Several factors in human serum are capable of precipitating soluble 125I-labelled heat aggregated IgG (agg IgG*). A study of the nature of these factors resulted in the development of two new methods for the detection and assay of anti-IgG-immunoglobulins (rheumatoid factors) in serum. One method detected rheumatoid factors of the IgG and the IgA classes which are capable of binding and coprecipitating with Clq and agg IgG* in an EDTA milieu. In a second method the serum was first heat inactivated and the assay was then made in a polymeric milieu where rheumatoid factors of the IgM as well as the IgG and IgA classes could be detected. 67 sera from patients with rheumatoid arthritis were tested with this method and rheumatoid factors were detected in all seropositive (as assayed with conventional rheumatoid factor tests) sera and in 58% of the seronegative sera. In the presence of certain anti-IgG-immunoglobulins or polymers, the precipitation of Clq and soluble agg IgG* is greatly enhanced, and we suggest that this can be used as a basis of a sensitive method for the assay of agg-IgG-binding activity of Clq.
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