Expression of Three Immunoglobulin Isotypes by Individual B Cells During Development: Implications for Heavy Chain Switching*

ABSTRACT: Previous studies in mice and humans have shown that the first IgA⁺ and IgG⁺ B cells appearing during ontogeny also bear IgM on the surface. The acquisition of IgD seems to occur at a later stage in differentiation. In this study we have combined autoradiography with two-color immunofluorescence to directly detect human B cells expressing three surface isotypes. We report that the vast majority of IgA⁺ cells in the neonate also bear IgM and IgD; the same holds true for IgG⁺ cells. This phenotype is peculiar of newborns, while in the adult the majority of IgA⁺ and IgG⁺ cells are single. We discuss the genetic implications of such a finding for heavy chain switching.