Lymphotoxin and tumor necrosis factor provide essential signals for the formation of secondary lymphoid tissue structures. Lymphotoxin in its membrane form (LTα1β2 heterotrimer) is required for the development of lymph nodes and Peyer's patches and supports the development of normal spleen structure. In the spleen, lymphotoxin acts during embryonic development to support the formation of distinct B and T cell zones. Lymphotoxin also acts in a tonic fashion - supporting the formation and maintenance of the follicular dendritic cell network and of primary B cell follicle structure. The cells that deliver the tonic lymphotoxin signal supporting follicular dendritic cell structure are B cells; thus, B cells participate fundamentally in the development of the lymphoid tissue structure in which they subsequently mature.