For several decades it has been known that IgE is central to the allergic response. In recent years, however, IgE has been described as having several actions in addition to passively arming the mast cell or basophil by binding to their high affinity IgE receptors. Recent studies have revealed roles for IgE in the regulation of its own receptor expression and in contributing to cell survival. Furthermore, the presence of trimeric high-affinity IgE receptors on leukocytes other than mast cells and basophils, together with the discovery of previously unknown functions of the beta subunit of the complex, has added to the complexity of IgE modulation of the allergic response.