Graft-versus-host disease after bone marrow transplant results from donor immune cells attacking host tissues. This is best described as which type of hypersensitivity reaction?

• A. Type II hypersensitivity
• B. Type I hypersensitivity
• C. Type III hypersensitivity
• D. Type IV hypersensitivity

Answer: (D)

Graft-versus-host disease is a T-cell–mediated, delayed hypersensitivity reaction. After a bone marrow transplant, donor T lymphocytes recognize the host’s tissues as foreign and mount an immune attack, causing tissue injury—especially in skin, liver, and gut. This injury is driven by T cells and macrophages releasing cytokines, not by antibodies, so it fits the delayed-type (Type IV) pattern. The other hypersensitivity types involve antibodies or immune complexes (Type II and III) or immediate IgE-mediated reactions (Type I), which is why they don’t describe GVHD.