Platelet aggregation

Once a primary layer of adhesive platelets have covered the exposed subendothelial matrix, subsequent adhesion will continue but in the form of aggregation, where platelets from flowing blood bind to adhered platelets at the wound site. The mechanism of platelet aggregation is mainly attributed to the integrin αIIbβ3 (also known as GPIIb/IIIa) receptor. The main ligand of integrin αIIbβ3 is fibrinogen but the receptor also have affinity for multimeric vWF, vitronectin, fibronectin and thrombospondin [22]. The mechanism of aggregation involves two integrin αIIbβ3 receptors on different platelets that bind to the same fibrinogen molecule. The αIIbβ3 receptor has a low and a high affinity state for binding fibrinogen. The high affinity state is achieved by inside-out signaling when platelets are activated [23]. The αIIbβ3 receptor can also mediate outside-in signaling by binding fibrinogen [24].