Since the beginning of this century, allergen immunotherapy has been widely used to treat allergic disorders. In addition to the long-term clinical efficacy of this therapy, there are immediate beneficial effects as observed in rush immunotherapy for which there is no clear mechanism. We investigated the direct impact of an Ag on its specific IgE in terms of IgE measurability in immunoassays and subsequent binding of IgE to the high-affinity receptor for IgE. As a model we used a chimeric IgE specific for NIP that exhibits similar biologic properties as serum or myeloma IgE. To mimic particulate and soluble allergens we coupled 15 NIP molecules to BSA. Using this "artificial allergen" we could show that the presence of the Ag reduced the IgE measurability in immune assays. Furthermore, IgE binding to the Fc epsilon RI was 60% inhibited in the presence of the Ag shown with an optical biosensor that monitors molecular interactions. In normal basophils passively sensitized with IgE that was preincubated with the Ag, no sulfidoleukotriene release could be induced. Rush immunotherapy may invoke a similar phenomenon, resulting in the short-term alteration of symptoms by blocking or substantially reducing binding of IgE to its high-affinity receptor. Thus, our result may explain some of the short-term beneficial effects observed in rush immunotherapy.