This review highlights the role of a CD4(+)CD3(-) accessory cell in the development of organized lymphoid infrastructures as well as in the development of high-affinity antibody responses and T-cell memory. These 2 functions are linked in the development of the vertebrate immune system and are effected by the constitutive expression of 2 sets of tumor necrosis factor (TNF) family members. The expression of lymphotoxin 3 (LT3), LT3, and TNF-3, which are closely linked genetically, affects the organization of lymphoid structures into B-cell and T-cell areas; the dual expression of OX40 ligand (TNFSF4) and CD30 ligand (TNFSF8) influences both the survival of T-cells within germinal centers and T-cell memory.