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 CD11c

CD11c

CD11c, also known as Integrin, alpha X , is a human gene.CD11c is a type I transmembrane protein found at high levels on most human dendritic cells, but also on monocytes, macrophages, neutrophils, and some B cells that induces cellular activation and helps trigger neutrophil respiratory burst;...

 or CD14

CD14

Cluster of differentiation 14 also known as CD14 is a human gene.The protein encoded by this gene is a component of the innate immune system. CD14 exists in two forms. Either it is anchored into the membrane by a glycosylphosphatidylinositol tail or it appears in a soluble form...

, which distinguishes them from conventional dendritic cells or monocytes, respectively. Mouse pDC express CD11c, B220, BST-2 (mPDCA) and Siglec-H and are negative for CD11b.
As components of the innate immune system

Innate immune system

The innate immune system, also known as non-specific immune system and secondary line of defence, comprises the cells and mechanisms that defend the host from infection by other organisms in a non-specific manner...

, these cells express intracellular Toll-like receptors 7 and 9 which detect ssRNA and CpG

CpG

CpG can be:*CpG site - methylated sequences of DNA significant in gene regulation*CpG Oligodeoxynucleotide - unmethylated sequences of DNA that have immunostimulatory properties*CpG island - regions of DNA that contain several CpG sites...

 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.

The number of circulating pDCs are found to be decreased during chronic HIV

HIV

Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...

 infection as well as HCV

HCV

HCV may mean the following:*Hepatitis C virus*Human Coronavirus*Hypersonic Cruise Vehicle, a target for DARPA's FALCON program*Heavy commercial vehicle*Higher Calorific Value...

infection.