Interleukin-28B (IL-28B) also known as Interferon lambda-3 and IFN-lambda-3, belongs to the type III interferon family of cytokines and are highly similar to IL-29. IL-28B belongs to the newly described interferon lambda (IFNλ) family of cytokines. IL-28B is a cytokine with immunomodulatory activity. It functions in Up-regulating MHC class I antigen expression. IL-28B displays potent antiviral activity and antitumor activity. This cytokine serves as ligand for the heterodimeric class II cytokine receptor composed of IL1RB and IL28RA. The ligand/receptor complex seems to signal through the Jak-STAT pathway. IL-28B, like IL-12, is capable of robustly enhancing adaptive immunity. Moreover, we describe for the first time how IL-28B reduces regulatory T-cell populations during DNA vaccination, whereas IL-12 increases this cellular subset. We also show that IL-28B, unlike IL-12, is able to increase the percentage of splenic CD8+ T cells in vaccinated animals, and that these cells are more granular and have higher antigen-specific cytolytic degranulation compared with cells taken from animals that received IL-12 as an adjuvant.