As biologic therapies enter the mainstream for cancer and HIV treatments, clinicians need the knowledge and expertise to safely and competently care for their patients who are undergoing these therapies. This article provides an overview of the immune system, emphasizing the elements that are affected by the biologic agent interleukin 2 (lL-2), lL-2 has been approved for use in the treatment of metastatic renal cell carcinoma and metastatic melanoma. Clinical trials currently are being conducted to determine its use in treating other cancers. The severity of side effects of lL-2 varies with the dose, route, and schedule of administration. The most common effects with all methods of administration are flu-like symptoms. Because the side effects of lL-2 are relatively predictable, clinical pathways offer practical tools for anticipating and managing the toxicities associated with lL-2 administration.