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When an organ is transplanted, the immune system of the recipient will most likely recognize it as foreign tissue and attack it. The destruction of the organ will, if untreated, end in the death of the recipient.
To prevent that, radiation therapy was used in the past, but now immunosuppressant drugs are used to inhibit the reaction of the immune system. The downside is that with such a deactivated immune system, the body is very vulnerable to opportunistic infections, even those usually considered harmless. Also, prolonged use of immunosuppressants increases the risk of cancer.
Cortisone was the first immunosuppressant identified. The more effective azathioprine was identified in 1959, but it was not until the discovery of cyclosporine in 1970 that transplant surgery found a sufficiently powerful immunosuppressive.
A list of immunosuppressant drugs is given on the transplant rejection page.
A person who is undergoing immunosuppression, or whose immune system is weak for other reasons (e.g. chemotherapy and HIV) is said to be immunocompromised.
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