Lymphocytes. 2. Differentiation. Differentiation of lymphoid precursor cells.

The diversity of immunoglobulin (Ig) molecules and antigen-specific receptors on lymphoid cells, the activation processes during the immune response, the clonal propagation of cells stimulated by antigens, and the sophisticated control mechanisms in the immune system are unparalleled in other biological systems (see A. R. Williamson at page 76, and P. C. L. Beverley at page 59). The emergence of early lymphoid precursors is nevertheless likely to be governed by developmental programmes similar to those observed in haemopoietic precursors and other differentiating cell types. The primary purpose of this paper is to summarise some of the salient observations about the development of the earliest identifiable human lymphoid precursors.