An electron micrograph (EM) showing the convolutions of the FDC dendrities (FDC-D) located among several germinal center B lymphocytes. The convolutions seen represent a section through FDC dendrites outlined in this electron micrograph by the electron dense precipitate formed during the reaction of the antigen, HRP, with its substrate. This precipitate represent the location of HRP (or Ag)-Ab-C' complexes. The FDC dendrites are tightly balled up as the result of the binding of the immune complex on the surface of the dendrites to Fc (CD32) and C' receptors (CD21). This happens when antibody levels to the antigen rise (as immunity develops). After a time, when antibody levels drop in the circulation, antibody dissociates from these immune complexes and the dendrites are freed. This will again expose the antigen on the dendrites to the surrounding lymphocytes and the FDC can enter a new cycle in the alternative antigen transport pathway. For more information on this FDC control of the retained antigen see: REF1. For the significance of this event consult: REF2