Graft-versus-host disease is a central concept within transplantation immunology and clinical medicine. It describes the immunological condition arising when immune cells from transplanted donor tissue or cells (the graft) recognize and attack the recipient's tissues (the host). This academic field investigates the complex alloimmune mechanisms underlying this host-versus-graft reaction, its variable clinical manifestations (acute and chronic), identification of risk factors, and the development of preventive and therapeutic strategies, holding critical significance for the success and safety of allogeneic transplantation procedures.