Immunosuppressant drugs are medications that reduce the ability of the immune system to recognize and respond to the presence of foreign material. Such drugs were developed and still have an important use as a means of ensuring that transplanted organs and tissues are not rejected by the recipient.

Rejection of transplanted organs or tissue is a natural reaction of a person's immune system. In a very real sense, the transplanted material is foreign and is treated, as would be an infectious microorganism. The immune system attacks and tries to destroy the foreign matter. Suppressing the immune system allows the transplanted material to be retained.

Drugs to suppress the immune system are available only with a physician's authorization. Some commonly prescribed drugs are azathioprine, cyclosporine, prednisolone, and tacrolimus. These can be taken orally, both in solid and liquid forms, or can be injected.

The main target of...