Cyclosporine, an immunosuppressant, is given to patients who have received kidney, liver and heart transplants. Without this drug, the patient would be much more likely to reject the new organ because of the body’s natural response to foreign objects. The main aim of an immunosuppressant drug is to suppress the action of helper T-cells (a type of lymphocyte) which act to influence other cells in the immune response, such as cytotoxic T-cells, and to increase the effectiveness of phagocytes such as macrophages.
Except for certain transplants such as heart valves from cows and pigs, the body will always recognize the new organ as a foreign body, and send leukocytes to attack it. Therefore, after receiving a new organ, the patient must take immunosuppressant drugs for life. In addition they must take prophylactic antibiotics to avoid infections. The correct balance of drugs will allow the person to keep their new organ for life. http://www.wisegeek.com/what-causes-transplant-rejection.htm