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.