Application of human erythrocytes to a radioimmune assay of immune complexes in serum.

An immune adherence receptor exists on the surface of primate erythrocytes, and has been characterized as a receptor for the activated third component of complement (C3b). We have applied human red blood cells (RBCs, blood group O) to a sensitive determination of complement-fixing, soluble immune complexes in serum. The method involved the binding of immune complexes with RBCs in the presence of complement and the detection of cell-bound IgG molecules by radiolabelled anti-human IgG antibodies. Since the binding of RBCs with monomeric IgG was minimal, cell-bound IgG molecules were taken as representing immune complexes. When aggregated human gammaglobulin (AHG) was used as a model of immune complexes, as little as 5 μg dissolved in 1 ml of normal human serum were detected. The binding of RBCs with AHG was inhibited in EDTA solution where the classical complement pathway could not be activated. The RBC radioimmune assay was successfully applied to the determination of soluble immune complexes in pathological serum samples obtained from the patients with systemic lupus erythematosus and those with fulminant Type B hepatitis. False-positive results by autoantibodies against RBCs could be excluded by performing a Coombs test and by comparing the binding in the presence of complement with that in EDTA solution. The ubiquitous availability of RBCs coupled with a high sensitivity would allow the RBC radioimmune assay to be added to the battery of previous methods to determine immune complexes in the serum.