Final answer:
The rejection of a transplanted organ or tissue due to different MHC I molecules occurs because the recipient's immune system identifies these molecules as foreign, leading to a cytotoxic T cell response and destruction of the organ.
Step-by-step explanation:
Different MHC I molecules between donor and recipient cells can lead to the rejection of a transplanted organ or tissue because the recipient's immune system recognizes these molecules as foreign. This recognition triggers a cytotoxic T cell response, which is very specific and leads to the destruction of the transplanted tissue. The presence of different MHC I on the donated organ prompts the immune system to mount a defense, resulting in the organ's failure to integrate into the recipient's body.
To mitigate this issue, MHC typing is conducted before organ transplantation to determine compatibility between the donor and the recipient. Despite typing and matches, patients still require immunosuppressants to reduce the chances of organ rejection. The complexity of MHC molecules, due to MHC polygeny and polymorphism, makes it so that there are numerous possible allele combinations in the human population, which complicates the matching process for transplantation.