Aim: the main aim of this study is to investigate if immune cells (regulatory T-cells, Th17 cells and other immune cell types) or biomarkers can be used to predict the response or lack of response to treatment with Infliximab. If so, characteristics of the immune cells may also unveil the mechanisms behind lack of response to Infliximab.||Design: a prospective, observational study with three arms. In the treatment group, 35 patients with Crohn's disease about to start Infliximab-treatment are recruited. They have blood samples drawn at day 1 before first treatment, after 6 week, and again after 22 weeks of treatment. 12 healthy volunteers serve as a control group. Controls are only investigated once. All treatment and follow-up are according to national guidelines, and data from this study is not used by the clinicians.||Methods: the number of regulatory T-cells and pro-inflammatory T-cells (Th17 cells) is investigated using flow cytometry. From plasma and serum samples, various proteins (biomarkers), such as transforming growth factor beta (TGF-beta) and tumour necrosis factor alpha (TNF-alpha), are measured using immunoassays. Patient data (demographics and medical history) are extracted from various registries.