Why do scabs form?
If blood was like water, it would drain away through cuts and grazes. Instead, small leaks are quickly plugged, bringing bleeding to a halt. This happens because blood contains chemical called fibrinogen. When a blood vessel is cut, the fibrinogen turn into another chemical, called fibrin, which creates a maze of sticky strands. Blood cells get trapped in the strands, making a solid plug called a clot. Once a clot has formed, the skin underneath starts to heal. Eventually, the clot dries out to form a scab, which later falls off. By this time, the injury has been fully repaired.