Cell surface receptors for IgE were isolated from detergent lysates of iodinated, IgE-saturated, rat basophilic leukemia cells by precipitation with anti-IgE antibodies followed by chromatography at acid pH. The isolated material showed a single 125I-band (m.w. ∼58,000) on gel electrophoresis in sodium dodecyl surfate and was used to immunize a rabbit. The resulting antiserum was reacted with lysates of surface iodinated mouse or rat tumor mast cells. Analysis of the precipitates on (10%) gel electrophoresis revealed one major peak comprising >80% of the detectable counts and having an estimated m.w. of ∼58,000. The antiserum reacted with detergent-solubilized and cell-bound receptors in the presence of absence of excess IgE; it also inhibited the binding of 125I-IgE. Cultured mouse mastocytoma cells never exposed to IgE released 3H-serotonin when incubated with F(ab′)2, but not Fab′ fragments of the antiserum, which had been rigorously freed of IgE and anti-IgE. The release was inhibited in the presence of excess IgE, was Ca++ dependent, and equaled 80% of the maximum obtained with IgE and anti-IgE. We conclude that aggregation of the receptors for IgE provides the critical signals for cell activation.

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