Role of IL-21 and CD40 ligation in IgG and antiplatelet-antibody production by splenic B cells. Sorted splenic CD19⁺ B cells were stimulated with a combination of anti-IgM antibody (αIgM), anti-CD40 agonist antibody (αCD40), and IL-21. At day 10, supernatants were collected to measure total IgG and anti-GPIIb/IIIa autoantibody secretion. (A) IgG production measured in 3 controls (open circles) and 5 ITP patients (black squares). P value derived by Wilcoxon matched-pairs test. Data are depicted in dot plots. The horizontal bar represents the median with interquartile range. (B) Anti-GPIIb/IIIa antibody secretion obtained from 2 controls and 5 ITP patients. P value derived by unpaired Student t test. Data are depicted in histograms representing the mean with standard error of the mean. NS, nonsignificant; *P < .05; **P < .01.