Previous studies that showed that complement (C) can convert insoluble Ag-Ab precipitates into soluble immune complexes suggested that activated C components might be able to dissociate Ab from Ag. The present study directly examined this possibility by measuring the dissociation of radiolabeled Ab from immobilized Ag in the presence or absence of active complement. Small polystyrene test tubes were coated with BSA (Ag), and the coating was cross-linked with glutaraldehyde to ensure its stability. Then, 125I-labeled 7S fraction of mouse anti-BSA was allowed to bind to the BSA-coated tubes; controls showed that the binding was specific. The release of labeled Ab from the tubes was measured in the presence of fresh serum or decomplemented serum at 37°C. Active complement markedly increased both the rate of release and the final amount of Ab released. Once begun, the release of Ab could continue for some time in the absence of fluid-phase complement. Controls included duplicate sets of tubes coated with radiolabeled BSA in which unlabeled Ab was bound. These controls showed that, although minute amounts of BSA were unavoidably lost from the tubes, this loss could not account for the observed release of Ab. The ability of the released Ab to rebind to fresh BSA-coated tubes was also examined; it was found that C-released Ab could rebind nearly as efficiently as spontaneously released Ab. The results are completely compatible with the “intercalation” hypothesis developed to explain the C-mediated solubilization of immune aggregates.

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This work was supported by United States Public Health Service Grants AI-11080 and AI-09268.

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