40.2D: Platelets and Coagulation Factors - Biology

40.2D: Platelets and Coagulation Factors - Biology

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Platelets and coagulation factors are instrumental in plugging damaged blood vessel walls and stopping blood loss.

Learning Objectives

  • Describe the roles played by platelets and coagulation factors

Key Points

  • Platelets (thrombocytes) are small, anucleated cell fragments that result from the disintegration of megakaryocytes.
  • Under normal conditions, blood vessel walls produce chemical messengers that inhibit platelet activation, but, when injured, they expose collagen, releasing factors that attract platelets to the wound site.
  • Activated platelets stick together to form a platelet plug, which activates coagulation factor proteins found in the blood to further enhance the response to injury by strengthening the plug with fibrin.
  • Vitamin K is necessary for the proper function of many coagulation factors; a deficiency is detrimental to blood clotting.
  • Platelets can become activated and form clots in situations with non-physiological flow caused by disease or artificial devices.

Key Terms

  • collagen: Any of more than 28 types of glycoprotein that forms elongated fibers, usually found in the extracellular matrix of connective tissue.
  • clot: a solidified mass of blood
  • stenosis: an abnormal narrowing or stricture in a blood vessel or other tubular organ

Platelets and Coagulation Factors

Blood must form clots to heal wounds and prevent excess blood loss. Small cell fragments called platelets (thrombocytes) are formed from the disintegration of larger cells called megakaryocytes ( a). For each megakaryocyte, 2000–3000 platelets are formed with 150,000 to 400,000 platelets present in each cubic millimeter of blood. Each platelet is disc shaped and 2–4 μm in diameter. They contain many small vesicles, but do not contain a nucleus.

The inner surface of blood vessels is lined with a thin layer of cells (endothelial cells) that under normal situations produce chemical messengers that inhibit platelet activation. When the endothelial layer is injured, collagen is exposed, releasing other factors to the bloodstream which attracts platelets to the wound site. When the platelets are activated, they clump together to form a platelet plug (fibrin clot) ( b), releasing their contents. The released contents of the platelets activate other platelets and also interact with other coagulation factors. Coagulation factors (clotting factors) are proteins in the blood plasma that respond in a complex cascade to convert fibrinogen, a water-soluble protein present in blood serum, into fibrin, a non-water soluble protein, which strengthens the platelet plug. Many of the clotting factors require vitamin K to function. Vitamin K deficiency can lead to problems with blood clotting. The plug or clot lasts for a number of days, stopping the loss of blood.

Outside of the body, platelets can also be activated by a negatively-charged surface, such as glass. Non-physiological flow conditions (especially high values of shear stress) caused by arterial stenosis or artificial devices (e.g. mechanical heart valves or blood pumps) can also lead to platelet activation.

Blood Clotting: Mechanisms and Stages | Blood | Hematology | Biology

In this article we will discuss about the mechanisms and stages of blood clotting.

Mechanism of Blood Clotting:

Blood Clotting is one of three mechanisms that reduce the loss of blood from broken blood vessels.

The three Mechanisms are:

The smooth muscle in blood vessel walls contracts immediately the blood vessel is broken. This response reduces blood loss for some time, while the other haemostatic mechanisms become active.

ii. Platelet Plug Formation:

When blood platelets encounter a damaged blood vessel they form a “platelet plug” to help to close the gap in the broken blood vessel. (The key stages of this process are called platelet adhesion, platelet release reaction, and platelet aggregation)

Following damage to a blood vessel, vascular spasm occurs to reduce blood loss while other mechanisms also take effect. Blood platelets congregate at the site of damage and amass to form a platelet plug. This is the beginning of the process of the blood “breaking down” from its usual liquid form in such a way that its constituents play their own parts in processes to minimize blood loss.

Blood normally remains in its liquid state while it is within the blood vessels but when it leaves them the blood may thicken and form a gel (coagulation). Blood clotting (technically “blood coagulation”) is the process by which (liquid) blood is transformed into a solid state.

This blood clotting is a complex process involving many clotting factors (incl. calcium ions, enzymes, platelets, damaged tissues) activating each other.

Stages of Blood Clotting:

1. Formation of Prothrombinase:

Prothrombinase can be formed in two ways, depending of which of two “systems” or “pathways” apply.

This is initiated by liquid blood making contact with a foreign surface, i. e. something that is not part of the body or

This is initiated by liquid blood making contact with damage tissue.

Both the intrinsic and the extrinsic systems involve interactions between coagulation factors. These coagulation factors have individual names but are often referred to by a standardised set of Roman Numerals, e.g. Factor VIII (anti-haemophilic factor), Factor IX (Christmas factor).

2. Prothrombin Converted Into the Enzyme Thrombin:

Prothrombinase (formed in stage 1.) converts prothrombin, which is a plasma protein that is formed in the liver, into the enzyme thrombin.

3. Fibrinogen (Soluble) Converted to Fibrin (Insoluble):

In turn, thrombin converts fibrinogen (which is also a plasma protein synthesized in the liver) into fibrin.

Fibrin is insoluble and forms the threads that bind the clot

There are two pathways that lead to the conversion of prothrombin to thrombin:

(1) The intrinsic pathway and

(1) Intrinsic Pathway:

The intrinsic pathway, which is triggered by elements that lie within the blood inself (intrinsic to the blood), occurs in the flowing way. Damage to the vessel wall stimulates the activation of a cascade of clotting factors (for the sake of simplicity we will not consider the individual factors). This cascade results in the activation of factor X.

