Interferons (IFNs) are best known for their critical functions in host defense mechanisms. The type I IFNs include the IFN-alpha family, IFN-beta, -omega, -kappa and Limitin/IFN-zeta and are important for regulating the immune response to viral infection. The type II IFN group includes only IFN-gamma. IFN-gamma is a multifunctional cytokine that exerts anti-proliferative, immunoregulatory, and pro-inflammatory effects important for numerous aspects of the immune response. The type III IFN family includes IL-29/IFN-lambda 1, IL-28A/IFN-lambda 2, IL-28B/IFN-lambda 3, and IFN-lambda 4. Similar to the type I IFNs, type III IFNs have anti-viral, anti-proliferative, and immunoregulatory effects.