The present invention comprises a fusion molecule comprising a Fc.epsilon. fragment sequence including functionally active CH2, CH3 and CH4 domains of the constant region of an IgE heavy chain (CH.epsilon.2-CH.epsilon.3-CH.epsilon.4 sequence) linked at its C-terminus to the N-terminus of a second polypeptide including functionally active hinge, CH2 and CH3 domains of the constant region of an IgG.sub.1 heavy chain (.gamma.hinge-CH.gamma.2-CH.gamma.3 sequence), pharmaceutical compositions comprising the fusion molecule and methods of treatment using the fusion molecule.