Schematic of B-cell development in relation to IL-7Rα expression and IL-7 responsiveness.

B-cell development proceeds from a common lymphoid progenitor (not shown) that is characterized by the expression of IL-7Rα and c-kit but that lacks lineage-specific markers (eg, B220). The first identifiable progenitor committed to the B lineage is the pre–pro-B cell expressing B220 and low levels of heat stable antigen. Transition to the pro–B-cell stage involves a period of proliferation, probably in response to factors other than monomeric IL-7, and the beginning of immunoglobulin heavy chain rearrangement. Heavy chain rearrangement is completed at the early pre–B-cell stage. This stage also involves the expansion of successfully rearranged cells in response to IL-7 and other factors. By the late pre–B-cell stage, IL-7Rα expression ceases. Fractions listed correspond to those described by Hardy et al58 in the mouse.