The assembly of early components of complement on antibody-antigen aggregates and on antibody-coated erythrocytes

Abstract

Radioimmune assays were developed to assay the binding of [human and guinea pig] C1q [q fragment of complement component 1]. C1s and C4 to antibody [Ab] aggregates and to cell-bound Ab. The binding of the components was compared with the hemolytic activity and with the capacity to form the C3 convertase activity in the presence of excess C2. The destruction of whole complement and of C4 activity is similar per 1000 molecules of Ab in aggregates and cell-bound Ab, as is the binding of C1q and C1s, the latter being in a 1:2 molar ratio. The binding of C4 is about 12 times greater, per 1000 molecules of Ab, on cells than in aggregates. The effective C4 molecules, as judged by the formation of C3 convertase activity, are much more similar on cells and aggregates. An assembly mechanism of the early components of complement on [rabbit] Ab-coated [rat or sheep erythrocytes] was suggested.