Background: As a result of the very efficient capture of allergens by IgE that focuses to CD23 on B cells or Fc epsilon RI on dendritic cells, allergen-specific T cells can be activated after exposure to very low levels of allergens. This IgE-mediated allergen presentation is 100- to 1,000-fold more efficient than fluid phase endocytosis. The aim of the present study was to determine whether humanized anti-IgE mAb Hu-901 can prevent the activation of allergen-specific T cells by inhibiting IgE-mediated allergen presentation. Methods: A house dust mite major allergen Der p 1-specific T cell line was generated from an allergic asthma patient, and a model was set up to show IgE-facilitated allergen presentation via CD23 on EBV-transformed B cells. In addition, experiments were performed by FAGS analysis, detecting the presence of IgE-allergen complexes bound to EBV-B cells by polyclonal FITC-labeled anti-IgE antisera. Results:The anti-IgE mAb Hu-901 inhibited proliferation of allergen-specific T cells at low allergen concentrations, inhibition was dose-dependent, This effect could be explained by Hu-901 inhibition of binding of allergen-IgE complexes to CD23 expressed on EBV-transformed B lymphocytes. Conclusions: These data clearly indicate that anti-IgE antibodies for the treatment of allergy exert their effect not only by inhibiting mast cell/basophil degranulation, but also by preventing T cell activation, which possibly explains the effect of anti-IgE treatment on late-phase reactions noted in clinical studies. Copyright (C) 2001 S. Karger AG, Basel.