Platelets are tiny disc-shaped pieces of cells in your blood that help stop bleeding when you get cut or injured. When you scrape your knee or get a paper cut, platelets rush to the damaged spot and stick together like microscopic sandbags, forming a plug that blocks the wound. This plug, combined with other blood proteins, creates what you see as a scab.

Platelets are much smaller than red blood cells (the cells that carry oxygen) or white blood cells (the cells that fight infection). Your body produces them in your bone marrow, and you have about 150,000 to 450,000 platelets in every microliter of blood. They only live for about ten days, so your body constantly makes fresh ones.

Without enough platelets, even small injuries could be dangerous because your blood wouldn't clot properly. People with certain medical conditions might have too few platelets and bruise easily, or they might need platelet transfusions before surgery.