The levels of immune complex (IC) were studied by using two different methods in order to analyze the activation process of C 1 in IC diseases. In vitro, heat aggregated IgG (HAG) in various concentrations was added to normal sera, and was incuvated at 37°C for 48 hours. IC levels were determined both by the C 1 q solid phase assay (ELISA) and by the anti-C 1 q antibody assay (ELISA). Meanwhile, changes in the concentration of (C 1 r-C 1 s) C 1 inhibitor2 complex were also determined (ELISA) as the indications of the levels of activated C 1. The levels of IC obtained by the C 1 q solid phase assay decreased in the temporal course after adding HAG. On the other hand, a gradual increase in the temporal course after adding HAG was observed in the levels of IC determined by the anti-C 1 q antibody assay, and in those of (C 1 r-C 1 s) C 1 inhibitor2 complex. The larger the amounts of added HAG, the greater the increase in the levels of IC and (C 1 r-C 1 s) C 1 inhibitor2 complex. The activation of C 1 occurred after the connection of C 1 with HAG, which was demonstrated by the clear differences in the aspect of C 1 activity between the levels of IC measured by the C 1 q solid phase assay and by the anti-C 1 q antibody assay. These results suggest that the participation of C 1 in the development and progress of IC diseases can be examined more clearly by both assays. One ITP-patient was treated with steroid whose levels of IC determined by the anti-C 1 q antibody assay were continuously high and by the C 1 q solid phase assay were normal. During and after the treatment, his levels of IC, (C 1 r-C 1 s) C 1 inhibitor2 complex, and platelet-associated IgG were lowered, and the platelet counts and CH 50 levels were gradually improved. The results obtained by the in vitro study with HAG explain the discrepancies between the levels of IC determined by the C 1 q solid phase assay and by the anti-C 1 q antibody assay.
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