IL2 and hairy cell leukemia: In 1986, the U.S. Food and Drug Administration (FDA) agreed to the use of the immunotherapeutic cytokine interferon-α (IFN-α) for the therapy of hairy cell leukemia, and several clinical trials ranging from cytokines to interleukin-2 (IL-2) have demonstrated some efficacy and a high level of toxicity [26,27].