Fetal B-cell lymphopoiesis and the emergence of B-1-cell potential

Two populations of B cells, B-1 cells and B-2 cells, have been described in mice. B-2 cells are produced in the bone marrow during postnatal life. Most B cells in peripheral lymphoid tissues are B-2 cells and they are generally involved in adaptive immune responses. B-1 cells are a minor population of B cells; they constitute a high proportion of B cells in serous cavities and are generally effectors of innate immune responses.B-cell development initiates during embryogenesis. Cells with B-cell potential are present in multiple intra-embryonic and extra-embryonic tissues. However, B-cell potential seems to be initially associated with the intra-embryonic para-aortic splanchnopleura.There is strong circumstantial evidence that the potential of haematopoietic precursors to produce B-1 cells arises before B-2-cell potential.There has been considerable debate regarding the origin of B-1 cells. The selection model proposes that they are generated from surface-IgM-expressing B-2 cells in response to specific types of antigen. The lineage model proposes that B-1 and B-2 cells are distinct lineages with separate progenitors. The recent description of a population of CD19+CD45−/low cells that can only produce B-1 cells provides support for the lineage model.Progenitors with B-1-cell potential are preferentially produced in the embryo and B-2-cell progenitors arise late during embryogenesis and are preferentially produced in the adult. It is not clear why this is the case, although differences in the fetal and adult environment may be a factor.Much needs to be learned about human B-cell development. There is circumstantial evidence that B-1 cells are present in humans, but further investigation of this issue is needed.