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Secreted extracellular vesicles are categorized according to their physical characteristics. Exosomes are generally 50–100 nm in diameter and are released as a result of the fusion of a multivescular endosome with the plasma membrane.
Microvesicles are much larger, up to 1 µm (although they can be as small as 100 nm) and bud directly from the plasma membrane. Finally, membrane particles are smaller (50–80 nm) and are characterized by the presence of CD133 on their membranes.
Extracellular vesicles play important roles in adaptive immunity; B cells secrete endosomes containing peptide-MHC-II complexes, which selectively bind to T cells. In the nervous system, neurons, oligodendrocytes and microglia secrete extracellular vesicles, which are thought to play important roles in neuronal survival, myelin formation and neurite outgrowth. Finally, the presence of extracellular vesicles in most bodily fluids has generated a lot of interest in their use as disease biomarkers.
This pathway card lists the most important types of extracellular vesicles and summarizes the biogenesis mechanism, physical characteristics and most important protein markers for each type.