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-21-R 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. rHu IL-21 produced in E. coli is a single, non-glycosilated polypeptide chain that contains 133 amino acids and has a total molecular mass of 15.46 kDa.