Fibrinogen

Chemicals released by platelets lead to soluble fibrinogen in the blood plasma being converted into insoluble fibrin. Fibrin forms a mesh that traps blood cells, making a clot. The clot reduces blood loss and can dry into a scab, which helps prevent pathogens entering while new skin grows beneath it.

Platelets contain and release substances that start a series of clotting reactions when a blood vessel is damaged. These substances cause fibrinogen, a soluble protein in the plasma, to be converted into fibrin, an insoluble protein.