Platelet adhesion is the process by which platelets adhere to exposed collagen and other components of the extracellular matrix at sites of vascular injury, primarily mediated by von Willebrand factor. This initial step is crucial for the formation of a platelet plug, which prevents excessive bleeding. Impaired platelet adhesion can lead to bleeding disorders, such as von Willebrand disease, highlighting its importance in maintaining hemostasis and vascular integrity.