Sera of stem cell–transplanted patients contain DC-reactive antibodies. Allogeneic immature (top panels) and mature (bottom panels) mdDCs were probed with sera (final concentration 1:200) of stem cell–transplanted persons by FACS (gray histograms). Sera of healthy persons (thick line) and phosphate-buffered saline (thin line) were used as controls. Bound antibodies were detected with PE-labeled goat anti–human IgG antibodies.