Interleukin-21 is a cytokine that has potent regulatory effects on cells of the immune system, including natural killer (NK) cells and cytotoxic T cells that can destroy virally infected or cancerous cells. This cytokine induces cell division/proliferation in its target cells. IL-21 is expressed in activated human CD4+ T cells but not in most other tissues. In addition, IL-21 expression is up-regulated in Th2 and Th17 subsets of T helper cells, as well as T follicular cells. Interleukin-21 is also produced by Hodgkin's lymphoma (HL) cancer cells. Targeting IL-21 may be a potential treatment or possibly a test for HL. When bound to IL-21, the IL-21 receptor acts through the Jak/STAT pathway, utilizing Jak1 and Jak3 and a STAT3 homodimer to activate its target genes. A study using mice with peanut allergies showed that systemic treatment of IL-21 was an effective means of mitigating the allergic response. IL-21 is also noted to have anti-tumour effects through continued and increased CD8+ cell response to achieve enduring tumor immunity. IL-21 may be a critical factor in the control of persistent viral infections. This cytokine could potentially be useful for anti-HIV therapeutics.