The IgE antibody and total IgE responses were studied in three mouse strains immunized with low doses of ovalbumin. BALB/c and C57BL/6J mice were considered as ‘high’ and ‘intermediate’ IgE antibody responders, respectively; during immunization the peritoneal mast cells of mice from these two strains became sensitized. In identical conditions of immunization, very low IgE antibody response and lack of mast cell sensitization was observed in CFW mice. After exposure of these CFW mice to low-dose X-irradiation (250 R) or a moderate dose of cyclophosphamide (100 mg/kg), a detectable IgE antibody response appeared and peritoneal mast cells became sensitized. In all three strains the evolution of total serum IgE levels during immunization did not correlate with IgE antibody response; an early transient elevation of total IgE level appeared soon after first immunization, at a time when IgE antibody responses were not detectable or were very low. After X-irradiation or cyclophosphamide pretreatment, this elevation of total IgE was potentiated. The possibility of differences in regulation of specific IgE antibody response and IgE of unknown specificity in the mice is di
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