IL-21 induces IgE secretion by human transitional, naive, and memory B cells. Human CB (A) and PB (B) MNCs were labeled with mAb specific for CD20, CD10, and CD27. The frequencies of transitional (CD10⁺CD27⁻), naive (CD10⁻CD27⁻), and memory (CD10⁻CD27⁺) cells were determined. Values represent the mean percentages of B-cell subsets in 12 CB and 8 PB samples. (C) Transitional, naive, and memory B cells were sort-purified and then cultured (∼ 20 × 10³ cells/200 μL per well) with CD40L alone (□) or in combination with IL-4 (100 U/mL; ), IL-21 (50 ng/mL; ), or IL-4 plus IL-21 (■). After 12 to 14 days, supernatants were harvested and the amount of secreted IgE determined. The results are the means plus or minus SEM of data from 4 independent experiments using B-cell subsets isolated from 4 different blood donors.