C1 subcomponent complexes: basic and clinical aspects.

C1 subcomponents form a variety of complexes that can be detected in normal and pathological sera. Since aberrations of C1 subcomponents in disease could reflect in vivo interactions with influence on complement function, studies of C1 subcomponent complexes might provide insight into pathogenetic mechanisms. C1 inhibitor (C1Inh)-dependent dissociation of the C1q(C1r-C1s)2 complex gives rise to C1Inh-C1r-C1s or C1Inh-C1r-C1s-C1Inh complexes. Increased concentrations of C1Inh-C1r-C1s probably signify prevention of C1 activation, while C1Inh-C1r-C1s-C1Inh appears to be a clinically useful marker of efficient classical pathway activation. "Free" C1q as found in some pathological sera, and in joint fluids of patients with rheumatoid arthritis could be a result of C1Inh-dependent dissociation of C1q(C1r-C1s)2. The presence in serum of zymogen (C1r-C1s)2 is an expected finding in various conditions with low C1q concentrations without evidence of C1 activation. It is not excluded that circulating (C1r-C1s)2 might sometimes be acquired due to factors capable of interacting with the collagenous part of the C1q molecule.