IgG of paternal allotype first becomes detectable in the serum of (BALB/c x C57BL/6)F1 mice between day 12 and 14 after birth and reaches adult levels at an age of 5 wk. Since in mice there is a transfer of maternal IgG molecules through the placenta and via milk, F1 heterozygous at the allotype locus were used and the concentrations of IgG with paternal allotype were measured. This was done by a sensitive method capable of detecting IgG concentrations as low as 5 x 10–4 of normal adult serum levels. It is based on the quantitative inhibition of allotype-specific facilitation of hemolysis.

When lipid A or Salmonella bacteria were injected into neonatal mice, a stimulation of IgG synthesis was observed. Thus IgG levels were enhanced 10-30-fold compared to the nontreated mice. No increase in IgG levels was obtained in adult mice after treatment with lipid A. Whether the newborns were injected at birth, on day 2, 4, or 7, IgG was first demonstrable in the treated mice at an age of 6–11 days.

The increase in IgG levels was not paralleled by a demonstrable antibody activity against lipid A, SRBC, and LPS. Thus the bulk of newly induced IgG is probably a statistical distribution of different specificities.

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