Platelets Structures and Platelet Antigens
The glycoproteins of the surface coat are particularly important in the platelet reactions of adhesion and aggregation which are the initial events leading to platelet plug formation during haemostasis. Adhesion to collagen is facilitated by glycoprotein Ia. Glycoproteins Ib (defective in Bernard-Soulier syndrome) and IIb/IIIa (defective in thrombasthenia) are important in the attachment of platelets to von Willebrand factor and hence to vascular subendothelium. The binding site for IIb/IIIa is also the receptor for fibrinogen which is important in platelet-platelet aggregation.
The plasma membrane invaginates into the platelet interior to form an open membrane (canalicular) system which provides a large reactive surface to which the plasma coagulation protein may be selectively absorbed.
The membrane phospholipids (previously known as platelet factor 3) are of particular importance in the conversion of coagulation factor X and Xa and prothrombin (factor II) to thrombin (IIa), (Hoffbrand, 2010.).In platelet interior calcium, nucleotides (particularly adenosine disphosphate (ADP) and adenosine tri phosphate (ATP) and serotonin are contained in electron dense-granules.
The more frequent specific alpha granules contain a heparin antagonist, platelet-derived growth factor (PDGF), Beta-thrombolasbuin, fibrinogen, von Willebrand factor and other clotting factors. Dense granules are less common and contain APD, ATP, 5-hydroxytrypatamine (5-HT) and calcium.
Other specific enzymes and peroxisomes which contain catalase. During the release reaction contained below the contents of the granules are discharged into the open canalicular system.
PLATELET ANTIGENS in Platelets Structures
Several platelet surface proteins have been found to be important antigens in platelet specific autoimmunity and they have been termed human platelet antigens (HPA). In most cases two different alleles exist, termed a or b alleles, exampled HPA-Ia. Platelets also express ABO and human leukocyte antigen (HLA) class I but not class II antigens.