Biomarkers in Acute Graft-Versus-Host Disease and Extracorporeal Photopheresis Added to Investigator Chosen Therapies of Steroid Refractory Acute GVHD

Background: \- Some allogeneic stem cell transplant recipients get acute graft-versus-host disease (GVHD). They always get steroids as the first treatment, but this may not work. Those people where steroids are not enough may benefit from a treatment called extracorporeal photopheresis (ECP). ECP exposes white blood cells to ultraviolet light outside the body. Researchers want to study how certain markers in the blood predict the severity and outcome of acute GVHD and how ECP treatments work for people with acute GVHD. They will also study how these markers in the blood may help predict who should get ECP and its effects on the immune system. Objectives: \- To learn more about treatments for acute GVHD after allogeneic stem cell transplantation. Eligibility: \- Adults with acute GVHD enrolled in an National Cancer Institute (NCI) allogeneic transplantation protocol. Design: * Transplant physicians will confirm participant eligibility. * Participants will receive treatment with steroids for their acute GVHD as prescribed by their transplant physician. This will continue while they are enrolled on this study. * If steroids work in treating their acute GVHD, then every 28 days for 6 months, participants will have: * a physical exam. * blood tests. * If steroids do not work, participants will get additional treatments as prescribed by their transplant physician who may choose to use ECP as a part of this additional treatment. For ECP, blood is removed through an intravenous (IV) catheter. A machine separates the white blood cells from the other blood parts. Those cells are treated with methoxsalen and exposed to ultraviolet light. Then they are returned to the participant through their IV. * Participants who get ECP will over at least 6 months have: * veins researched. They may have a catheter placed in a larger vein in the chest or groin. * multiple blood tests. * multiple pregnancy tests (if needed). * multiple ECP procedures. * At the end of ECP treatment and 6 months after ECP, participants will have additional physical exams and blood tests.