The discovery of IgE was a major breakthrough in the development of allergology as a medical discipline. Exactly forty years ago two independent research teams in the USA and Sweden investigated and explained the role of an unknown factor called reagin in allergic hypersensitivity. Provided evidence suggested that reagins are immunoglobulins of a new class. Further research into IgE properties significantly improved our understanding of the mechanism of allergic reaction. Current diagnostics and therapy of allergic diseases are largely based on these findings