High doses of cyclophosphamide (high-CY), at a dose of 50 mg/kg/day X 4 days has become a standard dosing regimen in transplantation, a standard dosing for other transplant studies in Crohn's Disease, and the chemotherapeutic agent proposed for this clinical trial's Crohn's disease conditioning regimen. In this study, high-CY will be combined with an additional immunosuppressant, rabbit anti-thymocyte globulin (thymoglobulin, ATG), which has become part of the autologous transplant approach for autoimmune diseases including Crohn's disease. Thymoglobulin is a rabbit-thymocyte globulin obtained from the serum of rabbits immunized with human thymus lymphocytes. ATG is a lymphocyte-selective immunosuppressant that acts by causing in vivo T-cell depletion in blood and lymphoid tissues . There are sustained shifts in T and B cell subpopulations from memory to naïve cell dominance, supportive of thymic reprocessing and reeducation of the reconstituting immune system. This, in essence, resets the immune system of patients with autoimmune diseases, of which Crohn's disease may be considered. Thymoglobulin, the brand of ATG being used in this study, has a half-life of 2-3 days. This allows for further delay in the return of activating T cells which contribute to the pathophysiology of Crohn's disease.