B-cell development in the bone marrow proceeds through an ordered sequence of stages defined by immunoglobulin gene rearrangement and surface marker expression. In the pro-B stage, the cell commits to the B lineage and begins rearranging the heavy-chain locus (V-D-J recombination). Successful heavy-chain rearrangement produces the pre-B stage, where a surrogate light chain pairs with the heavy chain to form the pre-B-cell receptor; signaling through this receptor is the checkpoint that confirms a functional heavy chain and triggers light-chain rearrangement. After light-chain rearrangement, the immature B cell expresses surface IgM. If that IgM recognizes self-antigen in the marrow, the cell undergoes receptor editing or deletion, a central tolerance step. Cells that survive leave as mature naive B cells expressing IgM and IgD, ready to encounter antigen in secondary lymphoid tissue.
A block at any of these steps reduces the number of mature B cells and therefore the antibody repertoire. The clinical severity tracks the position of the block: a defect before heavy-chain rearrangement (for example, in the recombination machinery) causes a near-total absence of B cells and agammaglobulinemia, whereas a block after surface IgM expression may leave B cells present but unable to respond to antigen.