In this research proposal, the investigators will focus on methods to optimize the therapeutic response to anti-TNF antibodies, by determining a correlation of 6-mp metabolite levels with IFX trough levels, anti-IFX antibody levels and clinical response. The study will also evaluate (in vitro) the possible impact of vitamin D on the interaction of IFX with dendritic cells in both healthy subjects and patients with Crohn's disease (proliferation, maturation, cytokine profile, apoptosis, gene expression).