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Platelets, or thrombocytes, are cell-like structures that stick together to form blood clots. They join together when exposed to the air, as in a cut or disturbance of a blood vessel, and they also bind to the collagen that is exposed when endothelial cell lining a blood vessel are damaged. Platelets are not cells in the conventional sense, but are fragmented pieces of the cytoplasm of megakaryocytes in the bone marrow, released from the bone marrow into the bloodstream.

Platelets can be separated from donated blood by using a centrifuge. This is necessary because platelets do not survive the normal storage used for red blood cells, so they must be stored separately. People who need additional clotting agents (e.g. hemophiliacs) can benefit from such donations. A (see-through) bag of platelets is pale orange.

A normal platelet count in a healthy person is between 150 and 400 (x 10/L of blood). 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. Platelets can be transfused if a patient's platelet count falls too low. A low platelet count is called thrombocytopenia, having too many platelets is called thrombocytosis.

Thrombocytopenia may be caused by the drugs that stimulate the production of antibodies against platelets. This condition is called "Drug-induced immune thrombocytopenia"

See also:

• Hemostasis