Plasma Soluble FcyRIIa

Soluble FcyRIIa, which is released from a-granules on platelet activation by thrombin, has been demonstrated in plasma (Gachet et al., 1995). However, the relative amount of membrane versus soluble FcyRIIa is fixed (Keller et al., 1993). Gachet and colleagues (1995) reported that approximately 2 ng of soluble FcyRIIa is produced from 109 platelets. This value equals 0.002 ng, or 300 molecules per platelet compared with roughly 5-10 times as many molecules on the platelet surface. Due to the low affinity, a much larger amount of plasma-soluble FcyRIIa would be needed to inhibit significantly PF4-H immune complexes from binding to platelet FcyRIIa. Moreover, plasma levels of soluble FcyRIIa are higher in patients with HIT than in heparin-treated or other nonthrombocytopenic controls, presumably as a marker of in vivo platelet activation in HIT (Saffroy et al., 1997).

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