Characterization of splenic CD21hi T2 B cells.

B cell development is a highly regulated process that initiates in the bone marrow (BM) of adult mice. After reaching the IgM+ immature stage in the BM, these B cells migrate to the spleen to complete maturation and incorporation into the long-lived peripheral lymphocyte pool. Studies have identified these splenic immature B cells, and have further attempted to delineate the sequence whereby they transition into mature B cells. As such, these immature splenic populations are termed transitional B cells and have been the focus of intense study. The review summarizes the phenotype and currently known functions of the four putative transitional B cell subsets identified to date. Although most appear to represent short-lived transitional B cells, the CD21hi T2 B cell population exhibits a number of qualities that question its label as a transitional B cell subset.