Since transduction of human B cells with STAT5bER can be utilized to develop immunoglobulin producing B cells lines cultured at clonal density, the next step was to determine whether we could make antigen specific immunoglobulin producing B cells lines. TT is produced by the gram-positive bacterium Clostridium tetani. Infants are routinely vaccinated with a combination vaccine against diphtheria, pertussis, tetanus, and poliomyelitis (DPTP) [26] . As a consequence, most adults have memory B cells specifically directed against TT in their blood that can be detected using PE-conjugated recombinant TT C-Fragment (rTT.C) ( Figure S4 ) [15, 27] . Using flow cytometry rTT.C pos CD27 pos memory B cells were isolated from the peripheral blood of healthy donors and cultured on CD40L-L cells in the presence of IL-21. After 36 hrs the cells were transduced with STAT5bER-IRES-GFP and cultured again on CD40L-L cells, now in the presence of IL-2, IL-4 and tamoxifen. Seven days after transduction the cells were checked for TT binding (Figure 3a) , GFP sorted and maintained in microcultures of 1, 5 and 10 cells per well, again in the presence of CD40L-L cells, IL-2, IL-4 and tamoxifen. Within two weeks a number of rTT.C pos STAT5bER pos memory B cell clones could be selected (Table 1) and expanded further. Next we determined whether these rTT.C sorted cultures still had the capacity to produce immunoglobulins. After removal of tamoxifen from the cultures, rTT.C sorted STAT5bER pos clones resumed production of either IgM or IgG (Figure 3b and c). Next we determined the antigen specificity of the rTT.C pos STAT5bER pos memory B cell lines. The majority of clones expressing either IgM (54%) or IgG (69%) were specific for TT as determined by IgM and IgG specific TT ELISA (Figure 3d and e). Together these data show that the immortalizing capacity of STAT5bER can be used to obtain B cell lines with the capacity to secrete antigen specific immunoglobulins.