Interleukin-21 (IL-21) is an approximately 14 kDa four-helix-bundle cytokine in the family of cytokines that utilize the common gamma chain (gamma c) as a receptor subunit. Mature mouse IL-21 shares 66%, 59%, 58%, and 88% aa sequence identity with mature canine, human, rabbit, and rat IL-21, respectively. It can induce target cells division or proliferation. IL-21 elicits its effect through binding to IL-21R. IL-21 is a type I cytokine whose receptor is expressed on T, B, and NK cells. Within the B cell lineage, IL-21 regulates IgG1 production and cooperates with IL-4 for the production of multiple Ab classes in vivo. IL-21 has a significant influence on the regulation of B cell function in vivo and cooperates with IL-4. Additionally, IL-21 promoted tumor-specific CTL activity and enhanced memory responses to tumor rechallenge.