Kinetics of the agglutination of IgG-coated latex particles by C1q: the influence of heat-labile serum components.

Interaction between human C1q and IgG coated latex particles has been studied by means of a standard aggregometer equipment. A dose-dependent agglutination was observed and 100 ng of C1q were readily detected. The kinetics of the agglutination was also monitored. Serum, partially purified C1, and high molecular weight fractions from Sephadex G-200 fractionated serum produced agglutination only in the presence of EDTA. In the absence of this chelator these products disintegrated preformed C1q-IgG-latex particle agglutinates. This disagglutinating principle is heat-sensitive and tentatively macromolecular C1 dependent. The most probable basis of the activity is the competition between C1, with a high affinity for IgG particles, and C1q. The inability of C1 to induce particle agglutination might be caused by the C1 subunits C1r and C1s sterically inhibiting the subunit C1q to bridge between the particles.