Ultrastructure and immunogold labeling of MPs. Preparation of samples for electron microscopy is described in “Materials and methods.” MPs were isolated from PFP of a volunteer receiving endotoxin. In vivo-generated MPs are seen as vesicular structures with diameters ranging from 0.1 μm to 0.5 μm (A). MPs obtained by ultracentrifugation were incubated with rabbit polyclonal anti-TF antibody and/or anti-CD14 antibody or 1B10 anibody with appropriate secondary gold-labeled antibodies. In column i, positive gold-labeling to TF is demonstrated (B; 6 nm gold), CD14 (E; 12 nm gold), 1B10 (H; 12 nm gold). Microparticles double-labeled for 1B10 and TF (K), and CD14 and TF (N) are also shown. No labeling was observed with control antibodies (column ii) or secondary gold particle-labeled antibodies alone (column iii). Bars = 100 nm.