An enzymatically active antigen-antibody probe to measure circulating immune complexes. II. E. coli beta-galactosidase in the probe and C1q as the recognition unit.

An enzymatically active probe (beta-galactosidase-anti-beta-galactosidase complex) is used to measure circulating immune complexes (CIC), in a competition assay where probe and CIC are confronted with a 'recognition unit'. The latter is bovine conglutinin in the original description of this method. Here we describe a version utilizing human or bovine C1q. The two techniques are compared for their sensitivity and specificity, on both in vitro formed tetanus toxoid-anti-toxoid complexes and on sera from patients with selected diseases. The results confirm that the two recognition units are sensitive to families of CIC that only partially overlap. The parallel use of conglutinin and C1q yields both quantitative and qualitative information on the nature of CIC in individual sera.