Antibody responses in watermelon sensitivity

Watermelon-specific IgE (WM-IgE) appears to mediate the oropharyngeal symptoms after watermelon ingestion. However, less than one fourth of watermelon-sensitive individuals (i.e., those with detectable WM-IgE) develop such symptoms after watermelon ingestion. In an attempt to separate symptomatic from asymptomatic persons, we measured watermelon-specific-IgE and IgG₄ concentrations in the sera of 29 watermelon-sensitive patients, 6 of whom were symptomatic. Although the mean WM-IgE level of the symptomatic group exceeded that of the asymptomatic group (p = 0.04), individual WM-IgE values failed to distinguish symptomatic from asymptomatic persons. No differences in WM-IgG₄ levels between the two groups could be found. Watermelon extract proteins were separated by isoelectric focusing and blotted to nitrocellulose membranes. The membranes were reacted with the individual sera of 14 watermelon-sensitive patients and then stained for IgE, IgG₁, and IgG₄ binding. Great variations in IgE, IgG₁, and IgG₄ binding patterns were found, but no subtype restrictions were discovered. Although the pattern of IgE-stained bands in the absence of corresponding IgG₁-stained bands was seen more frequently in symptomatic individuals, immunoblot patterns failed to separate symptomatic from asymptomatic persons. We conclude that the pattern of IgE, IgG₁, and IgG₄ responses to separated watermelon allergens was not predictive of clinical symptoms in a group of watermelon-sensitive patients.