Background: We frequently encounter subjects without overt symptoms despite high IgE antibodies to egg white and its components. The measurements of these antibodies are not necessarily efficient for the diagnosis or the prediction of the outcome of egg allergy in children. Methods: Specific IgE antibodies to egg white and its components, including ovomucoid, ovalbumin, ovotransferrin and lysozyme, were measured by direct RAST assays. IgE-binding activity to ovomucoid degraded by pepsin, trypsin and chymotrypsin was examined by RAST inhibition. Thirty subjects were divided into two groups with positive (n = 18; mean age ± SD = 42 ± 25 months) and negative (n = 12; mean age ± SD = 48 ± 31 months) oral challenge tests with egg white antigens. The individuals with positive results to the first challenge tests were given the second provocation tests at mean intervals of 32 months. IgE-binding activity of the sera collected on the first challenge to these ovomucoid fragments was compared between subjects with positive and negative reactions to the follow-up challenge tests. Results: There were no significant differences in IgE antibody titers to egg white and its components between the positive and negative groups at the first and the second challenge tests. IgE-binding activity to ovomucoid digests after treatments with pepsin (p = 0.000008) and trypsin (p = 0.037), except chymotrypsin (p = 0.062), were significantly higher in subjects with positive challenge tests than in those with negative results. The difference was most remarkable in the IgE-binding to pepsin digests; the average concentrations (mean -SD and mean +SD) needed for 50% RAST inhibition in the positive group and in the negative group were 2.6 μg/ml (0.3 and 25) and 94.2 μg/ml (24.7 and 358.7), respectively. A significant difference was still observed in the inhibition tests using filtrates of pepsin digests with a membrane with MW 10,000 (p = 0.014) and 3000 (p = 0.042) of cutoff. The concentration (mean = 0.8, mean -SD = 0.2, mean +SD = 3.4; μg/ml) of pepsin-treated ovomucoid required for 50% RAST inhibition in the subjects with positive second challenge results was significantly (p = 0.033) lower than that (mean = 0.8, mean -SD = 0.6, mean +SD = 73.9) of the negative group. Conclusion: IgE-binding activity to pepsin-digested ovomucoid was of diagnostic value to distinguish the challenge-positive subjects from the negative subjects. Subjects with high IgE-binding activity to pepsin-treated ovomucoid are unlikely to outgrow egg white allergy.
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