Immunoglobulin E (IgE), and the interaction with it’s receptors expressed on mast cells and basophils, are main players in the pathomechanism of IgE mediated allergic reactions (allergic rhinitis, asthma, atopic dermatitis, food allergies). Purified human IgE protein, recombinant IgE receptors, and biochemically and functionally well characterized monoclonal antibodies, directed against different epitopes on these players, are tools of central importance for allergy research and diagnostics, and also for the identification and development of novel anti-allergic drugs.