Type I IFNs display multiple biological effects. They have a strong antiviral action, not only directly but also indirectly through activation of the immune system. They may also have actions that are deleterious for the host. The cells that produce type I IFN are mostly plasmacytoid dendritic cells (pDC), but this depends on the viral stimulus. The migration and distribution of pDC into lymphoid organs, driven by chemokine interactions with their ligands, determines interaction with different cell types. In HIV infection, IFN production in vitro is impaired during primary infection and later in association with opportunistic infections. Circulating pDC numbers are decreased in parallel. These parameters may be used to help assess the prognosis of the disease and to monitor treatment.