As early as 1919, from a case of transient asthma caused by blood transfusion, the presence in serum of a factor causing allergy was proposed. Despite intense search, this factor was not identified until 1968, when immunoglobulin E (IgE) was officially recognized by the World Health Organization. The knowledge about IgE mediating allergy has greatly improved our understanding of allergic inflammation and diagnoses and treatment of allergic diseases.