Biological therapy works with the immune system in order to help fight cancer or control side effects from other cancer treatments you are receiving. While treatments like chemotherapy attack cancer directly, biological therapy helps the immune system to fight cancer. Doctors aren’t sure precisely what it is that biological therapy achieves, but it may stop or slow the growth of cancer cells, making it easier for the immune system to destroy or get rid of cancer cells or keep the cancer from spreading to other areas of the body.