Tissue rejection, also known as transplant rejection, is primarily caused by the recipient's immune system identifying the transplanted tissue or organ as foreign and attacking it. This is due to differences in antigens, particularly Human Leukocyte Antigens (HLAs), between the donor and the recipient.
The Role of the Immune System
The immune system is designed to protect the body from foreign invaders like bacteria and viruses. It does this by recognizing specific molecules, called antigens, on the surface of cells. In the case of organ transplantation, the recipient's immune system may recognize the antigens on the donor's cells as "non-self."
Human Leukocyte Antigens (HLAs)
HLAs are proteins found on the surface of most cells in the body. They play a crucial role in the immune system's ability to distinguish between self and non-self. HLAs are highly variable from person to person. The closer the match between the donor's and recipient's HLAs, the lower the risk of rejection.
The Rejection Process
- Recognition: The recipient's immune cells, particularly T lymphocytes (T cells), recognize the donor's HLAs as foreign.
- Activation: This recognition triggers an immune response, activating the T cells.
- Attack: The activated T cells attack the cells of the transplanted organ or tissue, causing damage and inflammation.
- Antibody Production (in some cases): The immune system can also produce antibodies that target the donor's cells, leading to antibody-mediated rejection.
Factors Influencing Rejection Risk
- HLA Matching: The degree of HLA matching between the donor and recipient is a primary factor.
- Immunosuppressant Medications: These medications suppress the immune system to prevent it from attacking the transplanted organ. Dosage and type of immunosuppressant are critical.
- Recipient's Immune Status: A recipient's prior exposure to foreign antigens (e.g., through blood transfusions, pregnancies, or previous transplants) can influence their immune response and rejection risk.
- Type of Organ Transplanted: Some organs, such as the kidney, are more susceptible to rejection than others.
- Overall Health of Recipient: Recipient health status prior to transplant impacts risk of rejection and post-transplant outcomes.
Types of Rejection
- Hyperacute Rejection: Occurs within minutes to hours of transplantation due to pre-existing antibodies in the recipient that target the donor organ. This is rare due to pre-transplant crossmatching.
- Acute Rejection: Typically occurs within the first few months after transplantation and is mediated by T cells.
- Chronic Rejection: Develops over months or years and involves both cellular and antibody-mediated mechanisms, leading to gradual loss of organ function.
Conclusion
Tissue rejection is a complex immune response triggered by the recognition of the transplanted tissue as foreign. The primary cause is the difference in HLAs between the donor and the recipient, leading to an immune attack on the transplanted organ. Immunosuppressant medications are crucial in managing and preventing rejection.
