Immunologic Regulation of E. coli K1 by Serum from Neonatal Rats is Enhanced Following Intraperitoneal Administration of Human IgG

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Abstract
The effect of 1500 mg/kg intraperitoneal human IgG on the capacity of neonatal rat serum to opsonize and to kill Escherichia coli K1 and to deposit IgG and C3 onto this organism was investigated. Unlike serum from neonatal rats injected with albumin, serum from neonatal rats injected with human IgG opsonized and killed E. coli K1 efficiently. Heat treatment abolished the bactericidal effect of serum from IgG recipients, suggesting a role for complement. A radioimmunobinding assay demonstrated that the capacity of neonatal rat serum to deposit IgG and C3 onto E. coli K1 was impaired. However, intraperitoneal human IgG enabled serum from neonatal rats to deposit human IgG onto the bacteria andenhanced C3 deposition to a level equivalent to that observed with adult rat serum. Therefore, neonatal rats have a functionally competent classic pathway of complement. Human IgG ameliorates theopsonic and bacteriolytic inadequacies of neonatal rat serum that result primarily from a deficiency of antibodies to E. coli.