Interleukin-2 (IL-2) is an interleukin, which is a type of cytokine signaling molecule forming part of the immune system. It regulates the activities of white blood cells (leukocytes, often lymphocytes) that are responsible for immunity. IL-2 is part of the body's natural response to microbial infection, and in discriminating between foreign ("non-self") and "self". IL-2 mediates its effects by binding to IL-2 receptors, which are expressed by lymphocytes. The major sources of IL-2 are activated CD4+ T cells and activated CD8+ T cells. Put shortly, the function of IL-2 is to stimulate the growth of helper, cytotoxic and regulatory T cells.