Platelet

Platelets are the cells that stick together to form blood clots. Normally they don't stick together, but when there is a disturbance in the blood vessels (such as a broken vessel) then they stick together at that point and prevent blood from coming out of the vein or artery.

Platelets can be separated from the rest of donated blood and given to patients who need them. A (see-through) bag of them is pale orange.

A normal platelet count in a healthy person is between 200 and 400. (Actually that's 200 to 400 bazillion, maybe someone can clarify). People can live independently with a count as low as 20. People can live in hospital with a count as low as 5, but spontaneous bleeding gets to be a problem. That's when platelet transfusions have to be done.