The distribution of surface IgE on human basophils was studied using fluorescence microscopy and immunoferritin electronmicroscopy. Redistribution of the IgE was dose, time and temperature dependent and required divalent anti-IgE. Cells which can release histamine in vitro were indistinguishable in these respects from cells which cannot. The redistribution was unaffected by the presence or absence of Ca++.

No correlation between the conditions required for optimal histamine release and for redistribution was observed. At low doses, optimal histamine release occurred in the absence of, or before, redistribution. At higher doses redistribution occurred in the absence of histamine release. Antigen-induced histamine release was unaccompanied by gross redistribution of the surface IgE.

Since both histamine release and redistribution require bridging of IgE on basophils it is concluded that only certain kinds of cross-linking of the IgE effectively stimulates these cells.

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