Activated factor X is an enzyme that converts prothrombin to thrombin. Thrombin converts fibrinogen to fibrin monomers, which then polymerize in fibrin fibers. Fibirin fibers form a losse meshwork that is stabilized by crosslinks created by factor XIII. The stabilized meshwork of fibrin fibers ins now a clot that traps red blood cells and platelets and thus stops the flow of blood.

(2) Extrinsic Pathway:

The extrinsic pathway is triggered by tissue damage outside of the blood vessel. This pathway acts to clot blood that has escaped from the vessel into the tissues. Damage to tissue stimulates the activation of tissue thromboplastin, an enzyme that catalyzed the activation of factor X. At this point the intrinsic and extrinsic pathways converge and the subsequent steps are the same as those described above.

With advanced atherosclerosis take one baby asprin yet day to reduce the probability of heart attack and stroke.

Small tears of the capillaries and arterioles are happening all the time Platelets are responsible for quickly sealing these tears before the slower process of clotting completes the job.

In the absence of adequate numbers of platelets these micro blotches (thrombocytopenia purpura) visible on the skin. Thrombocytopenia can be acute or chronic and has many causes. Severe, untreated cases result in death.

The blood contains about a dozen clotting factors. These factors are proteins that exist in the blood in an inactive state, but can be called into action when tissues or blood vessels are damaged.

The activation of clotting factors occurs in a sequen­tial manner. The first factor in the sequence activates the second factor, which activates the third factors and so on. This series of reactions is called the clotting cascade.

Blood clotting is the transformation of liquid blood into a semisolid gel. Clots are made from fibers (polymers) of a protein called fibrin. Fibrin monomers come from an inactive precursor called fibrinogen.

The body of the fibrinogen molecule has caps on its ends that mast fibrin-to-fibrin binding sites. If the caps are removed then fibrin monomers polymerize to form fibrin polymers. This process required thrombin the enzyme that converts fibrinogen to fibrin.

This process also requires calcium, which acts as a kind of glue to hold the fibrin monomers to each other to form the polymeric fiber. The fibrin fibers form a loose mesh work that is stabilized by clotting factor XIII. The stabilized meshwork of fibrin fibers traps erythrocytes, thus forming a clot that stops the flow of blood.

Clot Busting Drugs:

Blood clots can be life-threatening if they form inappropriately in critically locations. Clots that block coronary arteries cause the heart attacks, while clots that block arteries in the brain cause stroke. Drugs that can mediate the removal of clots, “clot busters”, are used in cases of heart attract and stroke to decrease the damage caused by the clot.

Drugs used clinically to remove cots include:

1. Tissue plasminogen activator (TPA) was recently cloned and is now produced in mass quantities by the biotech fig, Amgen. It is used clinically to dissolved clots in coronary arteries following a heart attack. It is also used to dissolved clots in the brain following stroke.

2. Streptokinase is an enzyme that directly dissolved blood clots. It is produced by streptococcus bacteria. The bacteria use streptokinase to dissolve clots that nega­tively affect their growth in the human host. This clot dissolving enzyme is appar­ently as effective as recombinant TPA.

Streptokinase cost $2 dollars per does while TPA costs $2000 dollars per dose. Based on economic concerns, streptokinase is the drug of choice. However, streptokinase is not a human enzyme, therefore the immune system sees it as a foreign molecule that should be distorted.

The immune response increases with repeated use of this limits the effectiveness of the drug over time. TPA, on-the-other-hand is a huna molecule whole which the im­mune system does not destroy.

Blood Clotting or Coagulation

Blood clotting or coagulation is a biological process that stops bleeding. It&aposs vital that blood clots when we have a surface injury that breaks blood vessels. Clotting can prevent us from bleeding to death and protect us from the entry of bacteria and viruses. Clots also form inside our body when a blood vessel is injured. Here they prevent blood loss from the circulatory system.

Our body can both make clots and break them down once they&aposve done their job. In most people, a healthy balance is maintained between these two activities. In some people, however, abnormal blood coagulation occurs and their body may not be able to break clots down. A large clot inside a blood vessel is potentially dangerous because it can block blood flow in the vessel. Internal clots that form without an obvious injury or ones that travel through blood vessels are also dangerous.

Coagulation of blood is a fascinating and complex process that involves many steps. Proteins made by the liver and sent into the bloodstream are an essential part of the process. The proteins circulate around the body in our blood, ready for action at any time. An external or internal injury is the trigger that activates the proteins and sets the blood clotting process in motion.

Blood cells and platelets are sometimes referred to as formed elements in blood.

Red blood cells (or erythrocytes) carry oxygen to cells. The five types of white blood cells (leukocytes) fight infections in various ways. Platelets (thrombocytes) are cell fragments that play an essential role in the blood clotting process. They develop a spiky appearance when they&aposre activated.

