An enzyme-linked immunoassay for the measurement of circulating immune complexes

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 beta-D-galactosidase-conjugated monovalent fragment (Fab') of rabbit (anti-human IgG, gamma chain-specific) IgG. The enzyme activity bound to the tubes was determined fluorometrically using 4-methylumbelliferyl-beta-D-galactopyranoside as substrate. The sensitivity of this assay was as little as 100 ng/tube, which corresponded to 6 micrograms/ml when 16.7 microliter 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%).