immunosuppressive drug, any of a variety of substances used to prevent production of antibodies. They are commonly used to prevent rejection by a recipient's body of an organ transplanted from a donor. A transplant is rejected when the recipient's immune system acts against it; current methods aim at suppressing the activity of the lymphocytes, the cells that form antibodies (see immunity; transplantation). The steroids, such as cortisone, which suppress the antibody-forming lymphocyte cells, have been used to prolong human organ transplants. Steroids may also prevent antigens from entering cells and thereby prevent local allergic inflammation reactions. In another immunosuppressive method, human lymphocytes are injected into horses, stimulating the animals to produce antilymphocyte serum. The serum, administered to humans with transplanted organs, in some way inactivates lymphocyte cells. The procedure will not work effectively for more than a few injections of serum. Another group of immunosuppressive drugs act by interfering with the synthesis of nucleic acids and are especially effective against proliferating cells such as stimulated lymphocytes. Some of these are analogs of purines and pyrimidines, substances that are nucleic acid subunits; the purine analog azothioprine has been used to suppress rejection of transplanted human kidneys. Most substances that inhibit nucleic acid synthesis, such as nitrogen mustard, cyclophosphamide (Cytoxan), chloramphenicol, actinomycin, and colchicine, are not widely used clinically because they are too toxic. Many of the drugs that suppress the function of the immunological system are also used clinically to check growth of cancerous tissue, which is composed of rapidly dividing cells. The drugs currently used to suppress antibody formation also leave an individual susceptible to infection.