Plasmacytoid dendritic cells (pDC) are a rare subtype of circulating dendritic cells found in the blood as well as in peripheral lymphoid organs. These cells express the surface markers CD123, BDCA-2(CD303) and BDCA-4(CD304), but do not express CD11c or CD14, which distinguishes them from conventional dendritic cells or monocytes, respectively. As components of the innate immune system, these cells express intracellular Toll-like receptors 7 and 9, which enable the detection of viral and bacterial nucleic acids, such as ssRNA or CpG DNA motifs. 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.

The number of circulating pDCs are found to be declined during chronic HIV infection as well as HCV infection.