The coagulation cascade can be activated via extrinsic or intrinsic pathways. The extrinsic pathway is started by tissue factor (TF), which is normally expressed as a transmembrane protein in cells that are not directly in contact with the blood flow. When a blood vessel is damaged, TF is exposed to the blood and attaches to factor VII that is presented in blood and starts a common coagulation pathway. Activation of factor FXII initiates the intrinsic pathway upon binding to negatively charged surfaces, such as collagen and phospholipid. Subsequently, cleavage of FXI and FIX can use active FX and lead to fibrin formation [
28].
Plasmin degraded fibrin is the important regulator preventing clot formation and thrombosis. Fibrin clots like NETs can capture bacteria and prevent their invasion. In contrast to fibrin clots, NETs contain antimicrobial proteins that enable them to prevent the pathogens from spreading and kill them [
9,
29]. In an experiment in which whole blood was contained NETs, platelets recruited to NETs [
10]. The platelets recruitment may be due to binding to C3b deposits on NETs because platelets express complement receptor 1(CR1) on their membrane and or mediated by histones [
10,
30,
31]. NETs might lead to platelet aggregation as an essential step in clots formation. NETs contain histones, especially the type of H4 can activate platelets. Subsequently, the activated platelets releaseHMGB1that stimulate NETosis in a positive feedback loop. Additionally, histones stimulate platelets to secrete polyP from α-granules that activate FXII and blood coagulation's intrinsic pathway [
32,
33]. Decorated NETs with platelets may be suitable scaffolds for thrombus formation. In this regard, experiments on NETs in mice treated with DNase showed the formation of smaller thrombi [
34,
35]. The exact mechanism of coagulation activation by NETs remains unknown, and the separate components of NETs, including DNA and histones, have been demonstrated to induce thrombin formation [
32]. Experiments demonstrated that neutrophils treated with cytokines can upregulate TF mRNA and release TF on their NETs, resulting in thrombin formation [
36]. However, TF exposure on NETs apparently depends on the type of stimulation used to induce NETosis. Consequently, not all experiments have been shown NETs could produce TF and initiate coagulation [
9]. NETs can directly activate intrinsic coagulation cascade because NETs have negative charges and could bind and activate FXII and induce thrombin generation [
37]. In this regard, an in vitro study showed that inhibition of FXII or FXI in NETs reduced thrombin formation [
38]. So that NETs can start the initiation of coagulation via either intrinsic or extrinsic pathways[
9]. NE and, to a minor extent, cathepsin G as the NET-associated protein may contribute to fibrin formation on NETs because they could degrade TFPI. TFPI is the major extrinsic coagulation pathway inhibitor that recruited through nucleosome into the site of injury. [
39] The fibrin clots mixed with NETs have more resistant to fibrinolysis by plasmin that might be a critical point in NETs related thrombosis. [
40].