 IFN-α and IFN-β are secreted by many cell types including lymphocytes (NK cells, B-cells and T-cells), macrophages, fibroblasts, endothelial cells, osteoblasts and others.They stimulate both macrophages and NK cells to elicit an anti-viral response, involving IRF3/IRF7 antiviral pathways, and are also active against tumors.Plasmacytoid dendritic cells have been identified as being the most potent producers of type I IFNs in response to antigen, and have thus been coined natural IFN producing cells.