**1. Introduction**

Antiphospholipid syndrome (APS) is a systemic autoimmune disease characterized by thrombosis and/or obstetric complications and persistent presence of antiphospholipid antibodies (aPL) [1]. aPL cause the activation of cells involved in the vasculature (endothelial cells, platelets, monocytes) and the release of extracellular vesicles (EVs). EVs are submicron particles that are constitutively released from nearly all cell types [2] and circulate in plasma of healthy individuals in concentrations of approximately 1010 EVs/ml [3]. In response to stimuli, such as cell activation due to inflammation and/or apoptosis, increased amounts of EVs are released. The frequencies of plasma EVs, which originate from different cellular origins, can be altered in disease states [4]. Over the last decade, the number of scientific publications describing physiological and pathological functions of EVs has increased significantly. The term"extracellular vesicles" is a collective term that encompasses various subtypes of cell-releasing membranous structures called exosomes, microvesicles, microparticles, ectosomes, oncosomes, apoptotic bodies, and many others. The International Society for Extracellular Vesicles (ISEV) proposed Minimal Information for Studies of Extracellular Vesicles

("MISEV") guidelines for accurate isolation and characterizations of EVs [5]. MISEV2018 proposes the classification of EVs according to their physical properties (size and density), biochemical composition (protein marker positivity), cells of origin or based on the description of the conditions that induce their release. The heterogeneity of EVs research is, apart from nomenclature, also a reflection of poorly standardized methods of isolation and downstream analysis. Complex biological samples containing non-EV contaminants pose a challenge for both the isolation and characterization of EVs. Usually a combination of different methods is used to obtain good data quality. The most common EVs are of platelet or megakaryocyte origin (> 50%) [6], while about 5-15% of EVs are of endothelial origin [7]. An increase in circulating EVs, especially endothelial EVs, is considered a hallmark of vascular dysfunction and cardiovascular disease. Increased EVs are found particularly in patients with hypertension [8], diabetes [9], acute coronary syndromes [10] and cardiovascular disease [11]. EVs, especially medium to large endothelial EVs, have been studied in patients with APS, who had significantly higher levels of circulating endothelial and platelet EVs compared with healthy controls [12]. One study also reported increased levels of small EVs (sEVs), which are less than 200 nm in size, in the plasma of patients with APS [13]. In addition, they reported on an altered protein profile of sEVs, indicating platelet and endothelial activation. These results show that a complex systemic network that exists in the form of cell–cell communication via sEVs is altered in APS patients.
