['High-affinity IgE receptor (Fc epsilon RI) expression on blood dendritic cells reportedly correlates with serum IgE levels. Our studies demonstrate that plasmacytoid dendritic cells (pDCs) secrete pro-inflammatory cytokines (IL-6, TNF-alpha) following Fc epsilon RI stimulation - a mode of activation that simultaneously reduces expression of Toll-like receptor 9 (TLR9). Whether or not TLR9 and/or Fc epsilon RI levels and their function on dendritic cells relate to allergic status is unknown.', 'The aim of this study is to compare the innate (TLR9-mediated) immune response of human pDCs to TLR9 and Fc epsilon RI alpha receptor expression in allergic and non-allergic subjects.', 'Basophil-depleted mononuclear cell fractions containing pDCs were prepared from peripheral blood of allergic and non-allergic subjects. Intracellular TLR9 and surface Fc epsilon RI alpha expression in blood dendritic cell antigen-2-positive cells were determined by flow cytometry. Activating anti-IgE antibody, anti-Fc epsilon RI alpha antibody, and TLR9 agonist were used to stimulate cell suspensions, with cytokine levels determined by ELISA.', 'No difference in the frequency of pDCs was detected among allergic (n=9) vs. non-allergic (n=11) subjects (P=0.261). While there was also no difference in the baseline expression of TLR9, pDCs from allergic subjects produced sixfold less IFN-alpha when stimulated with CpG (P=0.002). Conversely, there was higher Fc epsilon RI alpha expression (P=0.01) on the pDCs of allergic subjects.'] Do human blood dendritic cells from allergic subjects have impaired capacity to produce interferon-alpha via Toll-like receptor 9?