Quantitation of specific IgG antibodies in rabbits by a solid-phase radioimmunoassay with 125I-protein A from Staphylococcus aureus.

Radioiodinated protein A (SpA) from Staphylococcus aureus was used to quantitate specific antibodies of the IgG class at the nanogram level in rabbit serum by a standardized radioimmunoassay with the antigen covalently bound to paper discs. Quantitation of IgG in micrograms of IgG per millilitre of serum was done by using a standard curve with purified anti-bovine serum albumin (BSA) IgG or by means of the calculated molar SpA to rabbit IgG ratio of 1:1, thus converting counts per minute of 125I-SpA to nanograms of IgG. The molar ratio for SpA to human IgG was also 1:1. The unspecific binding of IgG to the paper discs was studied and could be depressed by hen serum or by adsorption of the serum sample to cellulose before the assay. The affinity constants of the extracellular SpA and human, rabbit and guinea-pig IgG, respectively, were determined and found to be close to the values previously calculated for cell-wall-bound SpA. The applicability of the assay was demonstrated by quantitation of the specific IgG response on different days after immunization of rabbits with BSA and 3,4-dinitrobenzene (DNP)-BSA.