Platelets are disc-shaped cellular fragments that circulate in the blood and play an important role in clot formation. They help to prevent bleeding. If a blood vessel is damaged (e.g. by a cut), the platelets gather at the site of injury, stick together and form a plug to help stop the bleeding.
The normal adult platelet count varies between 150 and 400 x 109/L
Thrombocytopenia is the term used to describe a reduction in the platelet count to below normal. If your platelet count drops below 20 (20 x 109/L), you are at risk of bleeding and you tend to bruise easily. Platelet transfusions are sometimes given to bring the platelet count back to a safe level.