Exosomes: cell-derived vesicles that are present in many and perhaps all eukaryotic fluids, including blood, urine, and cultured medium of cell cultures. Exosomes are either released from the cell when multivesicular bodies fuse with the plasma membrane or released directly from the plasma membrane. Evidence is accumulating that exosomes have specialized functions and play a key role in processes such as coagulation, intercellular signaling, and waste management.