Background: Type 2 T-helper cells (Th2) are involved in the regulation of the humoral immune response against antigens and allergens and directly affect which isotype will be produced. The mechanism that regulates antigen-specific IgE secretion and immune deviation is still not known. Objectives: To delineate mechanisms behind antigen-specific IgE secretion we have used in vitro immunization and focused on T-cell phenotype and the activation status of the transcription factor NFkB. Methods: Peripheral blood lymphocytes (PBMC) from seronegative donors were immunized in vitro with a peptide consisting of both a T-cell and a B-cell epitope. Results Antigen-specific IgE antibodies could be detected after a primary immunization, during which T-helper cells secreted type 2 cytokines. Specific IgE was also detected in the secondary immunization, but due to a rapid polarization from Th2 to Th1 phenotype, exogenous IL-4 was required for the specific IgE secretion. Analysis of NFkB activation in B and T cells during primary and secondary immunization showed that NFkB could be detected in both B and T cells during primary immunization, but was dependent on exogenous IL-4 in the secondary immunization. Conclusion: This is the first evidence of antigen-specific IgE induction in vitro using naive B cells, demonstrating the involvement of T-helper cell phenotype and NFkB and demonstrates the usefulness of in vitro cultures to study the effect of antigens on human immunocytes.