Formation of α–granules

Vesicle trafficking

The development of α–granules begins in the megakaryocyte, but continues in the circulating platelet. In the megakaryocyte, α–granules are derived in part from budding of small vesicles containing α–granule cargo from the trans-Golgi network ( Fig. 1 ). 6 , 7 In other cell models, an orchestrated assemblage of coat proteins (e.g., clathrin, COPII), adaptor proteins (e.g., AP-1, AP-2, AP-3), fusion machinery (e.g., soluble NSF attachment protein receptors [SNAREs]), and monomeric GTPases (e.g., Rabs) mediate vesicle trafficking and maturation. Clathrin coat assembly likely functions, too, in trafficking of vesicles from the trans-Golgi network to α–granules in megakaryocytes. The clathrin-associated adaptor proteins AP-1, AP-2, and AP-3 are found in platelets 8 , 9 and are proposed to function in clathrin-mediated vesicle formation in platelets. 10 Mutations in the gene encoding AP-3, for example, results in impaired dense granule formation. 9 Clathrin-mediated endocytosis also functions in the delivery of plasma membrane into α–granules ( Fig. 1 ). Vesicles budding off from either the trans-Golgi network or the plasma membrane can subsequently be directed to multivesicular bodies (MVBs).

α–Granule cargo derives from budding of the trans-Golgi network (TGN) and endocytosis of the plasma membrane. Both processes are clathrin-mediated. Receptor-mediated endocytosis is depicted in this figure however, pinocytosis of α–granule cargo can also occur. Vesicles can subsequently be delivered to multivesicular bodies (MVBs), where sorting of vesicles occurs. It is possible that vesicles may also be delivered directly to α–granules. Some vesicles within MVBs contain exosomes. MVBs can mature to become α–granules.

MVBs found in most cells are endosomal structures containing vesicles that form from the limiting membrane of the endosome. 11 , 12 They are typically transient structures involved in sorting vesicles containing endocytosed and newly synthesized proteins. In megakaryocytes, MVBs serve in an intermediate stage of granule production. 13 Both dense granules and α–granules are sorted by MVBs. 13 , 14 Vesicles budding from the trans-Golgi network may be delivered directly to MVBs ( Fig. 1 ). 13 Kinetic studies in megakaryocytes have demonstrated that transport of endocytosed proteins proceeds from endosomes to immature MVBs (MVB I, with internal vesicles alone) to mature MVBs (MVB II, with internal vesicles and an electron dense matrix) to α–granules. α–Granules within MVBs contain 30� nm vesicles, termed exosomes. 13 Some exosomes persist in mature α–granules and are secreted following platelet activation. 15 Although it is unknown whether all or most vesicle trafficking to α–granules proceeds through MVB, these observations indicate that MVB represent a developmental stage in α–granule maturation.

Maturation of α–granules continues in circulating platelets by endocytosis of platelet plasma membranes. 16 – 18 A clathrin-dependent pathway leading to the delivery of plasma membrane to α–granules has been described, as has a clathrin-independent pathway that traffics vesicles to lysosomes. 18 Unlike other cells, coated vesicles in platelets retain their clathrin coat throughout trafficking and for a period following fusion with α–granules. 17 Platelet endocytosis appears to be a constitutive activity of resting platelets. The molecular control of endocytosis in platelets is not known, but may involve the Src family receptors Fyn, Fgr, Lck, and/or Lyn based on colocalization studies, their tyrosine-phosphorylation status, 19 , 20 and evidence of a role for Src family receptors in lymphocyte endocytosis. 21 Studies performed in dogs evaluating the accumulation of fibrinogen and immoglobulin, which are endocytosed by circulating platelets, show that levels of endocytosed, but not endogenous, α–granule proteins increase as platelets age. 22 This observation confirms that constitutive trafficking to α–granules continues throughout the lifespan of the platelet.

Protein Sorting

Many α–granule proteins are produced by megakaryocytes and sorted to α–granules via a regulated secretory pathway. These proteins are synthesized in the endoplasmic reticulum, exported to the Golgi for maturation, and subsequently sorted at the trans-Golgi network. 23 Trafficking of some well-known α–granule proteins synthesized in megakaryocytes, such as P-selectin, has been evaluated. Initial studies in heterologous cells indicated that the sorting sequence for P-selectin is contained within its cytoplasmic tail. 24 – 27 Subsequent studies, however, indicated that the cytoplasmic tail of P-selectin targets this adhesion molecule to storage granules in endothelial cells, but not in platelets. 25 , 28 This observation demonstrates that while some principles of protein sorting can be generalized among cell types, the mechanism of sorting of a particular protein can vary between cell types.

Trafficking of soluble proteins has also been evaluated. Study of the targeting of CXCL4 (also known as platelet factor 4) to α–granules has led to the identification of a signal sequence responsible for sorting chemokines into α–granules. 29 , 30 These experiments demonstrate that a four amino acid sequence within the exposed hydrophilic loop is required for sorting of CXCL4 into α–granules. 30 An analogous sequence was identified in the platelet chemokines RANTES and NAP-2. 30

Soluble proteins must be incorporated into vesicles formed at the trans-Golgi network to become cargo within mature α–granules. A mechanism involving binding to glycosaminoglycans has been proposed for sorting small soluble chemokines. Mice that lack the dominant platelet glycosaminoglycan, serglycin, fail to store soluble proteins containing basically charged regions, such as CXCL4, PDGF, or NAP-2, in their α–granules. 31 This observation suggests that glycosaminoglycans may serve as a retention mechanism for chemokines possessing an exposed cationic region. A mechanism to incorporate larger soluble proteins into α–granules is by aggregation of protein monomers. 32 Although not formally proven to sort by aggregation, large, self-assembling proteins such as multimerin have been proposed to sort into immature vesicles by homoaggregation. 33 vWf self-assembles into large multivalent structures and is packaged into a discrete tubular structure within α–granules. 34 , 35 Heterologous expression of vWf can drive the formation of granules in cell lines possessing a regulated secretory pathway (e.g., AtT-20, HEK293, or RIN 5F cells), but not in cells lines that lack such a pathway (CHO, COS, or 3T3 cells). 36 , 37 While aggregation and glycosaminoglycan binding represent plausible mechanisms for sorting soluble proteins, alternative sorting receptors must exist for other endogenous α–granule proteins.

