A drug that reduces the body's resistance to infection and other foreign agents. It does so by suppressing the activity of the immune system (see IMMUNITY). Examples of such drugs are AZATHIOPRINE, CYCLOPHOSPHAMIDE and CICLOSPORIN A. Immunosuppressants are used to help transplanted organs and tissues to survive the potential immune reaction from the host. They are also used to treat AUTOIMMUNE DISORDERS such as RHEUMATOID ARTHRITIS.