Used primarily as an immunosuppressant to prevent organ rejection in patients receiving kidney, heart, or liver transplants. It works by inhibiting the proliferation of T and B lymphocytes, which are key components of the immune response. Commonly prescribed in combination with other immunosuppressants like cyclosporine and corticosteroids. Also used in the treatment of autoimmune diseases such as lupus and certain types of vasculitis when conventional therapies are ineffective. Available in oral and intravenous forms, it is favored for its targeted mechanism and relatively favorable side effect profile compared to older immunosuppressive agents.