Abstract
A solid-phase, enzyme-linked immunoassay for the quantitation of circulating immune complexes was developed using C1q. In this assay, the immune complexes in human serum are adhered to C1q-coated polystyrene tubes and then detected by a β-D-galactosidase-conjugated monovalent fragment (Fab′) of rabbit (anti-human IgG, γ chain-specific) IgG. The enzyme activity bound to the tubes was determined fluorometrically using 4-methylumbelliferyl-β-D-galactopyranoside as substrate. The sensitivity of this assay was as little as 100 ng/tube, which corresponded to 6 μg/ml when 16·7 μl of serum was assayed. The specificity of the assay was demonstrated by the following observations: (1) absence of cross-reaction of monomeric IgG; (2) non-detectability of immune complex in most of the sera from normal subjects; (3) parallelism of the standard curve with dilution of reference serum. The precision of the assay was proved by the demonstration of sufficient within-assay coefficients of variation (10·7–14·9%).