An immunofluorescence assay for complement activation by the classical pathway.

The functional integrity of classical complement pathway components was determined by an immunofluorescence (IFL) assay based on the capacity of cytoskeletal intermediate filaments (IMF) to bind C1q and to activate the complement pathway. The assay uses IMF-rich capillary endothelium of human term placentae as complement-activating substrate. IFL staining for bound C1q, C4 and C3 was demonstrated after incubation of the tissue sections with normal human sera in dilutions up to 1 : 80. Known inhibitors of C1q binding and inhibitors fo C3 convertase formation prevented binding of complement components. Eight of 100 sera from patients showed negative or reduced binding of complement whereas all of 100 control sera from healthy individuals were positive in the assay. Four negative patients' sera were studied further: 3 had markedly reduced hemolytic activity and normal levels of C3 and C4. The IFL assay for complement activation provides a simple method of evaluating complement deficiencies and of studying mechanisms and inhibitors of complement activation.