Platelet aggregation in vivo is mediated by ?
Ans. is 'b' i.e, Ig mediators Platelets and hemostasis o Platelets play a central role in normal hemostasis. o After endothelial injury platelets come in contact with subendothelial ECM especially collegen. o On contact with ECM, platelets undergo three general reactions. Adhesion o Platelets adhesion to extracellular matrix is mediated largely interaction with Von-willebrand factor (VWF), which acts as a bridge between platelet surface receptor GPlb and exposed collegen. o Although platelets can also adhere to other components of ECM (fibronectin), GPIb - vWf association is the only interaction suffciently strong to overcome the high shear force of flowing blood. Secretion o Platelets contain two specific types of granules : - A)Alpha granule --> Contains fibrinogen, fibronectin, factors V & VIII, PDGF, TGF-(3 and platelet factor 4. B) Dens bodies or 5granules --> Contain ADP, Ca+2, histamine, serotonine, and epinephrine. Soon after adhesion, secretion of the contents of both granules occurs and secretion of thromboxane A2 also takes place. 3. Platelet aggregation It follows adhesion and secretion. The most impoant endogenous stimulus for platelet aggregation are ADP & TxA 2. o ADP & TxA, set up an autocatalytic reaction leading to buildup an enlarging platelet aggregate Primary hemostatic plug. o This primary plug is reversible. o Coagulation system is activated and thrombin is generated. o Thrombin binds to platelet surface receptors and along ADP & TXA2 cause furher aggregation of platelets and thrombin also conves fibrinogen to fibrin. o This followed by platelet contraction, creating an irreversible fused mass of platelet entangled in fibrin --> definitive secondary hemostatic plug. Factors that promote platelet aggregation (Pro-aggregatory factor) o ADP o Epinephrine o VWF o Thrombospondin o Immune complex o TXA2 o Serotonine o Fibrinogen o Collegen o Thrombin Now see each option of the question separately. o There is platelet contraction (not swelling). o Immune complex (Ig mediator) promote platelet aggregation. o Interaction among the leukocye contributes the inflammation that accompanies thrombosis, but has no role for platelet aggregation. o Interaction among platelet cause fuher aggregation of platelet. o Macromolecules (GPIIb/IIIa) promote platelet aggregation. Also know Platelet antiaggregatoiy factors PGI2 Endothelin NO 3 Bradykinin
Swelling
Ig mediators
Interaction among the leukocyte
All