CD38⁺⁺CD20^± B cells express and produce large amounts of Ig. (A) Spleen or BM MNCs were stained with anti-CD38 and CD20 mAb and then stained for surface or cytoplasmic immunoglobulin using heavy and light chain–specific antibodies. Results are representative of 3 different donor spleens or BM samples. (B) CD38⁺⁺CD20^± B cells were sorted and cultured (approximately 5 × 10³ cells/200 μL) in medium only. After 5 days, the amount of secreted Ig was determined using ELISA. Values represent the mean ± SEM of 10 experiments using cells from 8 different donors. (C) Splenic naive, memory, and CD38⁺⁺CD20^± B cells, as well as BM PCs, were sorted directly into wells of ELISPOT plates precoated with anti-IgM, IgG, or IgA antisera. Percentages of IgM, IgG, and IgA ISCs within the different B-cell populations were then enumerated. Values represent the mean of 6 (spleen) or 3 (BM) experiments using cells from different donors.