MZB cells, not B1b cells, are responsible for rapidly induced IgM xenoantibody formation. (A) MACS-enriched CD19^pos B cells from BALB/c nude spleens (left panel) were purified using FACS sorting into CD21^highCD23^low MZB cells (middle panel) and CD21^negCD23^neg B1b cells (right panel). Representative plots are shown from 3 identical experiments. (B) CB17 SCID mice were given 2 to 5 × 10⁶ subset B cells intravenously on day 0, and 200 μL whole hamster blood on day 7. Serum IgM xenoantibody titers in an unreconstituted SCID mouse (left panel), a SCID mouse given CD19^posCD21^highCD23^low MZB cells (middle panel), and a SCID mouse given CD19^posCD21^negCD23^neg B1b cells (right panel). Representative plots are shown from 1 of 3 to 4 mice from 3 identical experiments.