Interleukin-29 (IL-29), also known as cytokine Zcyto21, Interferon lambda-1, IFN-lambda-1, and IFNL1, is a secreted protein that belongs to the IL-28 / IL-29 family. IL-29 is a cytokine with immunomodulatory activity. IL-29 is highly similar in amino acid sequence to the IL-28. IL-28 and IL-29 are induced by a viral infection and showed antiviral activity. IL-28 and IL-29 interacted with a heterodimeric class II cytokine receptor that consisted of IL-10 receptor beta (IL-10Rbeta) and an orphan class II receptor chain, designated IL-28Ralpha. IL-29 plays an important role in host defenses against microbes and its gene is highly upregulated in cells infected with viruses. IL-29 may play a role in antiviral immunity. IL-29 up-regulates MHC class I antigen expression. It 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.