CD38 and CD19 are part of a supramolecular functional complex in human B cells. CD38 ligation in human B cells is followed by Ca²⁺ fluxes (A) and by ERK1/2 phosphorylation (B). CD19- and sIgM-mediated signals were comparatively assayed. Tonsillar B lymphocytes were labeled with Fluo 3-AM, washed, and analyzed continuously using a FACSort. The primary antibody was added 10 seconds after beginning the analysis and was followed by a cross-linker antibody (for anti-CD38 and anti-CD19) after approximately 2 to 300 seconds. ERK1/2 phosphorylation was checked 1 and 5 minutes after receptor ligation by using specific reagents in a Western blot system. (C-D) Ca²⁺ fluxes and phosphorylation of ERK1/2 proteins induced by CD38, CD19, and sIgM ligation were checked in a panel of cell lines using the same experimental approaches as in (B). In both cases, bars represent the percentage of increase over background levels, obtained after incubating the cells with an irrelevant antibody.