Receptors for IgM were detected on peripheral blood human eosinophils by a rosette technique with ox red blood cells coated with the IgM fraction of the specific immunserum. Between 14% and 43% (mean 27%) FcμR positive cells were found after an overnight incubation period at 37°C by using this technique. The specificity of the receptors for IgM was assessed by studying the inhibitory capacity of purified human IgM in the rosette assay. From an ultrastructural point of view, the EA(M) rosette-forming cells are mature eosinophilic granulocytes characterized by a nucleus with a variable number of lobes and a certain number of 'first type' granules partially or totally devoid of their content.
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