B cells as effectors.

B cells act as immune effectors, primarily through antigen-specific clonal expansion and plasma-cell differentiation. B1 (CD5(+)) B cells and marginal zone B cells dominate T-cell independent humoral responses under the molecular control of activated dendritic cells. Helper T cell-regulated B-cell responses draw on follicular B cells as precursors and rely on qualitatively different patterns of immune synapse formation to regulate B-cell fate. These activities culminate in the germinal center reaction, during which somatic hypermutation and antigen-driven selection produce and preserve high-affinity plasma cells with extended longevity and memory B cells as the sensitized precursors for antigen recall.