A re-evaluation of the role of C3 in B-cell activation.

There is considerable evidence that, in vivo, C3 is required for the induction and maintenance of memory cells of the B-cell lineage. Recent data from in-vitro systems indicate that ligation of C3 receptors on B cells can activate these cells, and that appropriately presented C3 split products can act as B-cell growth factors. Here, Gerry Klaus and John Humphrey propose that in vivo these growth-promoting effects of C3 may operate within the microenvironment of germinal centres, where B cells encounter antigen-antibody-C3 complexes on the surfaces of follicular dendritic cells.