Janus-faced B cells. Janus, the Roman god of beginnings and endings, is most often depicted as having 2 faces facing opposite directions. Like him, B cells can either elicit (right panel) or terminate (left panel) an immune response, after their activation by combination of ligation of the B-cell receptor, CD40, and Toll-like receptors (TLRs). (Left panel) B cells present the antigen along with costimulatory signals, leading to the activation and the proliferation of T effectors. In turn, activated T cells provide CD40 ligand (CD40L) for the differentiation of B cells into antibody-producing plasma cells. The cytokines produced by B cells may participate into the polarization of T effectors. Finally, B cells play a critical role for the development of T-cell memory. (Right panel) B cells regulate immune response by provision of IL10 that suppresses the activation and the expansion of T effectors directly, and indirectly through the differentiation of T regulatory cells and the suppression of dendritic cell function.