**2.3.2 Molecular interactions of Platelet MP**

Platelet MP also bear a number of antigens such GPIIbIIIa, GPIa, von Willebrand's factor and arachidonic acid which may all be important effectors in the clotting mechanism. The understanding of the molecular mechanisms of haemostasis has now led to the thinking that coagulation can be described as an interaction between p-selection, tissue factor thrombin and microparticles (Furie and Furie 2004). P-selectin is an adhesion molecule expressed at the platelet endothelial interface which is thought to be critical for tissue factor activity and leukocyte adhesion in the thrombus (Myers, 2003). Some authors have even described Pselectin on microparticles, tissue factor and clotting proteins as being the molecular triad for coagulation (Polgar, 2005).

Another potential role of MP may be in the interaction of endothelium, von Willebrand's factor and platelets.The platelet derived microparticles could interact with the protease ADAMTS-13 (**A D**isintegrin **A**nd **M**etalloproteinase with **T**hrombo**S**pondin-1-like motifs, member **13** of this family of metalloprotease) , which regulates the activity of high molecular weight von Willebrand's factor. Increased microparticles in circulation could potentially compete in binding ADAMTS-13, reducing its interaction with the endothelium and influencing multimer cleavage (Jy*, et al* 2005). This may then contribute to the increased rates of thrombosis observed in these patients with thrombotic thrombocytopaenic purpura though the evidence for this process is very preliminary.
