Final answer:
Transplanted organ rejection is often due to the recipient's immune system recognizing donor MHC I molecules as foreign, triggering an attack by cytotoxic T cells.
Step-by-step explanation:
One way the immune system is involved in the rejection of transplanted organs is through the host's recognition of MHC I molecules on the donor organ as foreign. The immune system, primarily through cytotoxic T cells, mounts an attack against these non-self MHC I molecules, leading to the eventual rejection of the transplanted tissue. This immune response can be highly specific and robust, causing significant harm to the recipient if not managed with immunosuppressive therapy.