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

Abstract

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.