GVHD occurs when immunologically competent cells or their precursors are transplanted into immunologically crippled recipients, and the transferred cells recognize alloantigen's in the host and attack host tissues. It is seen most commonly in the setting of HSC transplantation but, rarely, may occur following transplantation of solid organs rich in lymphoid cells (e.g., the liver) or transfusion of unirradiated blood. When immune-compromised recipients receive HSC preparations from allogeneic donors, the immunocompetent T cells present in the donor inoculum recognize the recipient's HLA antigens as foreign and react against them. To try to minimize GVHD, HSC transplants are done between donor and recipient that are HLA matched using precise DNA sequencing-based methods for molecular typing of HLA alleles. Acute GVHD Chronic GVHD Within 100 days After 100 days It is believed that in addition to direct cytotoxicity by CD8+ T cells, considerable damage is inflicted by cytokines released by sensitized donor T cells. Grafted CD4+ helper T cells reacting with host B cells an stimulating these cells, some of which are capable of producing auto-antibodies. S: Skin: generalized rash (most earliest and most common) I: Intestine: bloody diarrhea L: Liver/bile duct : jaundice S: Skin : destruction og skin appendage and fibrosis of dermis I: Intestine : esophageal stricture L: Cholestatic jaundice GVHD in case of animal is called runt disease.