 The objective of this NIH-specific substudy is immunologic monitoring of cytokine and immune cell responses in subjects undergoing treatment with AIN457 human monoclonal anti-human interleukin-17A for moderate to severe Crohns disease. Recent data suggests that interleukin-17 IL-17 is an important mediator of inflammation in certain animal models of Crohns disease, and treatment aimed at blocking the IL-23-IL-17 axis can ameliorate the inflammatory changes. In addition, elevated expression of IL-l7 has been found in the gut tissue of patients with active Crohns disease. This substudy will measure changes in cytokine production, relevant RNA expression, and immune cell populations in the periphery and lamina propria for correlation with clinical outcomes in order to explore the mechanisms of therapeutic response. 