IgE-binding activity to enzyme-digested ovomucoid distinguishes between patients with contact urticaria to egg with and without overt symptoms on ingestion

Background: We occasionally see egg-allergic children who develop contact urticaria to hen's egg despite the absence of the overt symptoms on ingestion. The mechanisms remain to be elucidated. Methods: Twenty-one subjects with positive reactions to 20-min patch tests for egg-white antigens were divided into subgroups with positive (n = 10) and negative (n = 11) results to oral challenge tests by the same antigens. We measured IgE antibody for egg white and its components, and IgE-binding activities to digestive enzyme-treated ovomucoid by RAST inhibition. Results: There were no significant differences in IgE antibody titers to egg white (positive vs negative: 30.3% vs 15.3%, P = 0.130), ovomucoid (21.5% vs 10.2%, P = 0.078), ovotransferrin (9.9% vs 3.7%, P = 0.105), and lysozyme (3.4% vs 2.9%, P = 0.944), except ovalbumin (16.8% vs 5.6%, P = 0.024), between the positive and negative subjects in the provocation tests. In contrast, the concentration (1.93 μg/ml) of pepsin-treated ovomucoid needed for 50% RAST inhibition in the challenge-positive subjects was significantly (P = 0.0003) lower than that (114.9 μg/ml) of negative subjects. Similar but less significant differences were obtained when ovomucoid fragments treated with chymotrypsin (0.91 μg/ml vs 6.86 μg/ml, P = 0.014) and trypsin (0.75 μg/ml vs 4.67 μg/ml, P = 0.041) were used as inhibitors. Conclusions: We suggest that IgE antibodies from subjects showing contact urticaria despite the absence of reactions to the ingestion of egg white recognize the epitope(s) unstable to digestive enzymes.