An immunoglobulin isotype (subclass). This isotype has the unique ability to bind with high affinity to IgE receptors (FceRI) on mast cells and basophils and induce degranulation and cytokine production by these cells when they are crosslinked by antigen. In addition to a central role in atopy and allergic responses, it has been speculated that IgE-mediated mast cell degranulation, by its release of mediators that increase vascular permeability, has an important role in the initiation of immune response in general. [ ]