Measurement of IgE antibody by an antigen-binding assay: Correlation with PK activity and IgG and IgA antibodies to allergens

A quantitative radioimmunoassay for IgE antibodies (ab) is described. The assay measures IgE antigen-binding activity (BA) by using purified allergens radiolabeled with ¹²⁵I, IgE myeloma serum as a source of carrier protein, and monospecific goat anti-IgE (anti-IgE) to precipitate the IgE. IgE BA for the Group I protein of rye grass pollen (Rye I) was measured in the serum of patients with grass pollen hay fever who had never received immunotherapy. Values for IgE BA were found to correlate well with radioallergosorbent technique (RAST) results (r=0.89, p<0.001). By assaying IgE-rich fractions, pure IgG, and whole serum, we found a good correlation between IgE BA, Prausnitz-Kustner (PK) activity, and total IgE. A positive PK test could be elicited with 0.05 ml of dilutions containing as little as 1 or 2 units IgE BA for Rye I/ml. PK testing was more sensitive than either RAST or the antigen-binding assay. Estimates based on the quantity of antigen bound in antigen excess suggested that 1 unit IgE BA represented 0.1 ng IgE ab. With higher concentrations of antigen (up to 1 μg/ml), however, estimates suggested that 1 unit IgE BA represented as much as 0.4 ng IgE ab. These higher estimates appeared to correlate reasonably well with estimates based on RAST elution or RAST absorption. The assay for IgE BA was directly comparable with assays for IgG BA and IgA BA. Results of these assays on sera from the patients showed that IgG BA for Rye I was detectable in 25 of 25 IgE BA in 24 of 25, and IgA BA in 21 of 25. Quantitatively we found a characteristic pattern IgG BA > IgE BA > IgA BA (geometric mean values 862, 214, and 60.2 units/ml, respectively). In keeping with this there was a highly significant quantitative correlation between IgG BA and IgE BA (r = 0.72, p<0.001). None of the sera from nonallergic persons contained detectable IgA or IgE BA for Rye I; however, 20% of these sera (10 of 48) contained IgG BA.