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Plasmacytoid dendritic cells (pDCs) are innate immune cells that circulate in the blood and are found in peripheral lymphoid organs. They constitute < 0.4% of peripheral blood mononuclear cells (PBMC). In humans these cells express the surface markers CD123, BDCA-2(CD303) and BDCA-4(CD304), but do not express high levels of CD11c or CD14, which distinguishes them from conventional dendritic cells or monocytes, respectively. Mouse pDC express CD11c, B220, BST-2/Tetherin (mPDCA) and Siglec-H and are negative for CD11b. As components of the innate immune system, these cells express intracellular Toll-like receptors 7 and 9 which detect ssRNA and CpG DNA motifs, respectively. Upon stimulation and subsequent activation, these cells produce large amounts of type I interferon (mainly IFN-α (alpha) and IFN-β (beta)), which are critical pleiotropic anti-viral compounds mediating a wide range of effects.