• bone marrow;
  • CD21;
  • flow cytometry;
  • human;
  • transitional B cells


B cells originate from precursors in the bone marrow, and the first cells which migrate to the peripheral blood have been classified as ‘transitional B cells’. Transitional B cells have been characterized in human blood with stage 1 (T1) and stage 2 (T2) subsets being proposed. In the present study, 27 normal human bone marrow samples were analysed for transitional B cell markers by eight-colour flow cytometry. T1 transitional B cells (CD45+CD19+CD10+IgM+IgDlo) and T2 transitional B cells (CD45+CD19+CD10+IgM+IgD+) were identified in normal bone marrow samples at a mean frequency of 3·2 and 3·1% of total B lineage cells, respectively. A majority of the bone marrow transitional B cells were CD24hiCD38hi, the phenotype of blood transitional B cells. Consistent with recent peripheral blood data, T2 B cells had a significantly higher CD21 expression compared with T1 B cells (72·4 versus 40·9%) in the bone marrow. These data raise the possibility that transitional B cells are capable of differentiating from T1 to T2 B cells within the bone marrow. Furthermore, transitional cells at either stages 1 or 2 might be capable of migrating out of the bone marrow.