Serum levels of C1 subunits in rheumatoid arthritis

Modifications of radial immunodiffusion and of hemolytic assays of C1q are described, which enable the results of these assays to be in agreement with those obtained by hydroxyproline assay. Using these assays, we show that C1q serum levels are significantly increased in rheumatoid arthritis (RA) and that the excess C1q levels in this disease are not accompanied by increased levels of C1r and C1s. Active RA is therefore characterized by increased levels of hemolytically active C1q that has a physiologically active stem region unbound to C1r and C1s.