Diverse, potent, and extremely complex chemical messengers secreted by the cells of the immune system. They stimulate the production of other substances to help protect the body. Cytokines encourage cell growth, promote cell activation, direct cellular traffic, and destroy target cells—including cancer cells. Interleukins, transforming growth factor, and interferon are types of cytokines. It is also important to note that cytokines can also have unwanted, potentially serious side effects (Sources: www.medlineplus.com; and the National Cancer Institute, www.nci.nih.gov or www.cancer.gov).