Solid phase radioimmunoassay for quantitation of antigen-specific IgG in human sera with 125I-protein A from Staphylococcus aureus.

Radiolabeled protein A from Staphylococcus aureus (Staph A) has been used to develop a solid phase, noncompetitive radioimmunoassay for quantitation of specific IgG antibody. The assay involves two incubations: First, agarose-insolubilized antigen is mixed with serum samples for 1 to 4 hr during which specific antibody is bound; second, after a washing procedure, the solid phase immune complexes are incubated for 4 to 18 hr with 125I-Staph A, during which the radiolabeled detection protein binds to the insolubilized specific IgG antibody. In a comparative study of the IgG antiphospholipase A antibody content of 23 human sera drawn from honeybee venom-sensitive patients, resulted of the Staph A assay correlated highly (r = 0.981, p less than 0.001, N = 23) with those obtained from a liquid phase, competitive radioimmunoprecipitation (double antibody) assay. The two assays demonstrated comparable precision, sensitivity, and reproducibility. In contrast, the use of 125I-Staph A in the solid phase radioimmunoassay was superior to 125I rabbit anti-human IgG because of lower negative serum (blank) values, shorter time required to reach equilibrium binding, and greater precision and reproducibility. In principle, the 125I Staph A assay may be applied ot IgG quantitation for crude allergen extracts as well as purified antigens. Furthermore, the sera of a number of mammalian species may be studied without further modification.