Regeneration of B cells in mice after depletion. Old mice were treated for multiple rounds of B-cell depletion and were followed for the kinetics of B-cell return by analysis of peripheral blood. The second and the third depletion events were introduced only after > 80% reconstitution was achieved. Gray bars represent the average time for B-cell reconstitution after each depletion, and black bar, the time needed for B-cell reconstitution in young mice. These data summarize the experiments of Keren et al.⁴⁸