Functionally distinct stages in B-lymphocyte maturation.

Evidence is reviewed that supports the concept of extrinsic stem cell colonization of inductive micro-environments for B-cell formation in the bursa of Fabricius of chickens and foetal liver of mammals. In these organs, the first distinctive marker of B-lineage differentiation (readily detectable immunoglobulin) is synthesized and displayed on cells which have extensive proliferative and differentiative capacity. The degree of commitment of these cells to the synthesis of trace or large quantities of immunoglobulin of the respective major classes when they colonize peripheral lymphoid tissues and the role of antigen in this process are considered.