The ontogeny and cloning of B cells in the bursa of Fabricius.

A pluripotent or multipotent stem cell must undergo two major changes on the way to becoming an immunoglobulin (Ig) secreting B lymphocyte. Thefirst is the commitment to the lineage of a B cell rather than a T lymphocyte or other cell type. The second is a commitment to the production of antibody with a particular specificity. The latter involves selection of germ-line Ig variable region genes and the somatic rearrangements required for their expression (reviewed in detail elsewhere(1-4)). Since the selection of variable region genes for both Ig heavy and light chains (VH and VL) precedes the expression of Ig on the surface of the B cell(3), a sIgM(+) cell is committed both to the B-cell lineage and to the use of a particular combination of germ-line VH and VL genes. The chicken is one of the most convenient species with which to study lymphoid ontogeny. In this review Michael Ratcliffe examines recent evidence about the commitment of avian lymphoid cells as they progress from immature precursors to mature antibody producing cells.