<prompt> True about GVHD is?Occurs when host is immunocompetentMost common organ involved is lungMost common in renal transplantOccurs when donor cells are immunocompetent  <response>: Graft versus host GVH Disease:Graft 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 un-irradiated 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 recognizes the recipients HLA antigen as foreign antigen and react against them. Both CD4 and CD8T cells recognize and attack host tissuesIn clinical practice, GVH can be so severe that bone marrow transplants are done only between HLA matched donor and recipientThere are three principal target tissues affected in GVHD : Skin, liver, and gut 