PDC and viral infectious disease: In this prospective study, using the Simoa technology, we confirmed that type I IFN response was elevated in patients with severe and critical COVID-19 pneumonia, compared with healthy controls (14–17, 19, 20, 29) correlated with plasma viral load, and associated with a decrease in pDC in blood, perhaps due to migration of these cells into the lung, the active site of the viral infection, as demonstrated for HIV infection (30, 31).