Exosomes are small vesicles originating from late endosomes and are released in the extracellular medium by a broad array of cells. They were initially described by Johnstone et al.[1] in reticulocytes where multi-vesicular bodies (MVBs) fused with the plasma membrane in an exocytic manner. This exosome secretion was later shown to occur in a wide variety of cell types, including B lymphocytes, mastocytes, platelets, cytotoxic T lymphocytes and epithelial cells [2–6]. The most striking finding of initial reports focusing on exosome biology was that they concentrate molecules involved in the function of the cells. For example, exosomes derived from red blood cells concentrate transferrin receptor, while exosomes derived from B cells concentrate major histocompatibility (MHC) class II molecules. From these data the hypothesis emerged that dendritic cells (DC) could release immunogenic exosomes and that tumor cells could release exosomes enriched in tumor antigens.