Which of the following poses a formidable obstacle to successful orgen transplant?

Tissue rejection poses a formidable obstacle to successful organ transplant.

When an organ is transplanted from one person (the donor) to another (the recipient), the recipient's immune system recognizes the donor organ as foreign and mounts an immune response against it. This immune response can damage or destroy the transplanted organ, leading to transplant rejection.

Tissue rejection can be prevented by immunosuppressive drugs, which suppress the recipient's immune system and prevent it from attacking the transplanted organ. However, immunosuppressive drugs can also have side effects, such as an increased risk of infection and cancer.