AbstractTo examine the physiological role of maternal natural IgG antibodies on the development of B lineage cells of the progeny, we have bred homozygous μMT/μMT or heterozygous μMT/+ females to μMT/μMT or μMT/+ males, respectively. We could thus compare normal or B cell‐deficient mice born from Ig‐deprived (Ig−) or phenotypically normal mothers (Ig+). B cell‐deficient progeny of heterozygous mothers contain no detectable serum IgA or IgM, but IgG concentrations that peak at 2 mg/ml by 7–21 days of age, decay after weaning with a half‐life of 7 days, and remain detectable for 2 months after birth. At 7 days after birth, μMT/+ progeny born of Ig+mothers contain two‐ to threefold higher numbers of bone marrow (BM) pre‐B and B cells, and of splenic B cells, compared to mice of the same age born from Ig−mothers. In contrast, the former progeny exhibit two to four times lower numbers of Ig‐secreting plasma cells in spleen and thymus, and contain sixfold lower serum IgM concentrations. A similar maternal IgG‐dependent stimulation of BM B cell precursors is also observed in μMT/μMT progeny. No significant differences were detected between the groups on day 3 after birth, suggesting the requirement for a minimal
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