Platelet Factor 4 (PF4) (Last Updated - 3/6/2005)

-pro-coagulant
-found on the platelet surface
-can cause Heparin resistance via competitive inhibition of heparin (or heparan sulfate) / anti-thrombin binding
-promotes localized clot formation @ the site of vessel injury

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References:

Goodman & Gilman's: The Pharmacologic Basis Of Therapeutics - 10th Edition - 2001. Chapter 55.