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By substituting the heavy chain constant region of IgM and IgD with that of IgG, IgA or IgE, immunoglobulin class switching endows antibodies with novel effector functions that enhance the ability of the immune system to effectively clear invading pathogens. Plasmacytoid dendritic cells critically link innate immunity with adaptive immunity by producing massive amounts of type 1 IFN in response to viruses. We have recently found that type 1 IFN triggers class switching by inducing myeloid dendritic cells to upregulate the expression of BAFF and APRIL, two powerful B cell-activating molecules. In this paper, we propose that IFN-producing plasmacytoid dendritic cells modulate class switching by activating B cells through both T cell-dependent and T cell-independent pathways. A better understanding of these pathways may facilitate the development of novel antiviral vaccine strategies and aid in identifying new therapies for antibody-mediated autoimmune disorders, such as lupus. [ABSTRACT FROM AUTHOR]