Platelets play a significant role in primary hemostasis, as they are the "bricks", or building blocks of the developing primary platelet plug, the forerunner to the end stage fibrin clot. Platelets have inherent adhesive properties, which are essential for adherence to the site of vascular damage, and for binding to one another in aggregation activities.
Platelets must be present in sufficient numbers, and be functionally active for optimal clotting to occur. Platelet functionality tends to be more crucial than the number of platelets available however, as patients with moderately decreased platelet counts can still form clots relatively effectively as compared to those patients with functional platelet defects.
The image to the right shows a highly magnified cluster of platelets. Notice their spiny shapes, which allow them to adhere to one another and a damaged vessel wall.