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Platelets, or thrombocytes, are cell-like structures that stick together to help form blood clots. They activate when brought into contact with collagen exposed when the endothelial blood vessel lining is damaged. Once activated they release a number of coagulation factors and bind together and to the forming fibrin clot.

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 are separated from donated blood using a centrifuge. This is necessary because platelets do not survive the cold temperatures used to keep red blood cells, so they must be stored separately. People with few platelets or platelets that do not work well benefit from such donations. Platelet concentrate 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"

platelet diseases

• disorders of platelet number
  • thrombocytopenia
  • thrombocytosis, including benign essential thrombocytosis
• disorders of platelet adhesion
  • Von Willebrand's disease
  • Bernard-Soulier syndrome
• disorders of platelet aggregation
  • Glanzmann's thrombasthenia
• disorders of platelet release
  • decreased cyclooxygenase activity, induced or congenital
  • granule storage pool defects, acquired or congenital
• disorders of platelet coagulant activity
  • Scott's syndrome
• related disorders
  • idiopathic thrombocytopenic purpura
  • thrombotic thrombocytopenic purpura

See also:

• hemostasis

• Blood and immune system cells
• Coagulation system