Immunoelectron microscopy analysis of pDCs during activation by flu. Ultrathin cryosections from pDCs exposed for 0, 3, or 24 hours to flu were prepared. Sections were then labeled using 10-nm and 15-nm immunogold particles for MHC II and LAMP-1 molecules, respectively. The 3 large sections of pDC presented (time 0, 3, and 24 hours) are electron microscopy images, which have been obtained using the Multiple Image Alignment function of the iTEM software (Eloïse; Electron Optics Instrument Service, West Orange, NJ). The 2 images on the bottom right show enlargements of typical MHC II⁺ compartments after 24 hours of flu exposure.