Background: Increasing evidence suggests that the low-affinity receptor for IgE, CD23, plays an important role in controlling the activity of allergen-specific T cells through IgE-facilitated allergen presentation. Objective: We sought to determine the number of CD23 molecules on immune cells in allergic patients and to investigate whether the number of CD23 molecules on antigen-presenting cells is associated with IgE levels and influences allergen uptake and allergen-specific T-cell activation. Methods: Numbers of CD23 molecules on immune cells of allergic patients were quantified by using flow cytometry with QuantiBRITE beads and compared with total and allergen-specific IgE levels, as well as with allergen-induced immediate skin reactivity. Allergen uptake and allergen-specific T-cell activation in relation to CD23 surface density were determined by using flow cytometry in combination with confocal microscopy and T cells transfected with the T-cell receptor specific for the birch pollen allergen Bet v 1, respectively. Defined IgE-allergen immune complexes were formed with human monoclonal allergen-specific IgE and Bet v 1. Results: In allergic patients the vast majority of CD23 molecules were expressed on naive IgD (+) B cells. The density of CD23 molecules on B cells but not the number of CD23 1 cells correlated with total IgE levels ( RS50.53, P5.03) and allergen-induced skin reactions (R-S = 0.63, P = .008). Uptake of allergen-IgE complexes into B cells and activation of allergen-specific T cells depended on IgE binding to CD23 and were associated with CD23 surface density. Addition of monoclonal IgE to cultured PBMCs significantly (P =.04) increased CD23 expression on B cells. Conclusion: CD23 surface density on B cells of allergic patients is correlated with allergen-specific IgE levels and determines allergen uptake and subsequent activation of T cells.