Interleukin-28B, also known as Cytokine Zcyto22, Interferon lambda-3, Interferon lambda-4, IFNL3, IFNL4, ZCYTO22 and IL28B, is a secreted cytokine which belongs to the IL-28/IL-29 family. IL-28 has also been shown to play a role in the adaptive immune response. IL28B has immunomodulatory activity and up-regulates MHC class I antigen expression. IL28B displays potent antiviral activity and antitumor activity. In addition, IL28B is a ligand for the heterodimeric class II cytokine receptor composed of IL10RB and IL28RA. The ligand/receptor complex seems to signal through the Jak-STAT pathway.