A topic allergy is a genetically determined immunodisorder that affects almost 20% of the population worldwide. Immediate symptoms of type I allergy are caused by the release of biologic mediators from effector cells induced by IgE-allergen complexes that cross-link the high-affinity receptor for IgE (FcεRI). Chronic disease manifestations result from allergen-specific T-cell activation, a process that is enhanced when allergens are presented via FcεRI-bound IgE. We report the baculovirus expression, as soluble recombinant proteins, of the minimal units required for human IgE and FcεRI interaction: Cε3 represents the third constant domain of the IgE heavy chain, and α2 is the membrane-proximal Ig-like module from FcεRIα. Native overlay experiments showed binding of human FcεRIα to recombinant Cε3 and of natural or recombinant human IgE to recombinant α2. Moreover, recombinant Cε3 inhibited binding of natural IgE antibodies to α2, and preincubation of human IgE with α2 inhibited anti-IgE–triggered histamine release from human basophils. Isolated Cε3 and α2 can now be used for the molecular and structural analysis of the IgE-FcεRI interaction, as well as for diagnostic and therapeutic applications.