Interference by rheumatoid factor and C1q in the detection of IgG complexes: studies of model systems by ELISA.

Interference by purified IgM rheumatoid factors (RFs) and C1q in the detection of model complexes was studied by enzyme-linked immunosorbent assay (ELISA). Soluble monoclonal and polyclonal cryoglobulin IgM RFs and human or porcine C1q inhibited dose-dependently the binding of human IgG complexes to solid-phase IgM RF, C1q, and bovine conglutinin (Kg). The inhibition patterns of the Kg binding were dependent on the order of confrontation between the reactants. To achieve inhibition, C1q or RF had to be offered to the complexes before treatment with fresh normal human serum (NHS); complexes pretreated with fresh NHS (alexinated complexes) were resistant to inhibition. Heating at 56 degrees C or 63 degrees C, which destroyed the alexinating capacity of NHS, did not affect the Kg binding of alexinated complexes. On the basis of present and previous findings it is concluded that intrinsic C1q and RFs are likely to interfere with detection of circulating immune complexes (CICs) in C1q- or RF-binding assays; they are less likely to interfere with detection of CICs in Kg-binding assays. The problems associated with selective removal of intrinsic C1q and RFs are discussed.