**2. Extracellular vesicles**

Extracellular vesicles are small particles composed of a phospholipid bilayer that encloses soluble cytosolic or endosomal material and nuclear components and, unlike a cell, are unable to replicate. EVs can be as small as the smallest physically possible unilamellar liposome (about 20-30 nm) or as large as 1 μm or more [14]. EVs serve as regulators of the transfer of biological information (proteins, nucleic acids, lipids and metabolites), which act both locally and remotely [15]. EVs are found in a variety of human biofluids including serum, plasma, urine, saliva, breast milk, amniotic fluid, ascites fluid, cerebrospinal fluid and even bile [16]. Under normal physiological conditions, they are continuously secreted into the extracellular environment, however, the amount of EVs is increased by activated and apoptotic cells and is associated with different pathologies, including thrombosis [7]. EVs are probably the most extensively studied in cancer and were also found to play a significant role in cancer-associated thrombosis [17]. Over the last decade, EVs have been extensively studied in the field of biomedical research to determine their biological role in normal physiology and in disease state as well as to exploit potential clinical applications in the diagnosis and prognosis of disease. EVs are considered a promising source of biomarkers since they carry different biological materials that reflect the status of the cell of origin. Nevertheless, EVs have also been considered as a therapeutic agent, as an alternative to their synthetic counterparts, such as liposomes [18].

### **2.1 Classification of EVs**

The classification and nomenclature of EVs is complicated and could be confusing due to overlapping definitions. The most common classification of EVs currently used in the literature is the classification of different EVs into subtypes, such as endosomal derived exosomes, membrane derived (microparticles, microvesicles or ectosomes) and apoptotic bodies. This classification is based on the *Extracellular Vesicles: Intercellular Communication Mediators in Antiphospholipid Syndrome DOI: http://dx.doi.org/10.5772/intechopen.97412*

assignment of a specific EV to a particular biogenesis pathway, which remains very difficult to assess [19]. Unless biogenesis is investigated directly, EVs are classified according to their a) physical characteristics such as size: "small EVs" (sEVs; size <100 nm or < 200 nm) and "medium/large" (m/lEVs; size >200 nm), and density; low, medium, high, with defined range, b) biochemical composition (surface expression or by the presence of a specific molecule within EVs), or c) description of a specific condition or cell of origin (**Figure 1**) [19].
