IL-28A (Interferon-λ2? IFN-λ2), IL-28B/IFN-λ3, and IL-29/IFN-λ1 are type III interferons which are distantly related to IL-10 family and type I IFN family cytokines. Mature human IL-28A is an approximately 22-25 kDa protein that shares 66% amino acid (aa) sequence identity with mouse and rat IL-28A and shows cross-species activity. It shares 96% and 70% aa sequence identity with human IL-28B and IL-29, respectively. IL-28A promotes the Th1 polarization of dendritic cells in the airway and inhibits Th2 and Th17 mediated inflammation. IL-28A additionally exhibits anti-tumor activity, in part by enhancing IL-12 dependent anti-tumor CTL responses in vivo. In contrast, it is up-regulated in invasive bladder cancer where it promotes tumor cell migration.