Plasma proteins are incorporated into α–granules via several distinct mechanisms of endocytosis. During receptor mediated endocytosis, a plasma protein is bound to a platelet surface receptor and subsequently internalized via a clathrin-dependent process. The most well-studied example is the incorporation of fibrinogen via integrin αIIbβ3. 38 – 42 Plasma proteins such as immunoglobulins and albumin incorporate into α–granules via pinocytosis. 43 The endocytosis of factor V by megakaryocytes involves two receptors. Following initial binding to a specific factor V receptor, subsequent binding to low-density lipoprotein receptor-related protein-1 (LRP-1) occurs and clathrin-dependent mediated endocytosis ensues. 44 , 45 Endocytosis of α–granule proteins may occur at the level of the megakaryocyte, the platelet, or both.

Transport of α–granules into platelets

α–Granules formed in megakaryocytes must be distributed to platelets during megakaryopoiesis. Two models to account for organelle delivery during megakaryopoiesis include the fragmentation model and the proplatelet model. The fragmentation model predicts that the megakaryocyte demarcation membrane system divides the cell into regions, each containing their allotment of organelles. 46 The proplatelet model predicts that, in a profound terminal reorganization of the megakaryocyte, platelets form along extended projections termed proplatelets. 47 – 49 Recent studies have indicated that platelets are produced in vivo via the formation of proplatelets 50 and that α–granules are transported from megakaryocytes to α–granules on microtubule bundles. 51 These studies demonstrate that organelles within the megakaryocyte move from the cell body to the nascent platelets on microtubule tracks, powered by the microtubule motor proteins. Organelles move at a rate of 0.1𠄲 μm/min in what appears to be a random direction. They are captured in developing platelets by virtue of microtubule coils, which persist in platelets. 51

Individual granules that move along proplatelet microtubule tracks appear to be heterogeneous with regard to cargo ( Fig. 2 ). 4 Some α–granules stained with antibodies directed against vWf, but not fibrinogen. Others stained with antibodies against fibrinogen, but not vWf. 4 Additional antigen pairs such as vascular endothelial cell growth factor (VEGF) and endostatin, as well as basic fibroblast growth factor (bFGF) and thrombospondin-1, were also found to reside in different α–granule subpopulations. 4 Differential staining of α–granule subpopulations is observed in mature platelets as well. 4 , 5 Whether different α–granule subpopulations represent α–granules derived from different sources (e.g., endocytosis versus regulated secretory pathway), differentially sorted in MVBs, or separated by yet unknown mechanisms remains to be determined.

Platelet α–granules are transported along microtubules from the megakaryocyte cell body through long pseudopodial extensions termed proplatelets. Platelets form as bulges along the length of these extensions. α–Granules are maintained in the nascent platelets by coiled microtubules. The insert demonstrates subpopulations of α–granules containing distinct cargos being transported along a proplatelet. α–Granules containing fibrinogen are shown in green, while those containing vWf are shown in red. (Insert from Italiano et al., Blood, 111:1227�.)

Defects in α–granule formation

Defects of α–granule formation have been described in both patients and mice. Gray platelet syndrome is the best known of the inherited disorders of α–granule formation (for review see 52 ). This syndrome is heterogenous and its genetic underpinnings have yet to be elucidated. α–Granules are also severely reduced in Medich giant platelet disorder and the White platelet syndrome. 53 , 54 The molecular defects resulting in these syndromes, however, have not been identified.

α–Granule deficiency can result from mutations or deletions of specific transcription factors. For example, mice that lack Hzf, a zinc finger protein that acts as a a transcription factor, produce megakaryocytes and platelets with markedly reduced α–granules, mimicking Gray platelet syndrome. 55 Fibrinogen, PDGF, and vWf are nearly absent from Hzf-deficient platelets. However, no mutations in the orthologous Hzf gene were identified in a series of patients with Gray platelet syndrome. 56 Mutations in GATA1 have been described in patients with thrombocytopenia and markedly reduced or absent α–granules. 57 , 58 The downstream regulators involved in granule formation, however, have not been characterized for these transcription factor mutants.

Some mutations resulting in markedly decreased or absent α–granules occur in genes encoding proteins involved in vesicular trafficking. ARC syndrome results from mutations in the VPS33B gene. 59 , 60 VPS33B is a membrane-associated protein that binds tightly to and regulates the function of SNAREs. 60 VPS33B associates with α–granules in platelets. 59 Patients with this mutation possess α–granule-deficient platelets ( Fig. 3 ), and their platelets possess no detectable PF4, vWf, fibrinogen, nor P-selectin. 59 This observation indicates that loss of VPS33B effects incorporation of both endogenous and endocytosed proteins, as well as both soluble and membrane-bound proteins, into α–granules. 59 The number of dense granules in VPS33B-deficient platelets is somewhat increased, indicating that VPS33B function is not critical for dense granule formation. Isolated deficiencies of dense granule formation with normal α-granule formation, such as in the Hermansky-Pudlak syndrome, are well-described. These observations further support the premise that dense granule and α–granule formation require distinct membrane trafficking machineries.

