Exosomes, 40-100 nm sized extracellular vesicles, are enriched in endosome-derived components. 1,2 Exosomes can be secreted by almost all types of cells such as fibroblasts, macrophages, tumor cells, and mesenchymal stem cells, and they can be found in amniotic fluid, milk, urine, ascites fluid, blood, cerebrospinal fluid, and even saliva. 3,4 Exosomes were first discovered in mature mammalian reticulocytes by Johnstone in 1987. 5 After decades of researches, scientists have discovered that exosomes have unique structures. First, exosomes are mostly "cup-shaped" or "circular," with a density of 1.13-1.19 g/ml. 3 Second, membranes of exosomes are rich in lipid rafts, cholesterol, sphingomyelins, and ceramides and have a variety of transmembrane proteins on its surface. Third, as an endogenous vector, exosomes contain functional proteins, mRNAs, and microR-NAs (miRNAs), DNAs and other signaling molecules. 6,7 Fourth, surface proteins and other related receptors of exosomes can reflect specific biological sources, for instance, CD9, CD63, and CD81. 8 With the gradual understanding of exosomes, their functions have gone from "junk" to "supernova". 9 The reasons are as follows:Foremost, in the body's immune response, exosomes are responsible for antigen presentation, stimulation of T-cell proliferation, and activation of immune response. 10 Next, miRNAs carried by exosomes can be used as biomarkers for the early diagnosis and prognosis of diseases, for instance, breast cancer, colorectal cancer, and prostate cancer. [11][12][13] Furthermore, because of the absence of cellular structure, exosomes are considered as ideal biological drug delivery
