The discovery of immunoglobulin E (IgE) was a breakthrough in the field of allergy and immunology. Our understanding of mechanisms of allergic reactions and the role of IgE in these disorders has paralleled to the discovery of treatment modalities for patients with allergy. The first clue to the existence of a substance responsible for hypersensitivity reactions was demonstrated in 1921 by Prausnitz and Kustner, and after four decades it was identified as an immunoglobulin subclass by Ishizakas and co-workers. In 1968, the WHO International Reference Centre for Immunoglobulins announced the presence of a fifth immunoglobulin isotype, IgE.