Thin-section transmission electron micrographs of platelets (A) from a fetus with a mutation in VPS33B and (B) platelets from an unaffected fetus. Abundant α–granules indicated with white arrows in control platelets are lacking in platelets with mutant VSP33B. Bar, 500 nm. (Adapted from Lo et al., Blood, 106:4159�).

That said, some mutations in vesicle trafficking proteins result in defects in both dense granule and α–granule formation, indicating aspects of commonality between the two pathways. Gunmetal mice with a mutant Rab geranylgeranyl transferase demonstrate a macrothrombocytopenia with a significant defect in both α–granule and dense granule production. 61 , 62 While the substrates of Rab geranylgeranyl transferase involved in granule formation have not all been elucidated, Rab27 is hypoprenylated in gunmetal mice and associates with both α–granules and dense granules. 61 , 63 , 64 Rab27 is a small GTP binding protein that regulates membrane trafficking. 65 Mice deficient in Rab27b demonstrate reduced numbers of α–granules and dense granules in their megakaryocytes and impaired proplatelet formation. 63 These observations suggest that Rab27b may coordinate proplatelet formation with granule transport. Other Rab proteins, including Rabs 1a, 1b, 3b, 5a, 5c 6a, 7, 8, 10, 11a 14, 18, 21, 27a, 27b, 32, 37 are present in platelets, associated with membranes, and may function in membrane trafficking and granule formation. 8 , 66

Defects in membrane composition can also result in aberrant α–granule formation. Mice lacking the ATP-binding cassette half-transporter, ABCG5, suffer sitosterolemia, an accumulation of circulating plant sterols, and a macrothrombocytopenia characterized by large platelets with decreased granules. 67 Sitosterolemia secondary to ABCG5 deficiency also occurs in humans and results in macrothrombocytopenia. 68 The reason why the megakaryocyte membrane system is more sensitive than other cells to plant sterols is unknown. However, the observation that α–granule formation is impaired in this condition may inform strategies for studying how α–granule membranes form.


Those more sophisticated and more durable repairs are collectively called coagulation, the formation of a blood clot. The process is sometimes characterized as a cascade, because one event prompts the next as in a multi-level waterfall. The result is the production of a gelatinous but robust clot made up of a mesh of fibrin—an insoluble filamentous protein derived from fibrinogen, the plasma protein introduced earlier—in which platelets and blood cells are trapped. Figure 1 summarizes the three steps of hemostasis.

Figure 1. (a) An injury to a blood vessel initiates the process of hemostasis. Blood clotting involves three steps. First, vascular spasm constricts the flow of blood. Next, a platelet plug forms to temporarily seal small openings in the vessel. Coagulation then enables the repair of the vessel wall once the leakage of blood has stopped. (b) The synthesis of fibrin in blood clots involves either an intrinsic pathway or an extrinsic pathway, both of which lead to a common pathway. (credit a: Kevin MacKenzie)

Clotting Factors Involved in Coagulation

In the coagulation cascade, chemicals called clotting factors (or coagulation factors) prompt reactions that activate still more coagulation factors. The process is complex, but is initiated along two basic pathways:

  • The extrinsic pathway, which normally is triggered by trauma.
  • The intrinsic pathway, which begins in the bloodstream and is triggered by internal damage to the wall of the vessel.

Both of these merge into a third pathway, referred to as the common pathway (see Figure 1b). All three pathways are dependent upon the 12 known clotting factors, including Ca 2+ and vitamin K (Table 1). Clotting factors are secreted primarily by the liver and the platelets. The liver requires the fat-soluble vitamin K to produce many of them. Vitamin K (along with biotin and folate) is somewhat unusual among vitamins in that it is not only consumed in the diet but is also synthesized by bacteria residing in the large intestine. The calcium ion, considered factor IV, is derived from the diet and from the breakdown of bone. Some recent evidence indicates that activation of various clotting factors occurs on specific receptor sites on the surfaces of platelets.

The 12 clotting factors are numbered I through XIII according to the order of their discovery. Factor VI was once believed to be a distinct clotting factor, but is now thought to be identical to factor V. Rather than renumber the other factors, factor VI was allowed to remain as a placeholder and also a reminder that knowledge changes over time.

Table 1. Clotting Factors
Factor number Name Type of molecule Source Pathway(s)
I Fibrinogen Plasma protein Liver Common converted into fibrin
II Prothrombin Plasma protein Liver* Common converted into thrombin
III Tissue thromboplastin or tissue factor Lipoprotein mixture Damaged cells and platelets Extrinsic
IV Calcium ions Inorganic ions in plasma Diet, platelets, bone matrix Entire process
V Proaccelerin Plasma protein Liver, platelets Extrinsic and intrinsic
VI Not used Not used Not used Not used
VII Proconvertin Plasma protein Liver * Extrinsic
VIII Antihemolytic factor A Plasma protein factor Platelets and endothelial cells Intrinsic deficiency results in hemophilia A
IX Antihemolytic factor B (plasma thromboplastin component) Plasma protein Liver* Intrinsic deficiency results in hemophilia B
X Stuart–Prower factor (thrombokinase) Protein Liver* Extrinsic and intrinsic
XI Antihemolytic factor C (plasma thromboplastin antecedent) Plasma protein Liver Intrinsic deficiency results in hemophilia C
XII Hageman factor Plasma protein Liver Intrinsic initiates clotting in vitro also activates plasmin
XIII Fibrin-stabilizing factor Plasma protein Liver, platelets Stabilizes fibrin slows fibrinolysis
* Vitamin K required.

