IgG on Infants' B Lymphocytes: Enhanced Binding of IgG by IgM‐Bearing Lymphoid Cells in Early Childhood

IgM/IgD-bearing lymphocytes (B cells) from children in the first few weeks of life were found to also have surface IgG, unlike most normal adult B cells. The IgG was loosely bound to the lymphocyte surface and was partially or completely removed by incubation at 37 degrees C or by trypsinization. When F(ab')2 antisera were employed, very few infant B cells had surface IgG, although the IgM staining was similar to that obtained with the native antisera. IgM/IgD-positive cells bound IgG anti-Rh-coated (Ripley) erythrocytes, unlike most adult B lymphocytes. Capping experiments suggested that an Fc receptor on the cells could be redistributed by the anti-IgM-surface IgM complex. These data indicate that, during infancy, B-lymphocyte receptors for IgG are of altered affinity, frequency, or availability compared with adult B-lymphocyte Fc receptors and resemble the Fc receptors found on "third population" (Fc + Ig-) mononuclear cells and monocytes.