35] The [125I]C1q binding assay for the detection of soluble immune complexes

Abstract

This chapter describes the [125I]Clq binding assay for the detection of soluble immune complexes. The radiolabeled Clq binding assay allows for a rapid quantitation of soluble immune complexes in native serum or other biological fluid. The principle of the test is to measure the amount of [125I]Clq bound to the macromolecular complexes in a given sample. The Clq binding assay has been applied to study the occurrence and clinical relevance of immune complexes in a variety of diseases, including immunological disorders, some infectious processes, and malignancies. Samples, with a Clq BA, increased by more than 2 or 3 SD values when compared to normal sera, are considered to contain detectable immune complexes. The results of the Clq binding assay might also be expressed in equivalents of AHG. Complexes formed of at least three IgG molecules are able to give a positive Clq binding test. Some immune complexes, such as those involving only IgA antibodies, would not be detected by a Clq binding assay. It is found that the Clq precipitation, by single-stranded deoxyribonucleic acid (DNA) at an optimal concentration of 60 μg/ml of serum does not exceed 10% in the test system, and addition of bacterial lipopolysaccharides to normal human serum does not lead to an increase of the serum Clq BA.