BAFF (B-cell activating factor), also known as BLyS, TALL-1, THANK, and TNFSF13B, is a TNF superfamily ligand that plays a critical role in the development and survival of B lineage cells. It can be expressed as a homotrimer or as a heteromer in association with the related protein APRIL. It is produced by many hematopoietic cell types, and a soluble form can be released by activated neutrophils. Both BAFF and APRIL signal through the receptors BCMA and TACI, and BAFF additionally signals through BAFF R. Mice that overexpress BAFF exhibit elevated B cell numbers, increased formation and size of germinal centers, and symptoms of autoimmunity. In addition, BAFF costimulates T cell activation, promotes a Th1 biased immune response, and promotes the expansion of Treg cells.