The relationship between the classical complement pathway and the alternative complement pathway was studied in 30 SLE patients. The activities of both pathways were measured by chemotaxis assay, and the following results were obtained. 1) The classical pathway-derived chemotactic index (CCI) was correlated with the level of CH50, C4 and C3, but not with those of C5. 2) The alternative pathway-derived chemotactic index (ACI) was correlated with the level of CH50, but not with those of C4, C3 and C5. 3) CCI was positively correlated with ACI. Both indices were low but CCI was more depressed than ACI. These findings suggest that the alternative complement pathway is activated mainly by the C3b positive feedback mechanism which is probably triggered via the classical complement pathway, and that the alternative complement pathway probably play a part in defence mechanism instead of reduced classical complement pathway in SLE