Extrinsic Pathway

The quicker responding and more direct extrinsic pathway (also known as the tissue factor pathway) begins when damage occurs to the surrounding tissues, such as in a traumatic injury. Upon contact with blood plasma, the damaged extravascular cells, which are extrinsic to the bloodstream, release factor III (thromboplastin). Sequentially, Ca 2+ then factor VII (proconvertin), which is activated by factor III, are added, forming an enzyme complex. This enzyme complex leads to activation of factor X (Stuart–Prower factor), which activates the common pathway discussed below. The events in the extrinsic pathway are completed in a matter of seconds.

Intrinsic Pathway

The intrinsic pathway (also known as the contact activation pathway) is longer and more complex. In this case, the factors involved are intrinsic to (present within) the bloodstream. The pathway can be prompted by damage to the tissues, resulting from internal factors such as arterial disease however, it is most often initiated when factor XII (Hageman factor) comes into contact with foreign materials, such as when a blood sample is put into a glass test tube. Within the body, factor XII is typically activated when it encounters negatively charged molecules, such as inorganic polymers and phosphate produced earlier in the series of intrinsic pathway reactions. Factor XII sets off a series of reactions that in turn activates factor XI (antihemolytic factor C or plasma thromboplastin antecedent) then factor IX (antihemolytic factor B or plasma thromboplasmin). In the meantime, chemicals released by the platelets increase the rate of these activation reactions. Finally, factor VIII (antihemolytic factor A) from the platelets and endothelial cells combines with factor IX (antihemolytic factor B or plasma thromboplasmin) to form an enzyme complex that activates factor X (Stuart–Prower factor or thrombokinase), leading to the common pathway. The events in the intrinsic pathway are completed in a few minutes.

Common Pathway

Both the intrinsic and extrinsic pathways lead to the common pathway, in which fibrin is produced to seal off the vessel. Once factor X has been activated by either the intrinsic or extrinsic pathway, the enzyme prothrombinase converts factor II, the inactive enzyme prothrombin, into the active enzyme thrombin. (Note that if the enzyme thrombin were not normally in an inactive form, clots would form spontaneously, a condition not consistent with life.) Then, thrombin converts factor I, the insoluble fibrinogen, into the soluble fibrin protein strands. Factor XIII then stabilizes the fibrin clot.

Types - Bleeding Disorders

Bleeding disorders can be inherited , or they can be acquired, meaning you develop them during your lifetime. Acquired bleeding disorders are more common than inherited bleeding disorders.

You may develop a bleeding disorder if something in your body, such as a disease or a medicine, causes your body to stop making blood clotting factors or causes the blood clotting factors to stop working correctly. In addition, problems with your blood vessels can lead to bleeding.

Acquired bleeding disorders include:

Inherited bleeding disorders include the following:

  • Combined deficiency of the vitamin K–dependent clotting factors (VKCFDs), caused by a problem with clotting factors II, VII, IX, and X.
  • HemophiliaA, a condition in which you are missing clotting factor VIII or have low levels of clotting factor VIII. Hemophilia A is the most common type of hemophilia.
  • Hemophilia B, a condition in which you are missing clotting factor IX or have low levels of clotting factor IX.
  • Hemophilia C, a rare condition also known as factor XI deficiency.
  • Von Willebrand disease (VWD), the most common inherited bleeding disorder. The different types of VWD are numbered based on how common the condition is and how severe the symptoms are. For example, VWD 1 is the most common, and symptoms are usually mild, and VWD 3 is uncommon with symptoms that are usually severe.
  • Other inherited bleeding disorders include other factor deficiencies, such as I, II, V, V + VIII, VII, X, XI, or XIII deficiencies. These rare bleeding disorders are named by the clotting factor causing the problem. is a rare inherited condition in which your blood vessels get tangled in different parts of the body, which can lead to bleeding.


The gene for human PF4 is located on human chromosome 4. [6]

Platelet factor-4 is a 70-amino acid protein that is released from the alpha-granules of activated platelets and binds with high affinity to heparin. Its major physiologic role appears to be neutralization of heparin-like molecules on the endothelial surface of blood vessels, thereby inhibiting local antithrombin activity and promoting coagulation. As a strong chemoattractant for neutrophils and fibroblasts, PF4 probably has a role in inflammation and wound repair. [5] [7]

PF4 is chemotactic for neutrophils, fibroblasts and monocytes, and interacts with a splice variant of the chemokine receptor CXCR3, known as CXCR3-B. [8]

