Patients will have blood drawn to collect lymphocytes causing GvHD. This may be done with a special needle or catheter (tube inserted into a vein) or for patients who need or prefer it with a temporary central venous catheter similar to that used for the stem cell transplantation. Patients will have three 2- to 3-hour treatments a week for the first week and two treatments a week after that for a total of 25 treatments over 3 months. Patients who do not tolerate the treatment...