TLR9 and viral infectious disease: TLR7 and TLR9 are selectively expressed by plasmacytoid dendritic cells (pDCs; also known as interferon (IFN)-producing cells), which are a subset of DCs with a plasmacytoid morphology and unique in their capacity to rapidly secrete large quantities of type I IFN in response to viral infection, including response to influenza (39).