Organ and tissue rejection occurs because the immune system is able to recognize transplants as foreign to the body. Normally, foreign entities are seen as threats, and in the case of viruses, bacteria, or parasites, this is a good thing! Recognizing and destroying infection quickly prevents you from developing serious symptoms. In the case of transplants, we must trick the immune system into tolerating a foreign object in the body.
While tissue rejection is quite uncommon, there is still a low risk of rejection when it comes to organ transplants. A graft is the transplantation of an organ or tissue to a different location, with the goal of replacing a missing or damaged organ or tissue.
Grafts are typically moved without their attachments to the circulatory system and must reestablish these, in addition to the other connections and interactions with their new surrounding tissues.
There are different types of grafts depending on the source of the new tissue or organ. Tissues that are transplanted from one genetically distinct individual to another within the same species are called allograft s.
An interesting variant of the allograft is an isograft , in which tissue from one twin is transplanted to another. As long as the twins are monozygotic therefore, essentially genetically identical , the transplanted tissue is virtually never rejected.
If tissues are transplanted from one area on an individual to another area on the same individual e. If tissues from an animal are transplanted into a human, this is called a xenograft. The different types of grafts described above have varying risks for rejection. Cytotoxic T cells then target and kill the grafted cells through the same mechanism they use to kill virus-infected cells; helper T cells may also release cytokines that activate macrophages to kill graft cells.
This is why a parent or a sibling may be the best donor in many situations—a genetic match between the MHC genes is much more likely and the organ is much less likely to be rejected. Although matching all of the MHC genes can lower the risk for rejection, there are a number of additional gene products that also play a role in stimulating responses against grafted tissue.
Because of this, no non-self grafted tissue is likely to completely avoid rejection. However, the more similar the MHC gene match, the more likely the graft is to be tolerated for a longer time. Most transplant recipients, even those with tissues well matched to their MHC genes, require treatment with immunosuppressant drugs for the rest of their lives.
This can make them more vulnerable than the general population to complications from infectious diseases. A form of rejection called graft-versus-host disease GVHD primarily occurs in recipients of bone marrow transplants and peripheral blood stem cells. GHVD presents a unique situation because the transplanted tissue is capable of producing immune cells; APCs in the donated bone marrow may recognize the host cells as non-self, leading to activation of the donor cytotoxic T cells.
Acute GVHD typically develops within weeks after a bone marrow transplant, causing tissue damage affecting the skin, gastrointestinal tract, liver, and eyes. In addition, acute GVHD may also lead to a cytokine storm , an unregulated secretion of cytokines that may be fatal.
The likelihood of rejection decreases as the kidney continues to function well. Chronic rejection happens over time and is due to scarring within the transplanted kidney. It may occur within months to years after your transplant. It is thought that controlling blood pressure, blood sugar, and cholesterol levels can help prevent chronic rejection.
Because there usually are no symptoms, it is often diagnosed by changes in your laboratory tests and a kidney biopsy. To date, there is no medication used to reverse this type of rejection. Kidney function generally lasts for months or even years after the diagnosis is made. If the kidney transplant is rejected, you may be able to receive another transplant in the future.
Sometimes the transplanted kidney must be surgically removed.
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