For patients who have undergone a bone marrow transplant, there is a chance they might develop Graft vs Host Disease (GvHD) – a condition born when the donor’s bone marrow or peripheral blood stem cells, view the recipient’s body as foreign, and the donated cells/bone marrow attack the body. Symptoms of GvHD can be severe and long lasting, especially in children. However, with a procedure called photopheresis, this condition can be successfully prevented and treated.