39] C3c-binding ELISA for the detection of immunoconglutinins and immunoglobulin aggregates

Abstract

This chapter discusses the C3c-Binding enzyme-linked immunosorbent assay (ELISA) for the detection of immunoconglutinins and immunoglobulin aggregates. The third component (C3) of the human complement system is composed of two polypeptide chains, α and ß, which are linked by disulfide bonds and noncovalent forces. Fractions containing C3c and C3d were identified, with rabbit antisera, against C3c and C3d, by immunodiffusion. The C3c pool contained a small amount of C3b, because a faint precipitation line was obtained by immunodiffusion against anti-C3d antiserum. The probable explanation for the observed binding of heat-denatured IgG and myeloma IgA to solid-phase C3c in the same way as to the solid-phase C3b is that the interaction between C3b and immunoglobulins depends on sites present on both C3c and C3d portions of the C3b molecule. Activated C3b probably binds to immunoglobulins through the labile binding site located on the C3d portion of the molecule. The results suggest that the binding of heat-denatured IgG to solid-phase C3c may be mediated through the Fab portion of the IgG molecules, but do not necessarily imply that heat-denatured Fab and IgG have the same C3c binding sites.