Follicular dendritic cells in the alternative antigen transport pathway: microenvironment, cellular events, age and retrovirus related alterations.

Follicular dendritic cells (FDC) are located in lymphoid follicles of secondary lymphoid tissues and play a pivotal role in the initiation and maintenance of the secondary antibody response. FDC are an integral part of the microenvironment of the follicle and function as members of the 'alternative antigen transport pathway.' This pathway consisting of the antigen transport cell-FDC-ICCOSOME-B cell axis, leads to the formation of germinal centers where antibody-forming cell and memory B cell development are initiated through interaction of FDC-retained antigen, B cells and T helper cells. Evidence suggests that these interactions are down-regulated through antibody feedback, or as needed, reactivated with the utilization of FDC-retained antigen for the maintenance of antibody levels. Age- and retrovirus-related FDC defects seriously compromise the capacity of this pathway to maintain immunity.