A 24-year-old woman with leukemia receives an allogeneic bone marrow transplant. Three weeks later, she develops a skin rash and diarrhea. Liver function tests show elevated serum levels of AST and ALT. A skin biopsy discloses a sparse lymphocytic infiltrate in the dermis and epidermis, as well as apoptotic cells in the epidermal basal cell layer. Skin rash and diarrhea in this patient are caused primarily by which of the following cells?

The advent of transplantation of bone marrow into patients whose immune system has been ablated or into otherwise immunodeficient patients has resulted in the complication of graft-versus-host disease (GVHD). GVHD occurs when lymphocytes in the grafted tissue recognize and react to the recipient. GVHD can also occur when an immunodeficient patient is transfused with blood containing HLA-incompatible lymphocytes. The major organs affected in GVHD include the skin, gastrointestinal tract, and liver. Clinically, GVHD manifests as rash, diarrhea, abdominal cramps, anemia, and liver dysfunction. None of the other cells mediates GVHD. Diagnosis: Graft-versus-host disease