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How does exposed collagen help platelets adhere at an injury site?

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Fibrin in blood clotting — bleeding and blood clotting Red blood cells (erythrocytes) trapped in a mesh of fibrin threads. Fibrin, a tough, insoluble protein formed after injury to the blood vessels, is an essential component of blood clots.

When a blood vessel is injured, the protective endothelial lining is damaged, exposing the underlying connective tissue that contains fibrous collagen.

Platelets do not normally adhere to the vessel lining, but they are strongly attracted to exposed collagen at an injury site. This process is mediated by von Willebrand factor, a large plasma protein that acts as a bridge between the collagen and the platelets. Specifically, the von Willebrand factor binds to the collagen and to a receptor on the platelet membrane known as glycoprotein Ib.123

Once this initial adhesion occurs, the platelets undergo activation, changing their shape and extending pseudopodia to form a platelet plug. This interaction is a critical step in the hemostatic process, ensuring that platelets aggregate at the site of vascular injury to stop blood loss.12