It is not known exactly how Imuran works to prevent kidney transplant rejection or to treat rheumatoid arthritis. Imuran acts in a way similar to certain types of chemotherapy medications, killing cells by damaging DNA. Also, much like chemotherapy medications, Imuran suppresses the immune system (it is thought that the medication works mostly by suppressing the immune system). The immune system is responsible for rejecting transplanted organs, and an overactive immune system plays an important role in rheumatoid arthritis.