Immunoglobulin receptors on murine B lymphocytes Origins of Lymphocyte Diversity

Two new stages in B-cell differentiation have been described, that is, the authors have placed the cell bearing IgM and IgD and the cell bearing only IgD as successive stages between an earlier cell bearing only IgM and a later one bearing IgG (and possibly also IgD). It should be emphasized, however, that formal proof that these are successive stages of a single B-cell lineage is lacking. Second, the biologic reactivity of these different cell types differs in several important respects, e.g., neonatal B cells are exquisitely sensitive to induction of tolerance as compared to adult B cells, and LPS stimulates a different response in IgM⁺ versus IgM^- B cells. Whether these different biologic functions are caused by the different biological reactivities of the respective isotypes or by other cell-surface or metabolic differences is not known. However, these possibilities should be testable in in vitro models, most particularly since the recent availability of anti-mouse δ. Hopefully, such approaches will eventually answer the questions of the function of IgD, the roles played by IgM and IgD on the same cell, and the predominance of these two isotypes in B-cell populations.