Human immunoglobulin E and immunoglobulin G antibody responses to the "minor" ragweed allergen Ra3: correlation with skin tests and comparison with other allergens.

We report here the use of antigen-binding assay to measure serum IgE and IgG binding activity (BA) for the "minor" ragweed antigen, Ra3. These studies were carried out in ragweed-allergic individuals, many of whom had been skin tested with Ra3 as well as with the "major" ragweed allergen, antigen E (AgE). IgE BA for Ra3 showed a weak quantitative correlation with skin-test sensitivity to Ra3. More strikingly, there was a threshold of skin sensitivity (< 10(-3) microgram/ml) above which serum IgE BA was likely to be detectable. Serum IgG BA for Ra3 measured in parallel with IgE BA was found in all sera that showed detectable IgE BA for Ra3. By contrast, 11/13 ragweed-allergic individuals who were skin-test negative for Ra3 showed no detectable IgG BA for Ra3. These results support the view that the known genetic controls over Ra3 sensitivity control IgG antibody as well as IgE antibody responses. Individuals receiving injections of ragweed extract showed increases in IgG BA for Ra3 following therapy. Among nine persons who had no IgG BA for Ra3 before treatment, four out of nine developed a low degree of IgG BA for Ra3. However, the patients who developed IgG BA did not develop detectable IgE BA for Ra3.