Effect of intravenous immunoglobulin G on the deposition of immunoglobulin G and C3 onto type III group B streptococcus and Escherichia coli K1.

Seventeen neonates with suspected or proven sepsis received either a 750 mg/kg dose of intravenous immunoglobulin G (IVIG) or placebo. Compared with values in adult serum, the preinfusion serum concentrations of immunoglobulin G (IgG) and complement component C3 were diminished; the concentrations were unaffected by the administration of placebo to nine infants. Fifteen minutes after infusion in the eight IVIG recipients, the serum concentration of IgG increased from 3.66 mg/ml to 16.58 mg/ml but the C3 concentration of 540 micrograms/ml was unaffected. Similarly, a radioimmunoassay revealed that during incubation of bacteria with sera from the neonates, the quantities of IgG and C3 bound to type III group B streptococcus and Escherichia coli O7:K1: NM were low and were unaffected by the infusion of placebo. During incubation of bacteria with the postinfusion sera from the IVIG recipients, the amount of IgG, but not C3, deposited onto the bacteria increased to a level equivalent to that observed in adult serum. Therefore IVIG enhanced the capacity of sera from ill neonates to deposit IgG but not C3 onto bacteria. We speculate that in neonates with sepsis, a diminished capacity to deposit C3 onto bacteria may possibly limit the therapeutic efficacy of IVIG.