The STAT5b-ER transduced B cells only secrete antibody when STAT5b-ER is inactivated and in the presence of IL-21. IL-21 induces proliferation and plasma cell differentiation in B cells, both in mice as well as in humans [23, 24, 25, 36, 37] . This occurs at least partly through upregulation of Blimp-1 by STAT3 activation [18, 38, 39] . IL-21 can also activate STAT5 and thus counteract STAT3 induced differentiation but the activation of STAT5 by IL-21 is only transient (30 hrs) compared to STAT3 (days). Stimulation with IL-2 and IL-4 did not lead to antibody secretion, which is expected since both cytokines induce proliferation but do not plasma cell differentiation of B cells [16, 40, 41, 42] .