Platelets

When blood vessels are cut or damaged, the loss of blood from the system must be stopped before shock and possible death occur. This is accomplished by solidification of the blood, a process called coagulation or clotting. A blood clot consists of a plug of platelets (purple) and red blood cells enmeshed (trapped) in a network of insoluble fibrin molecules (blue).

http://www.med.upenn.edu/cellbio/faculty/weisel/