When you receive an organ transplant the donor cells, although matched as closely as possible to yours, will not be identical to yours. This means that your immune system will recognise the tissue as foreign and try to attack the transplanted organ. This can result in the transplant being rejected. Tacrolimus is used to prevent transplant rejection because it suppresses the activity of the cells in your immune system that would normally attack the transplanted tissue. Tacrolimus acts on the white blood cells. It works by blocking the action of a substance called calcineurin that is found in white blood cells called T-lymphocytes. This action prevents the T-lymphocytes from producing substances called lymphokines. Tacrolimus is used to prevent rejection of kidney, liver and heart transplants. It is used in combination with other immunosuppressant medicines, such as corticosteroids.