CD21^−/lo naive B cells fail to properly respond to BCR triggering. CD21^−/lo naive B cells from (A) RA01 and (B) CVID321 patients fail to up-regulate CD69 and CD25 expression after stimulation with F(ab′2) anti-IgM, recombinant human CD40L, and TLR9 agonist CpG. Total naive B cells containing both CD21^−/lo and CD21⁺ B cells were plated and dot plots show CD69 and CD25 expression after 2 days according to gated CD21 expression. (C) BCR triggering in CD21^−/lo naive B cells does not properly increase [Ca²⁺]_i. [Ca²⁺]_i was assessed by flow cytometry in CD21^−/lo and CD21⁺ naive B cells from a healthy donor (HD) and RA01 and CVID321 patients. Both B-cell populations were loaded with Fluo-3 and expressed similar levels of surface IgM expression when experiments were performed. Arrow indicates when F(ab′)2 anti-IgM was added. Data from each group of persons are representative of 2-3 independent experiments.