Evaluation of the Quantity and Affinity of Human IgG “Blocking” Antibodies

Open Access
Publisher Website
Google Scholar
Abstract
We have analyzed the quantity and avidity of IgG antibodies in human sera specific for allergenic proteins of clinical relevance. The methods used were saturation analysis, which provides quantitative estimates of antibody content and Scatchard and Sipps analyses of antigen-binding experiments, which provide estimates of antibody affinity and homogeneity and, by mathematical extrapolation, antibody quantity as well. We studied human IgG “blocking” antibody specific for three unrelated allergens: honeybee phospholipase A (PLA), ragweed antigen Ra5 (Ra5), and the benzylpenicilloyl (BPO) hapten. Antibody content determined by saturation analysis was highly correlated (r = 0.99; p < 0.001) with estimates obtained by mathematical extrapolation of Scatchard and Sipps plots. The average equilibrium constant (K0) for IgG anti-PLA was 3 × 1010 M-1; for IgG anti-Ra5, the average K0 was 0.34 × 1010 M-1; and the K0 of a single serum with IgG anti-BPO had a high binding constant (12 ± 1.3 × 1010 M-1). The heterogeneity coefficients obtained from the Sipps analysis were consistently high for all allergens studied, suggesting remarkable homogeneity of antibody. Since antibody affinities varied over a relatively narrow range, it was possible to obtain estimates of antibody content by interpolation from the dilution curve of a standardized serum that agreed very well (r = 0.99; p < 0.001) with values obtained by the more laborious saturation analysis technique. It appears that saturation analysis and the Scatchard and Sipps transformations can be usefully applied to the study of human IgG “blocking” antibodies against large and small m.w. allergens, as well as haptens. The average affinity constant of human IgG blocking antibody is high when compared with hyperimmune animal antisera. The affinity constant for each allergen system varied over less than a 10-fold range in the limited number of sera studied, suggesting that variability in K0 is not a major contributor to the allergen-binding capacity of most sera.