||P.304 2nd paragraph: "7-27 Most immature B cells arriving in the spleen are short-lived and require cytokines and positive signals through the B-cell receptor for maturation and survival. When B cells emerge from bone marrow into the periphery, they are still functionally immature, expressing high levels of sIgM but little sIgD. Most of these immature cells will not survive to become fully mature B cells bearing low levels of sIgM and high levels of sIgD. Fig. 7.39 shows the possible fates of newly produced B cells that enter the periphery. The daily output of new B cells from the bone marrow is roughly 5-10% of the total B-lymphocyte population in the steady-state peripheral pool. The size of this pool seems to remain constant in unimmunized animals, and so the stream of new B cells needs to be balanced by the removal of an equal number of peripheral B cells."