On the cover: Human tonsillar lymphoid tissue was stained using anti-IgD (green), anti-Ki-67 (red), and anti-IgM (blue) Abs and analyzed by fluorescence microscopy. A fraction of IgD +IgM+ B cells in follicular mantles and the majority of IgM +IgD− B cells in germinal centers show Ki-67 reactivity, a hallmark of active cell proliferation. Xu, W., P. A. Santini, A. J. Matthews, A. Chiu, A. Plebani, B. He, K. Chen, and A. Cerutti. 2008. Viral RNA triggers T cell-independent IgG class switch DNA recombination by activating mucosal B cells through TLR3. J. Immunol. 181: 276–287.