The interleukins are a broad family of well characterized cytokines, primarily of hematopoietic cell origin. As new cytokines are molecularly characterized, they are assigned an IL number to maintain a standard nomenclature. The interleukins are secreted by immune cells (mainly macrophages, B-cells or T-cells) that regulate a wide range of immune system functions. The functions of different interleukins vary from the regulation of inflammatory and immune responses to the regulation of other interleukins. IL-28 and IL-29 are induced by viral infection and show antiviral activity. IL-28 may play a role in antiviral immunity through up-regulation of MHC class I antigen expression. IL-28 is a ligand for the heterodimeric class II cytokine receptor composed of IL10RB and IL28RA.