Why Humans Reject Animal Organs for Transplants: An Immune System Perspective

When I taught young students about 'living things', there were always some who found it surprising to learn that humans are animals. In science class, we discussed that humans, like other animals, cannot get their energy directly from the sun but instead rely on processes within their cells.

One key aspect of human physiology is the presence of unique cell markers on every cell in the body. These markers are specific to an individual and serve as a form of identification for the immune system. The immune system is constantly monitoring these cell markers to ensure they are correct. If a cell is detected with incorrect markers, the immune system identifies it as foreign and removes it to protect the body from potential threats.

Cell Markers and Organ Transplants

Each human has a unique set of cell markers, making foreign organs appear distinct to the immune system. For example, when a human organ from another individual is implanted, the immune system recognizes it as a foreign entity and begins to reject it. This is a critical challenge in organ transplantation. Even with careful selection of donors and receivers to have as identical cell markers as possible, recipients still require lifelong immunosuppressive medications to keep the immune system from attacking the new organ.

The effectiveness of the immune response can vary depending on the type of organ. For instance, a liver transplant has a five-year survival rate of about 80%, but this can drop significantly if the donor and recipient have incompatible cell markers.

Immune Response Mechanisms

Immune cells are constantly on the lookout for proteins on the surfaces of cells they encounter. These proteins include cell markers and tissue type markers that help the immune system identify whether a cell is part of the body or not. If an immune cell encounters a protein it does not recognize, it undergoes processes such as tearing down, engulfing, and removing the foreign matter.

The severity of the immune response can vary based on how similar the proteins are to the individual's own. In a healthy person, the immune response to foreign cells is minimal, or sometimes even zero, for cells with compatible tissue types. Incompatible types may elicit a milder to severe response, and foreign animal cells are generally met with a more severe immune response, depending on the tissue type.

Some studies suggest that certain animal tissues might be tolerated better than human tissues due to their evolutionary compatibility. For example, animal proteins may be removed from graft materials to reduce the immune response, making transplants more successful.

Conclusion

Organ rejection in human recipients of animal organs is primarily due to the immune system's identification of these organs as foreign. This highlights the critical role of compatible cell markers and tissue type compatibility in successful organ transplants. Continued research into immunosuppressive medications and graft modifications may eventually lead to more successful transplantation of animal organs in humans.

Understanding the complexities of the immune system and the rejection of animal organs is crucial for advancing the field of organ transplantation. Further research could pave the way for more successful and long-lasting transplants, ultimately increasing the chances of a better quality of life for many patients.