Factor XI and Platelets: Evidence that Platelets Contain only Minimal Factor XI Activity and Antigen

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Abstract
Factor-XI activity of platelets [human] was studied in platelet-rich plasmas and isolated platelet suspensions. Fresh platelets in both environments had little or no measurable factor-XI activity. Frozen and thawed platelet-rich normal plasma had markedly elevated apparent factor-XI activity and factor-IX activity as compared to platelet-poor plasma. Frozen and thawed platelet-rich and platelet-poor normal plasmas had equivalent factor-XI antigen. Platelets isolated from normal blood and from factor-XI deficient blood had the same small amounts of apparent factor-XI activity, which increased slightly on freezing and thawing. Minimal factor XI is apparently associated with the platelet. The markedly elevated apparent factor-XI activity of frozen and thawed platelet-rich plasma reflects the interaction of a platelet activator with plasma clotting factors to produce a later activated-clotting-intermediate.