Interleukin-21 (IL-21), which is expressed by activated CD4(+) T cells, is the newest member of the common gamma-chain family of cytokines (IL-2, IL-4, IL-7, IL-9, IL-13, and IL-15). IL-21 receptor (IL-21R) 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 antibody classes in vivo. IL-21R has been shown to activate the Janus kinase/signal transducers and activators of transcription signaling pathway upon ligand binding. Initial studies have demonstrated that IL-21 has pleiotropic effects on the proliferation, differentiation, and effector functions of B, T, natural killer, and dendritic cells. Recombinant Human IL-21 produced in E. coli is a single non-glycosylated polypeptide chain containing 133 amino acids with a MW of 15,463 Da.