(red blood cells, plasma, and platelets) are transfused to address severe hemorrhage. platelet transfusion is contraindicated in thrombotic thrombocytopenic purpura (ttp), as it fuels the coagulopathy. collection platelet concentrate platelets are either isolated from collected units of whole blood and pooled to make a therapeutic dose, or collected by platelet apheresis: blood is taken from the donor, passed through a device which removes the platelets, and the remainder is returned to the donor in a closed loop. the industry standard is for platelets to be tested for bacteria before transfusion to avoid septic reactions, which can be fatal. recently the aabb industry standards for blood banks and transfusion services (5.1.5.1) has allowed use of pathogen reduction technology as an alternative to bacterial screenings in platelets. pooled whole-blood platelets, sometimes called "random" platelets, are separated by one of two methods. in the us, a unit of whole blood is placed into a large centrifuge in what is referred to as a "soft spin". at these settings, the platelets remain suspended in the plasma. the platelet-rich plasma (prp) is removed from the red cells, then centrifuged at a faster setting to harvest the platelets from the plasma. in other regions of the world, the unit of whole blood is centrifuged using settings that cause the platelets to become suspended in the "buffy coat" layer, which includes the platelets and the white blood cells. the "buffy coat" is isolated in a sterile bag, suspended in a small amount of red blood cells and plasma, then centrifuged again to separate the platelets and plasma from the red and white blood cells. regardless of the initial method of preparation, multiple donations may be combined into one container using a sterile connection device to manufacture a single product with the desired therapeutic dose. apheresis platelets are collected using a mechanical device that draws blood from the donor and centrifuges the collected blood to separate out the platelets and other components to be collected. the remaining blood is returned to the donor. the advantage to this method is that a single donation provides at least one therapeutic dose, as opposed to the multiple donations for whole-blood platelets. this means that a recipient is not exposed to as many different donors and has less risk of transfusion-transmitted disease and other complications. sometimes a person such as a cancer patient who requires routine transfusions of platelets will receive repeated donations from a specific donor to further minimize the risk. pathogen reduction of platelets using for example, riboflavin and uv light treatments can also be carried out to reduce the infectious load of pathogens contained in donated blood products, thereby reducing the risk of transmission of transfusion-transmitted diseases. another photochemical treatment process utilizing amotosalen and uva light has been developed for the inactivation of viruses, bacteria, parasites, and leukocytes that can contaminate blood components intended for transfusion. in addition, apheresis platelets tend to contain fewer contaminating red blood cells because the collection method is more efficient than "soft spin" centrifugation at isolating the desired blood component. storage platelets collected by either method have a very short shelf life, typically five days. this results in frequent problems with short supply, as testing the donations often requires up to a full day. since there are no effective preservative solutions for platelets, they lose potency quickly and are best when fresh. platelets are stored under constant agitation at 20–24 °c (68–75.2 °f). units can not be refrigerated as this causes platelets to change shape and lose function. storage at room temperature provides an environment where any bacteria that are introduced to the blood component during the collection process may proliferate and subsequently cause bacteremia in the patient. regulations are in place in the united states that require products to be tested for the presence of bacterial contamination before transfusion. platelets collected by using apheresis at an american red cross donation center delivery to recipients platelets do not need to belong to the same a-b-o blood group as the recipient or be cross-matched to ensure immune compatibility between donor and recipient unless they contain a significant amount of red blood cells (rbcs). the presence of rbcs imparts a reddish-orange color to the product and is usually associated with whole-blood platelets. an effort is sometimes made to issue type specific platelets, but this is not critical, as it is with rbcs. prior to issuing platelets to the recipient, they may be irradiated to prevent transfusion-associated graft versus host disease or they may be washed to remove the plasma if indicated. the change in the recipient's platelet count after transfusion is termed the "increment" and is calculated by subtracting the pre-transfusion platelet count from the post-transfusion platelet count. many factors affect the increment including the recipient's body size, the number of platelets transfused, and clinical features that may cause premature destruction of the transfused platelets. when recipients fail to demonstrate an adequate post-transfusion increment, this is termed platelet transfusion refractoriness. platelets, either apheresis-derived or random-donor, can be processed through a volume reduction process. in this process, the platelets are spun in a centrifuge and the excess plasma is removed, leaving 10 to 100 ml of platelet concentrate. such volume-reduced platelets are normally transfused only to neonatal and pediatric patients when a large volume of plasma could overload the child's small circulatory system. the lower volume of plasma also reduces the chances of an adverse transfusion reaction to plasma proteins. volume reduced platelets have a shelf life of only four hours. wound repair main article: wound repair the blood clot is only a temporary solution to stop bleeding; tissue repair is needed. small interruptions in the endothelium are handled by physiological mechanisms; large interruptions by the trauma surgeon. the fibrin is slowly dissolved by the fibrinolytic enzyme, plasmin, and the platelets are cleared by phagocytosis. platelets release platelet-derived growth factor (pdgf), a potent chemotactic agent; and tgf beta, which stimulates the deposition of extracellular matrix; fibroblast growth factor, insulin-like growth factor 1, platelet-derived epidermal growth factor, and vascular endothelial growth factor. local application of these factors in increased concentrations through platelet-rich plasma (prp) is used as an adjunct in wound healing. other animals instead of having platelets, non-mammalian vertebrates have nucleated thrombocytes, which resemble b lymphocytes in morphology. they aggregate in response to thrombin, but not to adp, serotonin, nor adrenaline, as platelets do. history george gulliver in 1841 drew pictures of platelets using the twin lens (compound) microscope invented in 1830 by joseph jackson lister. this microscope improved resolution sufficiently to make it possible to see platelets for the first time. william addison in 1842 drew pictures of a platelet-fibrin clot. lionel beale in 1864 was the first to publish a drawing showing platelets. max schultze in 1865 described what he called "spherules", which he noted were much smaller than red blood cells, occasionally clumped, and were sometimes found in collections of fibrin material. giulio bizzozero in 1882 studied the blood of amphibians microscopically in vivo. he named schultze's spherules (it.) piastrine: little plates. an article in scientific american suggests bizzozero proposed the name blutplattchen. william osler observed platelets and, in published lectures in 1886, called them a third corpuscle and a blood plaque; and described them as "a colorless protoplasmic disc". james wright examined blood smears using the stain named for him, and used the term plates in his 1906 publication but changed to platelets in his 1910 publication which has become the universally accepted term. the term thrombocyte (clot cell) came into use in the early 1900s and is sometimes used as a synonym for platelet; but not generally in the scientific literature, except as a root word for other terms related to platelets (e.g. thrombocytopenia meaning low platelets).: v3 the term thrombocytes are proper for mononuclear cells found in the blood of non-mammalian vertebrates: they are the functional equivalent of platelets, but circulate as intact cells rather than cytoplasmic fragments of bone marrow megakaryocytes.: 3 in some contexts, the word thrombus is used interchangeably with the word clot, regardless of its composition (white, red, or mixed). in other contexts it is used to contrast a normal from an abnormal clot: thrombus arises from physiologic hemostasis, thrombosis arises from a pathologic and excessive quantity of clot. in a third context it is used to contrast the result from the process: thrombus is the result, thrombosis is the process.