IL-21 stimulates proliferation of B cell stimulated by crosslinking of the CD40 antigen. It inhibits proliferation stimulated by IL-4 plus anti-IgM. IL-21 augments stimulation of the proliferation of naive CD45RA(+) but not memory CD45R0(+) T cells mediated by engagement of CD3. IL-21 stimulates the proliferation of bone marrow progenitor cells and the expression of the natural killer cell marker CD56 in the presence of IL-15. The Interleukin-21 receptor is closely related to human IL-2 beta receptor. The receptor contains a WSXWS motif in the extracellular region, typical of type 1 cytokine receptors. The receptor is expressed on natural killer cells, T cells, and B cell line.The functional signaling complex activates Janus kinases JAK1, JAK3, and the STAT proteins STAT1, and STAT3.