The phenomenon you're referring to is called graft rejection, and it occurs when the body's immune system recognizes transplanted tissues or organs as foreign and launches an immune response against them. Even when the transplant comes from the same individual (autograft), the immune system can still recognize it as non-self and initiate a rejection response.
The immune system's primary function is to protect the body from foreign invaders, such as bacteria, viruses, and other pathogens. It does this by recognizing specific markers on the surface of these invaders called antigens. Each individual's immune system is highly specific and can distinguish between "self" antigens (belonging to the person's own body) and "non-self" antigens (belonging to foreign substances).
In the case of organ transplantation, although the donor and recipient may be the same person, certain immune cells can recognize differences in the antigens on the transplanted tissue compared to the rest of the body. These differences can arise due to genetic variations, mutations, or other factors.
Even in the absence of external immune system activation (as is the case in autografts), some degree of rejection can still occur due to minor differences in the transplanted tissue compared to the rest of the body. This phenomenon is known as an autologous immune response.
To mitigate graft rejection in transplants, doctors use immunosuppressive medications. These medications help to dampen the immune system's response, reducing the likelihood of rejection and allowing the transplanted organ or tissue to function in the recipient's body. However, long-term use of immunosuppressive drugs can have side effects and increase the risk of infections and other complications. That's why organ transplantation requires careful consideration and close monitoring by medical professionals.