Direct intrafollicular differentiation of memory B cells into plasma cells.

Antibody-forming (plasma) cells and memory B cells are both generated during a humoral immune response in the spleen. Antibody-forming cells develop in the outer parts of the periarteriolar lymphocyte sheaths (PALS) while memory B cells develop in the lymphoid follicles. As soon as the first antibodies appear in the circulation, immune complexes are formed, the bulk of which are ingested by cells of the mononuclear phagocyte system. A small proportion, however, is immobilized on the processes of follicular dendritic cells (FDCs). These immune complexes are thought to play a key role in the generation of memory B cells in the follicles. In this article Nico van Rooijen postulates that, in the continuing presence of soluble antigen, the newly generated memory B cells in the follicles may continue to differentiate into antibody-forming cells. Experimentally such conditions are fulfilled when adjuvants that slowly release antigen are used and when antigens used for immunization can replicate in the body. The postulated synergistic action of immobilized immune complexes and soluble antigen in the follicles may represent an efficient mechanism for the rapid production of large numbers of plasma cells.