Biological therapy: A type of treatment that uses substances made from living organisms to treat disease. Some biological therapies stimulate or suppress the immune system to help the body fight cancer, infection, and other diseases. Other biological therapies attack specific cancer cells, which may help keep them from growing or kill them. Types of biological therapy include immunotherapy (such as vaccines, cytokines, and some antibodies), gene therapy, and some targeted therapies. Also called biological response modifier therapy, biotherapy, and BRM therapy.