation. Beside its major role in B cell biology, BAFF co-stimulates activated T cells. Deregulated expression of this membrane-bound protein, which can readily be released in a soluble form by proteolytic cleavage, leads to autoimmune disorders in mice. In the human, elevated levels of soluble BAFF have been detected in the serum of patients with various autoimmune diseases, such as rheumatoid arthritis (RA), Sjögren's syndrome (SS) and systemic lupus erythematosus (SLE). BAFF levels are also elevated in patients with multiple myeloma and B-cell chronic lymphoid leukemia (B-CCL).

BAFF ELISA determines soluble human BAFF in serum and in cell culture supernatant.