Platelets and Their Role in Immunity: Formation, Activation and Activity, and Biologically Active Substances in Their Granules and Extracellular Vesicles

When presenting the role of platelets in immunity, the organelles and processes occurring within them are listed, and due to their lack of a cell nucleus, their specific transcriptional activity is discussed. Their formation, activation, and functional activity are also described, along with the characterization of elements important for shaping their intravascular activity, including immunity, such as their granules and extracellular vesicles (EVs). Presenting platelets in the context of their immune role, it is indicated that their activation and activity are complex processes resulting from the binding of their receptors with the endothelium of blood vessels, pattern recognition receptors (PRRs) of immune system cells, pathogen-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPs), and lifestyle-associated molecular patterns (LAMPs). As a result of these interactions, the inflammatory phenotype of platelets is promoted, making them not only the fundamental elements of homeostasis in blood vessels but also, above all, of immunity. Discussing the immunological role of platelets in blood vessels, biologically active substances contained in their five types of granules (α, δ, lysosomes, peroxisomes, and T), and two subtypes of EVs (exosomes and ectosomes-microvesicles) that determine their activity, including the immunological status, are characterized. Moreover, describing the role of platelets in blood vessels, it has been demonstrated that these cells are not only effective sentinels and regulators of these vessels, as previously assumed, but also exhibit strong pro- and anti-inflammatory, immunomodulatory, and regenerative effects, making them fundamental cellular elements determining intravascular immunity. It is also pointed out that by inducing an inflammatory environment in blood vessels, platelets can not only cause potential tissue damage but also emerge as potential cellular candidates for treating inflammatory diseases.