ABSTRACT
In the past, B cells were considered a homogeneous population that gave rise to
Ig-secreting cells and memory B cells, following specific antigenic stimulation. In recent
years, this view has changed, and B cells are now documented as composed of different
subpopulations, each with special functions (Fig. 1). These concepts emerged from
observations in both humans and experimental animals suggesting that the B cell-rich
zone of peripheral lymphoid tissues segregates into functionally unique areas. For example,
B-cell proliferation and selection occur in germinal centers (GCs) of lymphoid follicles
during an antigenic response, promoting the specific expansion of the cells equipped with
B-cell antigen receptors (BCRs) of the highest affinity for the stimulating antigen. In the
mantle of lymphoid follicles, there is an accumulation of “virgin” (foreign antigen
inexperienced) cells that may be recruited into GCs by antigen stimulation. In contrast,
B cells localized in the splenic marginal zone (MZ) can respond in a T cell-independent
fashion by producing IgM antibodies against polysaccharide antigens of encapsulated
bacteria. B cells with similar features are detected in subepithelial areas of tonsils,
subcapsular areas of lymph nodes, and dome regions of Peyer’s patches. Cells of
lymphoid follicles and those of the MZ have dissimilar phenotypic and trafficking
features, mature by distinct pathways, and respond differently to cytokines and
chemokines. This further highlights the diversity of B-cell subsets.
