A transplant is the replacement of a diseased organ or tissue with a healthy, living substitute. The organ is usually taken from a person who has just died but may also come from a living donor (usually a relative of the patient). The results of transplant surgery have been improved by testing for histocompatibility antigens and tissue-typing.

Rejection is a major problem. A combination of a corticosteroid drug with an immunosuppressant drug is given in order to suppress this response. Every patient who has to go undergo an organ transplant procedure must be given immunosuppressant drugs for an indefinite period.