B lymphocyte stimulator (BLyS), a member of the tumor necrosis factor (TNF) superfamily of cytokines, induces B-cell proliferation and immunoglobulin secretion and is a key regulator of peripheral B-cell populations in vivo. BLyS is made by immune-cells called monocytes and macrophages. When monocytes and macrophages are activated, BLyS is released and binds to a receptor found only on B cells.