Itt írjon a(z) PlateletImmunity-ról/rő
Platelets
Introduction
Platelets Platelets are a unique type of mammalian blood cell. They are anucleate molecules which play an integral role in host homeostasis, immune defence and repair. At sites of infection or injury in a host organism, platelets will be deployed rapidly where they will display their role in defence and signalling. Platelets are activated by agonists, of which there are several. Thrombin, collagen, ADP, platelet activating factor (PAF) and other factors all promote the activation of platelets. There are two types of platelet activation, primary and secondary, both of which lead to platelet aggregation. Primary platelet aggregation occurs when the specialised surface receptors of platelets bind to the negative charges of injured endothelial area (exposed collagen) whilst secondary platelet activation refers to activation of the platelets by their actin and microtubule system as well as by their secretion of coagulation factors. The potency of platelet agonist varies depending on the activation response
Hemostasis
Platelets' foremost role is hemostasis. It not only helps the animal from blood loss but it also prevents microorganisms from entering the animal which may cause disease. The role of Platelets in homeostasis would suggest evolution from more primitive multifunctional innate defensive cells as although highly differentiated they have inflammatory and antimicrobial systems and link clotting and immune cascades (A.S Weyrich et al). Hemostasis is crucial to animals, when a blood vessel of theirs becomes damaged or injured. It is the first stage of wound healing. Platelets are present in the blood circulation at a volume of 2-8 x 1011/l, depending on the species. They are unable to move themselves so they passively move with the blood circulation (page 131). After an injury to a blood vessel occurs, vascular constriction is the organisms initial reaction. Platelets are not usually attracted to endothelial surfaces. When an injury of a blood vessel happens, platelets combine with their specialized surface receptors to the negative charges of the endothelial regions. This is primary aggregation (page 200-201). Also, Von Willebrand factor, which is a multimeric glycoprotein released by the endothelial cells plays a key role in the adhesion of platelets to the subendothelium.(M. Hermmann et al). This allows platelet aggregation to increase. Secondary activation follows. The microtubule system which is located beneath the plasma membrane and the actin cytoskeleton system of the platelets activate the reaction (S. Severin et al). This causes white thrombosis to arise. This happens when by filopodia are produced which provides the framework for platelets to attach to each other and also collagen fibres. Endothelial production of prostacyclines and nitrogen monoxyde ceases. Their role is to prevent the production of factors responsible for aggregation. Accordingly, the thrombocytes can now produce their own stimulating factors. These include TXA2, serotonin, adenosine-diphosphates that will stimulate other thrombocytes increasing the aggregation as well as the secretion of coagulation factors (page 201 and 202). This shows how platelets are pivitol in hemostasis. Without them, a relatively innocuous injury to a blood vessel may be fatal. They help heal wounds and by doing this help to prevent harmful microorganisms which could be disease causing from entering the animal. If these disease causing microorganisms were to off got into the body the animal would have to endure processes such as the production of antibodies to fight the infection. These processes are energy consuming and put the animal under stress. Therefore platelet's role in hemostasis can be looked upon as a temporary mechanical barrier between the organism and the outer environment, reducing the threat of disease when an animal is wounded.