Defective B cells are responsible for the failure to produce PF4/heparin-specific antibodies in B-cell–specific Notch2-deficient mice. Splenic B cells were isolated from CD19CreNotch2^+/+ or CD19CreNotch2^fl/fl mice, mixed with B-cell–depleted wild-type splenocytes at a 1:1 ratio, and then adoptively transferred into partially irradiated μMT mice. The recipients were immunized with mouse PF4/heparin complexes. Sera were collected at the indicated time points after immunization. Total PF4/heparin-specific IgG level was measured by ELISA. The horizontal lines indicate the mean values. Data shown are obtained from 5 recipients in each group.