The heparin:PF4 complex is the antigen in heparin-induced thrombocytopenia, an idiosyncratic autoimmune reaction to the administration of the anticoagulant heparin. [9] PF4 autoantibodies have also been found in patients with thrombosis and features resembling HIT but no prior administration of heparin. [10] Antibodies against PF4 have been implicated in cases of thrombosis and thrombocytopenia subsequent to ChAdOx1 nCoV-19 vaccination (AstraZeneca) as well as Janssen COVID-19 Vaccine (Johnson & Johnson). [11] [12] This phenomenon has been termed vaccine-induced immune thrombotic thrombocytopenia (VITT). [ citation needed ]

It is increased in patients with systemic sclerosis that also have interstitial lung disease. [13]

The human platelet factor 4 kills malaria parasites within erythrocytes by selectively lysing the parasite's digestive vacuole. [14]

  1. ^ abcGRCh38: Ensembl release 89: ENSG00000163737 - Ensembl, May 2017
  2. ^ abcGRCm38: Ensembl release 89: ENSMUSG00000029373 - Ensembl, May 2017
  3. ^"Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  4. ^
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This article incorporates text from the United States National Library of Medicine, which is in the public domain.

Blood Clotting

Blood clotting (coagulation) is the process by which blood vessels repair ruptures after injury. Injury repair actually begins even before clotting does, through vascular spasm, or muscular contraction of the vessel walls, which reduces blood loss. Clotting itself is a complex cascade of reactions involving platelets, enzymes , and structural proteins .

Platelets are not whole cells, but rather small packets of membrane-bounded cytoplasm . There are approximately one million platelets in a drop of blood. Damage to the lining of a blood vessel (the endothelial lining) exposes materials that cause platelets to stick to the endothelial cells additional platelets then stick to these. These aggregating platelets release factors that promote accumulation of fibrin, a circulating protein. A blood clot is a meshwork of platelets and blood cells woven together by fibrin.

Accumulation of fibrin must be tightly regulated, of course, to prevent clot formation where there is no wound. Thrombosis is an abnormal localized activation of the clotting system. Disseminated intravascular coagulation is a pathological condition in which the clotting system is activated throughout the circulatory system in response to bacterial toxins, trauma, or other stimuli. A clot may break off, forming an embolus, which can lodge in a small blood vessel, cutting off circulation. If this occurs in the heart, it may cause ischemia (lack of blood flow) or myocardial infarction (heart attack). In the lungs, it causes pulmonary embolism, with loss of capacity for oxygen exchange. In the brain, it can cause stroke.

Because of this need for tight regulation, and the need for rapid response, the clotting mechanism involves a multistep cascade of enzymes, most of whose jobs are to activate the next enzyme in the cascade. In this way, the effect of the initial stimulus (the damaged blood vessel) can be quickly magnified, as a single enzyme at the first stage activates many copies of another enzyme at the next stage, each of which activates many more at the next, and so on. At the same time, the many levels of interaction provide many points of control over the process. This coagulation cascade begins from thirty seconds to several minutes after the injury.

Coagulation can begin with either of two pathways, called the extrinsic and intrinsic pathway, both of which feed into a common pathway that completes the process. The extrinsic pathway begins with a substance called tissue factor (tissue thromboplastin) released by damaged blood vessels and surrounding tissues. In the presence of other plasma proteins (clotting factors) and calcium ions , this leads to the activation of a protein called factor X. The intrinsic pathway begins with a substance called factor XII, released by blood platelets. Through a series of additional clotting factors, and again in the presence of calcium ions, this pathway also leads to the activation of factor X. One of the necessary factors of the intrinsic pathway is called factor VIII. A mutation in the gene for this factor is the most common cause of hemophilia.

The common pathway begins with the activation of factor X. In the presence of calcium ions and other clotting factors, factor X activates an enzyme called prothrombin activator. This enzyme them converts the plasma protein prothrombin into thrombin. Thrombin is an enzyme that, in turn, converts fibrinogen to fibrin. Here the cascade ends, because fibrin is not an enzyme, but a fibrous protein. It forms strands that stick to the platelets and endothelial cells at the wound, forming a meshwork that, in turn, traps other cells.

Once the clot forms, contraction of the platelets pulls the edges of the wound closer together, and fresh endothelial cells then grow across it, repairing the damaged blood vessel. Over time, fibrin is degraded by plasmin. This enzyme is formed from circulating plasminogen by tissue plasminogen activator (t-PA). Synthetic t-PA is used to dissolve blood clots in stroke, myocardial infarction, pulmonary embolism, and other conditions.

Blood Coagulation and Atherothrombosis


Coagulation pathways are dependent on a group of proteins termed coagulation factors that are normally present in inactive proenzyme forms ( Table 28-1 ). 1–3 The sequential activation of coagulation factors forms a coagulation cascade that eventually results in fibrin clot formation. The blood coagulation system includes two pathways composed of distinct groups of coagulation factors: the extrinsic or tissue factor-dependent pathway and the intrinsic pathway. The final step in both the extrinsic and the intrinsic pathways is the activation of factor X (fX) to factor Xa (fXa). Generation of fXa merges both pathways in a common pathway that leads to the production of the multifunctional molecule thrombin ( Fig. 28-1 ). In addition, several mechanisms to counteract the coagulation cascade exist to maintain intact blood circulation and prevent clotting. These mechanisms, including antithrombin, the protein C/protein S/thrombomodulin system, and tissue factor pathway inhibitor (TFPI), regulate the coagulation cascade at different levels to mitigate clot formation under physiologic circumstances ( Fig. 28-2 ).

