<prompt> Acute GVHD is caused by-B lymphocyteT lymphocyteMacrophageNKcell  <response>: Graft versus host gvh diseasesGraft versus host disease occurs in any situation in which immunologically competent cells or their precursors are transplanted into immunologically crippled patients and the transferred cells recognize alloantigens in the host.Graft versus host disease occurs most commonly in the setting of allogeneic bone marrow transplantation but may also follow transplantation of solid organs rich in lymphoid cells e.g. the liver or transfusion of unirradiated blood.Recipients of bone marrow transplants are immunodeficient because of either their primary disease or prior treatment of the disease with drugs or irradiation.When such recipients receive normal bone marrow cells from allogeneic donors, the immunocompetent, T cells present in the donor marrow recognize the recipients HLA antigen as a foreign antigen and react against them. Both CD4 and CD8 T cells recognize and attack host tissues.In clinical practice, GVH can be so severe that bone marrow transplants are done only between HLA matched donor and recipient.Pathological changes in GVH diseasesThere are three principal target tissues affected in GVHD: Skin, liver, and gut. GVHD may be acute or chronic:-A Acute GVHDAcute GVHD occurs within 100 days usually 10-50 days of bone marrow transplantation. The manifestations of acute GVHD are1. SkinIt is the most commonly affected tissue in acute GVHD. There is generalized rash maculopapular.Histological findings arei. Perivascular mononuclear infiltrates.ii. Vacuolar degradation of dermo-epidermal junctioniii. Dyskeratotic or eosinophilic bodies in the epidermis.iv. Epidermolysis bullae.v. Denudation of epidermis separation of epidermis from dermis,2. GutThe primary clinical manifestation of gut GVHD is diarrhoea and abdominal pain. There is lymphocytic infiltrate at the crypts with accompanied necrosis and drop out of crypt cells.3. LiverLymphocytic infiltrates in the interlobular and marginal bile ducts are characteristic histopathologic findings. This results in hepatitis with necrosis of hepatocytes and bites duct epithelial cells. There is inflammation of the parenchyma and portal tracts. These findings lead to a clinically identifiable cholestatic picture.B Chronic GVHDIt occurs after 100 days of bone marrow transplantation. Manifestations are:-1. SkinExtensive cutaneous injury, with the destruction of skin appendages and fibrosis of the dermis.2. GutDamage to GIT may result in necrosis, ulceration fibrosis and strictures.3. LiverThe findings of liver damage are i Portal tract inflammation ii Selective bile duct destructioniii. Endothelitis of the portal vein and hepatitis vein radicles, in which a subendothelial lymphocytic infiltrate lifts the endothelium from its basement membrane.iv. Intrahepatic cholestasis 