Ans. is 'a' i.e., Immunocompetent T cells in graft tissue Graft versus host 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 allogenic 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 allogenic donors, the immunocompetent, T cells present in the donor marrow recognizes the recipient's HLA antigen as 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 ILIA matched donor and recipient. Acute GVH disease occurs within days to weeks after allogenic bone marrow transplantation. Although any organ may be affected, the major clinical manifestation results from involvement of the immune system and epithelia of the skin, liver and intestines. Involvement of the skin in GVH disease is manifested by a generalized rash leading to desquamation in severe cases. Destruction of small bile ducts gives rise to jaundice and mucosal ulceration of the gut results in bloody diarrhoea. Immunodeficiency is a frequent accompaniment of GVH disease. The immunodeficiency may be a result of prior treatment, myeloablative preparation for the graft, a delay in repopulation of the recipient's immune system and attack on host's immune cells by grafted lymphocytes. Affected individuals are profoundly immunosuppressed and are easy prey to infections. Although many different types of organisms may infect patients, infection with cytomegalovirus is paicularly impoant.