Regulatory T-cells are naturally produced by the immune system. These cells have a powerful immunosuppressive action; they prevent auto-immune diseases by suppressing the over-active response that the immune system mounts against the body in these diseases. In addition it is thought that in patients with active CD, other immune cells in the gut are resistant to the normal controlling action of Tregs. Finally, we have found that Tregs that are isolated from patients and then are grown in the laboratory are more suppressive than Tregs freshly isolated from patients' blood.