The precise ultrastructural morphology and functions In reactive conditions of lymphoid follicles (LF) and dendritic cells, including follicular dendrltic cells (FDC) are reviewed; as Well as the pathognomonic role of FDC In some disease conditions and finally, the cellular origin of FDC. In reactive conditions, FDC In each of the fivre follicular zones have distinct ultrastructural features, retlecting the different three dimensional structures and functions of these zones. The FDC framework may be supported by some characteristic factors, including desmosome‐like junctions between FDC and the expression of flbronectin and laminin receptors and caldeasmon on FDC. FDC, especially In the light zone, express various cytokine receptors, but produce only one cytokine, TGF‐β. The outer zone may not only be a cellular pathway in the LF, but may also provide a site for germinal center B cell prollferation, and the FDC‐lymphocyte cluster Is not the site of germinal center B cell division. In patients with autoimmune diseases, such as Hashimoto's thyroiditis and rheumatoid arthritis, FDC may be in a hyperfunctional state, whereas those In patients w*H immunosuppressive disorders, such as Kimura's disease and AIDS, may be in a dysfunctional state. FDC may be derived from fibroblastic reticulum cells in lymphatic tissues rather than in bone marrow cells. The data discussed In this retview provide fascinating insight into the roles of FDC, which are intimately related to the migration, proliferation, cell selection and differentiation of B cells in secondary LF.