True about GVHD is?
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 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 HLA matched donor and recipient
There are three principal target tissues affected in GVHD : Skin, liver, and gut
Occurs when host is immunocompetent
Most common organ involved is lung
Most common in renal transplant
Occurs when donor cells are immunocompetent