A new enzyme-linked immunosorbent assay (ELISA) for measuring immunoconglutinins directed against the third component of human complement. Findings in systemic lupus erythematosus.

A new enzyme-linked immunosorbent assay, using purified activation product of the third component of human complement (C3b), detects immunoglobulins of the IgG isotype which demonstrate affinity for C3b (C3b-immunoconglutinins; C3b-IK). This assay offers significantly improved specificity compared to previous immunoconglutinin (IK) assays in that it not only defines the isotype and antigenic specificity of the IK but also eliminates false positive results associated with immune complexes or aggregated human gamma globulin, or with natural antibodies directed at heterologous reagents. Using this assay, we observed elevated C3b-IK levels in serum of 34 systemic lupus erythematosus (SLE) patients when compared to serum of 13 healthy controls. Comparing sera from patients with clinically active and clinically inactive lupus showed greater immunoconglutinin levels in the active group. Immunoconglutinin levels did not correlate with the erythrocyte sedimentation rate, total hemolytic complement, or with circulating immune complex levels by the Raji cell and C1q-binding assays.