Inflammatory bowel diseases (IBD), including Crohn's disease and ulcerative colitis, is a group of immune-mediated disorders of the intestine, characterized by aberrant cytokine production by a multitude of immune cells in response to the microbiota. The interplay of these immune cell subsets (B cells, plasma cells, T cells and myeloid cell populations) is highly complex and poorly understood. We will apply a systemic approach to define an overall lymphocyte profile of immune cell subsets which contribute to a pro-inflammatory setting during chronic and acute IBD. We will especially focus on B cells in shaping disease outcome. Advancing our understanding of the underlying disease processes may eventually lead to the development of new diagnostic and therapeutic modalities.