An ex vivo method for producing a B cell capable of producing antibody for an antigen of interest comprising: a) selecting one B cell capable of producing antibody specific for said antigen of interest or one B cell capable of developing into a B cell capable of producing antibody specific for said antigen of interest; b) providing said B cell with a nucleic acid molecule encoding BCL6 and with a nucleic acid molecule encoding Bcl-xL, Mcl-1, Bcl-2, A1, Bcl-w or Bcl2L10, preferably Bcl-xL or Mcl-1; c) allowing expansion of said B cell into a first B cell culture in the presence of IL21 and CD40L; d) selecting at least one B cell from said first B cell culture with a binding avidity for said antigen of interest that is higher than the average binding avidity for said antigen of interest of B cells in said first B cell culture; e) preferably allowing expansion of said at least one B cell selected in step d) into a second B cell culture; f) determining the thermal stability and/or the resistance to aggregation over time after storage, and the affinity for said antigen of interest, of antibodies produced by said at least one B cell selected in step d) or by said second B cell culture; and g) selecting at least one B cell capable of producing antibodies with a higher thermal stability and/or with a higher resistance to aggregation, as compared to the average thermal stability or resistance to aggregation of antibodies produced by B cells of said first B cell culture, and with an affinity for said antigen of interest that is the same as, or less than, the average affinity for said antigen of interest of antibodies produced by said first B cell culture. a) selecting one B cell capable of producing antibody specific for said antigen of interest or one B cell capable of developing into a B cell capable of producing antibody specific for said antigen of interest; b) providing said B cell with a nucleic acid molecule encoding BCL6 and with a nucleic acid molecule encoding Bcl-xL, Mcl-1, Bcl-2, A1, Bcl-w or Bcl2L10, preferably Bcl-xL or Mcl-1; c) allowing expansion of said B cell into a first B cell culture in the presence of IL21 and CD40L; d) selecting at least one B cell from said first B cell culture with a binding avidity for said antigen of interest that is higher than the average binding avidity for said antigen of interest of B cells in said first B cell culture; e) preferably allowing expansion of said at least one B cell selected in step d) into a second B cell culture; f) determining the thermal stability and/or the resistance to aggregation over time after storage, and the affinity for said antigen of interest, of antibodies produced by said at least one B cell selected in step d) or by said second B cell culture; and g) selecting at least one B cell capable of producing antibodies with a higher thermal stability and/or with a higher resistance to aggregation, as compared to the average thermal stability or resistance to aggregation of antibodies produced by B cells of said first B cell culture, and with an affinity for said antigen of interest that is the same as, or less than, the average affinity for said antigen of interest of antibodies produced by said first B cell culture. <EOS>