The use of protein A‐containing Staphylococcus aureus as a solid phase anti‐IgG reagent in radioimmunoassays as exemplified in the quantitation of α‐fetoprotein in normal human adult serum

Abstract A new technique for the separation of antigen‐antibody complexes from free antigen in radioimmunoassays has been developed. The technique is based on the interaction between the Fc portion of IgG and staphylococcal protein A. One ml of a 10 % suspension of formaldehyde‐treated and heat‐killed staphylococci of strain Cowan I, particularly rich in protein A, at equivalence binds 1.2–1.5 mg IgG in a pure IgG solution and 0.8–1.0 mg of IgG present in serum. This staphylococcal solid phase anti‐IgG reagent is added in excess of the IgG content in the preincubated radioimmunoassay reaction mixture. Centrifugation can be started within minutes. The bacteria form a solid pellet, so the supernatant can be decanted without further washing being required. Nonspecific uptake of radioactivity is below 7 %. In our assay for human α‐fetoprotein using antiserum raised in the rabbit, completeness of separation is better than 98 %. The present technique will differentiate between the presence of antigen and homologous IgG antibody in a sample tested.