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

Radiolabelled protein A from Staphylococcus aureus (Staph A) has been used to develop a solid phase, non-competitive 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 radiolabelled 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, results of the Staph A assay correlated highly (r=0.981, p<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 to 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.