Final answer:
Transplanted organs can be rejected due to genetic differences, especially in the MHC genes. Cytotoxic T cells can mount an immune response against the transplanted organ if the MHC molecules are different from the recipient's. Immunossupressant drugs are often required to prevent rejection, but they can increase the risk of complications.
Step-by-step explanation:
When an organ is transplanted from an incompatible donor to a recipient, it can be rejected due to genetic differences, particularly in the MHC (HLA) genes. MHC molecules are responsible for histocompatibility and play a major role in transplant rejection. If the donor's MHC molecules are different from the recipient's, cytotoxic T cells can mount an immune response against the transplanted organ, leading to rejection.
Graft-versus-host disease (GVHD) can also occur in bone marrow transplants. In this case, the mature T cells in the transplant recognize the recipient's tissues as foreign and attack them.
Matching MHC genes can lower the risk of rejection, but no non-self grafted tissue is likely to completely avoid rejection. Most transplant recipients require immunosuppressant drugs, which can make them more susceptible to infections and can increase the risk of transplant-related malignancies.