Cytokine that binds to LTBR/TNFRSF3. May play a specific role in immune response regulation. Provides the membrane anchor for the attachment of the heterotrimeric complex to the cell surface (By similarity). Heterotrimer of either two LTB and one LTA subunits or (less prevalent) two LTA and one LTB subunits. Belongs to the tumor necrosis factor family.