Why are transplanted organs often rejected by the person that receives them?

Transplants are life-saving procedures that offer individuals the opportunity to receive healthy organs and tissues. However, one of the significant challenges in transplantation is the possibility of organ rejection. The body's immune system is responsible for recognizing and attacking foreign substances to prevent infection and disease. When an organ is transplanted into someone's body, the recipient's immune system may perceive the transplanted organ as foreign and launch an immune response against it. This can result in organ rejection.

There are several reasons why transplanted organs are often rejected by the recipient's body:

1. Immune System Response: The immune system is designed to protect the body by recognizing foreign invaders and eliminating them. When a transplanted organ is introduced, the immune system may not be able to distinguish it as part of the body and initiate an immune response against it. This reaction is called "acute rejection" and can occur within the first few weeks or months after the transplant.

2. Major Histocompatibility Complex (MHC) Mismatch: The MHC plays a crucial role in the body's immune system. Every individual has unique MHC molecules. When an organ is transplanted from one person to another, MHC incompatibility can cause the recipient's immune system to identify the transplanted organ as non-self and attack it. This can result in a form of rejection called "hyperacute rejection," which occurs within minutes to hours after transplantation.

3. T Cell Activation: T cells are a type of white blood cell that play a central role in the immune system's response. When the recipient's immune system detects foreign substances, T cells can become activated and target the transplanted organ. This can initiate rejection processes and lead to tissue damage.

4. Antibody Formation: The immune system produces antibodies, which are proteins that help protect the body against foreign invaders. In the case of organ transplantation, antibodies can be produced against the transplanted organ, marking it for destruction by the immune system. The formation of antibodies can result in rejection or damage to the transplanted organ over time.

5. Ischemia-Reperfusion Injury: Organ transplantation involves removing the organ from the donor and transporting it to the recipient. This process can result in ischemia, or lack of blood flow to the organ. Ischemia-reperfusion injury refers to the damage caused by the interruption and subsequent restoration of blood flow to the transplanted organ. This can contribute to rejection by damaging the organ's cells and making it more vulnerable to attack by the recipient's immune system.

Minimizing the risk of rejection is crucial for the success of organ transplantation. Various measures are taken to achieve this, including tissue matching, immunosuppressive medications, and close monitoring. However, despite these measures, rejection can still occur, underscoring the complexity of the immune system and the challenges faced in transplantation medicine.