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. Isoform 2 is probably non-functional. Heterotrimer of either two LTB and one LTA subunits or (less prevalent) one LTB and two LTA subunits. Spleen and thymus. Belongs to the tumor necrosis factor family.