Immunoglobulin isotype expression of normal pre-B cells as determined by immunofluorescence

We have undertaken an immunofluorescent analysis of the immunoglobulin heavy and light chains expressed by pre-B cells from normal human fetal and adult bone marrow using purified mouse monoclonal and goat antibodies to human immunoglobulin isotypes. Our results indicate that (i) the great majority of normal pre-B cells in both fetuses and adults expresses intracytoplasmic mu chains only, (ii) immunoglobulin light-chain synthesis may be initiated during a late stage in pre-B-cell development, and (iii) heavy-chain isotype switching at the pre-B-cell stage may not occur during normal B-cell development.