A simple immunoradiometric assay for the terminal SC5b-9 complex of human complement.

Complement activation in serum via classical or alternative pathway results in the formation of a water-soluble, macromolecular SC5b-9 protein complex consisting of non-covalently associated C5b-C9 complement components and serum S-protein. Characteristic neoantigens are expressed on this complex that permit its immunological differentiation from native complement proteins. The present paper describes a procedure for isolating radiolabelled, specific antibodies to C5b-9 neoantigens. With the use of these antibodies, a simple and rapid immunoradiometric assay for SC5b-9 has been developed that allows for detection of 3-4 micrograms SC5b-9 in 1 ml human serum or plasma. The method is suitable for routine screening analyses for SC5b-9 in patients' plasma, and for detection of SC5b-9 experimentally generated in vitro.