Monomeric IgE bound to the low-affinity receptor for IgE (FcERII-CD23) on EBV-transformed human B cells selectively enhances binding of antigen and therefore presentation to specific T-cell clones. To demonstrate the role of monomeric IgE in antigen focusing, we have made use of a system consisting of human T-cell clones specific for Der-P1 (major allergen of the Dermathophagoides pteronyssinus), Der-P1 coupled to NIP (Der-P1-NIP), and the commercially available chimeric (human-murine) monoclonal IgE antibodies with specificity for the hapten NIP. We have found that monomeric IgE binds to CD23 and remains detectable on the surface of the B cells for a period of at least 16 hours at 37 degrees C. Pulsing of these IgE-anti-NIP (1 microgram/ml) treated B cells for 1 hour at 37 degrees C with low amounts (10 ng/ml) of Der-P1-NIP antigen allows the B cells to stimulate Der-P1-specific T cells. Even with IgE concentrations as low as 20 ng/ml, which were not detectable by immunofluorescence, we were able to induce a significant T-cell response. Furthermore, ongoing specific T-cell-B-cell interactions were not inhibited by the presence of high concentrations of nonspecific IgE molecules (incubated with up to 25 micrograms/ml) on the surface of the B cells. Our data confirm the hypothesis that IgE, bound by either CD23 or the high-affinity receptor for IgE, potentiates the immune response. Therefore, IgE may be seen as the fourth general mechanism for antigen capture by (nonspecific) antigen-presenting cells.