In the 1950s, researchers discovered a class of cytokines called interferons that help the body to eliminate viruses. Researchers have also learned that cytokines can activate the body's natural immune system to help destroy certain types of tumors. Since the discovery of interferons in the 1950s, other cytokines with anti-tumor properties have been discovered including interleukins and tumor necrosis factor. The two most common cytokines used as immunotherapy for metastatic RCC are interleukin-2 and interferon-alpha.