IgE plays a critical role in hypersensitivity reactions such as asthma and allergy as well as poorly defined roles in immunity to parasitic helminth infections. The quantity of antigen-specific IgE is thought to affect the intensity of the allergic reaction as well as the perceived level of resistance to parasitic worms. Because most somatic IgE is bound by its receptors, Fc epsilon RI and Fc epsilon RII, and increased expression of IgE receptors also change with cellular activation status, the serum concentration of IgE may not necessarily reflect levels of systemic IgE. Accurate measures of IgE would help to define the bona fide role of this molecule in immunity. Furthermore, improved indicators of systemic antigen-specific IgE could better predict the risk for severe allergic responses. In this report, we demonstrate a simple method for measuring cell-bound IgE in whole blood using basic flow cytometry. This method demonstrates that, in general, cell-bound IgE correlates well with serum levels of IgE. However, discordance in serum and cell-bound IgE levels in some individuals illustrates the inadequacy of using serum levels of IgE as a systemic indicator and validates the need to assay both cell-bound and free IgE
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