Background: Anti−TNFα therapy is efficacious in Crohn's disease (CD), yet the mechanism of action is poorly understood. The effect of TNFα on germinal center formation, mature B−cell development, and immune mediated disease is a rapidly evolving, highly relevant area of research. In patients with CD responsive to anti−TNFα therapy, an augmentation in mature IgM+ B−cells has been described. Recent murine data indicate a sub-population of IgM+ B−cells with potential immuno−regulatory function (“marginal zone precursors”). Therefore, we performed a prospective, longitudinal study to precisely immunophenotype peripheral lymphocyte populations from patients with CD before and after anti−TNFα therapy. Methods: Ten patients with clinically active CD on no concomitant immunotherapy (corticosteroids, thiopurines, or methotrexate) in whom anti−TNFα therapy was clinically indicated were enrolled. A 50-ml blood sample was collected at baseline and 12 weeks post induction of anti−TNFα therapy. CLIA-validated immunological assays were performed for T, B and NK cell quantitation, detailed quantitative T and B cell immunophenotyping, functional CD8 T−cell immune competence and assessment of thymic output by TREC analysis. Results: There was no change in total T−, B− or NK−cell quantitation between pre and post treatment samples. B−cell subset analysis demonstrated a significant up−regulation in total peripheral memory B−cells (CD19+CD27+) (absolute numbers/ml 21.1 vs 53.6 and p=0.03). Of the three memory B−cell subsets (IgM only, class-switched, and marginal zone or non class-switched) only the marginal zone B−cell population (IgM+IgD+) demonstrated significant augmentation upon anti−TNFα therapy (absolute numbers/ml 4.5 vs 16.4 and p=0.03). Anti−TNFα therapy did not appear to affect thymic output or CD8 T−cell function, in terms, of mitogen-stimulated IFN-γ secretion or CD107a/b expression (a surrogate marker for cytotoxic function). As demonstrated in other reports, we also document augmentation of regulatory T−cells (CD4+CD25+) (p=0.01) in patients post-treatment. Conclusions: We demonstrate for the first time in patients with Crohn's disease a significant increase in peripheral marginal zone B−cells in response to anti−TNFα therapy. Given recent evidence that marginal zone B−cells arise from germinal centers, the regulatory role of marginal zone B−cells from mouse models and the critical role of TNFα in germinal center development, our data is significant and hypothesis generating. These results lay the foundation for future mechanistic studies related to human marginal zone B−cell development and function.