Enzyme Immunoassay for IgG Rheumatoid Factor Combining with Homologous IgG

An enzyme immunoassay (EIA) was developed to detect, in human sera, IgG rheumatoid factor (RF) combining with human IgG. Wells of the EIA plate were coated with the Fc fragment of human IgG. Binding of RF was detected by goat antibodies to F(ab')₂ of human IgG followed by rabbit anti-goat IgG conjugated with alkaline phosphatase and finally by the substrate. Using this procedure, RF was detected in 35%–85% of various pathological human sera, but only in 7% of normal human sera. An analogous procedure was also described for detection, in rabbit sera, of an RF combining with rabbit IgG.