Why do killer cells cause rejection of an organ transplant?

Killer cells, also known as cytotoxic T lymphocytes or natural killer cells, play a crucial role in the immune system's defense mechanisms against foreign invaders and abnormal cells. However, in the context of organ transplantation, the activity of killer cells can lead to the rejection of the transplanted organ. Here's why:

1. Foreign Antigen Recognition: After an organ transplant, the recipient's immune system recognizes the transplanted organ as foreign due to the presence of different antigens on its cells. These foreign antigens can be identified by the killer cells.

2. Activation of Killer Cells: The recognition of foreign antigens triggers the activation of killer cells. These activated killer cells become cytotoxic and develop the ability to destroy cells bearing the foreign antigens.

3. Attack on Transplanted Organ Cells: The activated killer cells then infiltrate the transplanted organ and target its cells that express the foreign antigens. They release cytotoxic substances, such as perforin and granzymes, which induce apoptosis (programmed cell death) in the transplanted organ cells.

4. Immune Response Cascade: The destruction of transplanted organ cells by killer cells initiates an immune response cascade, where other components of the immune system, such as macrophages and antibodies, become involved. This further amplifies the rejection process.

5. Graft-Versus-Host Disease (GVHD): In certain cases, such as bone marrow transplantation, the donor's immune cells (including killer cells) can attack the recipient's own tissues, leading to a condition called graft-versus-host disease (GVHD). GVHD contributes to the rejection of the transplanted organ and causes damage to various recipient tissues.

To prevent organ rejection, immunosuppressive drugs are typically administered to transplant recipients. These drugs suppress the activity of the immune system, including killer cells, and help prolong the survival of the transplanted organ. However, immunosuppression also increases the risk of infections and other complications. Balancing the immune suppression to prevent rejection while minimizing the side effects remains a challenge in transplantation medicine.