Stat3 is necessary for TD IgG plasma cell formation. Stat3^fl/flCD19^Cre/+ mice are capable of forming IgM plasma cells in response to either a TD or TI antigen. In addition, germinal center formation proceeds normally, leading to successful isotype switching to all 3 classes (IgG, IgA, IgE). It is likely that this occurs in distinct anatomic locations for each isotype (not depicted), with IgG predominating in the spleen and lymph nodes; IgA, in the gut and related mucosal tissues such as appendix, Peyer patch, and tonsils; and IgE, in still-unknown sites. These class-switched B cells (of any isotype) can subsequently be selected into the memory pool or terminally differentiate into plasma cells. However, in the absence of B-cell Stat3, IgG B cells are unable to respond to T-cell-derived cytokines that drive terminal differentiation. Thus, Stat3 is uniquely required only for terminal differentiation and only for IgG plasma cells.