Cytokine that binds to TNFRSF13B/TACI and TNFRSF17/BCMA. TNFSF13/APRIL binds to the same 2 receptors. Together, they form a 2 ligands -2 receptors pathway involved in the stimulation of B- and T-cell function and the regulation of humoral immunity. A third B-cell specific BAFF-receptor (BAFFR/BR3) promotes the survival of mature B-cells and the B-cell response. Isoform 2 seems to inhibit isoform 1 secretion and bioactivity. Homotrimer. Isoform 2 heteromultimerizes with isoform 1, probably limiting the amount of functional isoform 1 on the cell surface. Isoform 2 is expressed in many myeloid cell lines. The soluble form derives from the membrane form by proteolytic processing. Isoform 2 is not efficiently shed from the membrane unlike isoform 1. Belongs to the tumor necrosis factor family.