CD23 expression correlates with responsiveness to IL-4 plus CD40 stimulation in the BM and the periphery. The indicated BM and splenic B-cell subsets were sorted and cultured in triplicate with the indicated stimuli as described in “Materials and methods.” Fr E indicates sIgM⁺ AA4⁺ BM B cells. Data represent the mean and error bars indicate the SEM for each group.