A simplified method for measuring complement-mediated inhibition of immune precipitation (IIP) using peroxidase (PO) as an antigen was studied. IIP was measured in 48 patients with systemic lupus erythematosus (SLE). Immune precipitation of PO and anti-PO rabbit IgG at equivalence was inhibited principally via the classical complement pathway in the serum. When the substrates H(2)O(2) and 5-aminosalicylic acid were added to the supernatants (soluble PO immune complex and free PO) of antigen, antibody and fresh or heat-inactivated serum mixture, their absorbance (O.D.) at 450nm increased in a dose dependent manner. The IIp of the complement in the serum was shown by the difference in absorbance between fresh serum and heat-inactivated serum. The IIP in SLE was lower than that in normal subjects and patients with other collagen diseases. There were correlations between IIP and CH50 (r=0.47, p<0.01), C3 (r=0.67, p<0.01) and complement-mediated solubilization of immune complex (r=0.45,p<0.01). The IIP in SLE was reduced only in the earlier active state and normalized very rapidly.The above results suggest that the formation of an immune complex lesion in SLE is not due to the acquired depression of complement-mediated inhibition of immune precipitation as seen in the complement deficiency state.
classical complement pathway