Activation of the extrinsic pathway begins with binding of tissue factor (TF), a cell-surface glycoprotein, to an activated serine protease fVIIa. Exposure of TF to circulating blood is caused by disruption of the endothelial layer as in vascular injury or by heterotropic TF expression in different cell types in response to various stimuli. Small amounts of fVIIa are present (1% to 2%) and circulate in the blood. 4 TF on the cell surface binds to free fVIIa in the plasma to form the TF/fVIIa complex. 5–7 Both proteins possess low enzymatic activity in their free forms, but the TF/fVIIa complex acts as a potent enzyme to further activate free fVII to generate fVIIa, producing more TF/fVIIa complexes to amplify the initial trigger (TF-mediated fVII autoactivation). The TF/fVIIa complex then activates fX to yield fXa, either directly or indirectly by initially converting fIX to fIXa, which subsequently activates fX in the presence of fVIIIa.

The intrinsic pathway is triggered by the autoactivation of fXII to fXIIa, which subsequently initiates the cascade of sequential activation of fXI and fIX to generate fIXa. Then fIXa catalyzes the conversion of fX to fXa, but this reaction requires an activated form of another coagulation factor fVIIIa, which is generated by thrombin-mediated activation of fVIII.

Factor Xa, the end product of both the extrinsic and intrinsic pathways, triggers the common pathway of coagulation by converting prothrombin to thrombin, which in turn initiates formation of fibrin from fibrinogen. The conversion of prothrombin to thrombin requires a cofactor fVa, which is produced by thrombin-mediated activation of fV. Thrombin also activates fXIII to form fXIIIa, which catalyzes the formation of cross-linked fibrin polymer.

Antithrombin (antithrombin III) is a plasma protease inhibitor that inactivates thrombin and other activated coagulation factors in the intrinsic and common pathways by binding to the active site of these enzymes. The anticoagulant heparin's major mechanism of action is to accelerate the formation of these neutralizing complexes.

Protein C is a plasma glycoprotein that is activated by thrombin, and activated protein C (APC) is a potent anticoagulant that inactivates fVa and fVIIIa through the thrombin-thrombomodulin complex. The thrombin-induced activation of protein C occurs physiologically on thrombomodulin, a transmembrane proteoglycan-binding site for thrombin on endothelial cell surfaces. The rate of this reaction is increased by a cofactor, protein S, which increases the affinity of APC for phospholipids in the formation of the membrane-bound protein Case complex ( Fig. 28-3 ).

TFPI is the endogenous inhibitor of the TF/fVIIa complex ( Fig. 28-4 ). TFPI is a multivalent Kunitz-type serine protease inhibitor, consisting of three tandem Kunitz domains, which exerts inhibitory effects against the TF/fVIIa complex and fXa, thereby regulating the extrinsic pathway of coagulation. 8 Endothelial cells are the principal source of plasma TFPI. 9–11

It has recently been recognized that the coagulation mechanism involves the assembly of multiprotein complexes on the phospholipid cellular membrane, including the extrinsic and intrinsic Xase (tenase) complex, prothrombinase complex, and protein Case complex ( Fig. 28-3 , Table 28-2 ). Each complex consists of an enzyme, its zymogen substrate, and its cofactor on the phospholipid membrane surface. The formation of these complexes on the cell membrane surface promotes the reactions in the coagulation and anticoagulation pathways.

The Blood-Clot Problem Is Multiplying

For weeks, Americans looked on as other countries grappled with case reports of rare, sometimes fatal blood abnormalities among those who had received the AstraZeneca vaccine against COVID-19. That vaccine has not yet been authorized by the FDA, so restrictions on its use throughout Europe did not get that much attention in the United States. But Americans experienced a rude awakening this week when public-health officials called for a pause on the use of the Johnson & Johnson vaccine, after a few cases of the same, unusual blood-clotting syndrome turned up among the millions of people in the country who have received it.

The world is now engaged in a vaccination program unlike anything we have seen in our lifetimes, and with it, unprecedented scrutiny of ultra-rare but dangerous side effects. An estimated 852 million COVID-19 vaccine doses have been administered across 154 countries, according to data collected by Bloomberg. Last week, the European Medicines Agency, which regulates medicines in the European Union, concluded that the unusual clotting events were indeed a side effect of the AstraZeneca vaccine by that point, more than 220 cases of dangerous blood abnormalities had been identified. Only half a dozen cases have been documented so far among Americans vaccinated with the Johnson & Johnson vaccine, and a causal link has not yet been established. But the latest news suggests that the scope of this problem might be changing.

Whether the blood issues are ultimately linked to only one vaccine, or two vaccines, or more, it’s absolutely crucial to remember the unrelenting death toll from the coronavirus itself—and the fact that COVID-19 can set off its own chaos in the circulatory system, with blood clots showing up in “almost every organ.” That effect of the disease is just one of many reasons the European Medicines Agency has emphasized that the “overall benefits of the [AstraZeneca] vaccine in preventing COVID-19 outweigh the risks of side effects.” The same is true of Johnson & Johnson’s. These vaccines are saving countless lives across multiple continents.

But it’s also crucial to determine the biological cause of any vaccine-related blood conditions. This global immunization project presents a lot of firsts: the first authorized use of mRNA vaccines like the ones from Pfizer and Moderna the first worldwide use of adenovirus vectors for vaccines like AstraZeneca’s, Johnson & Johnson’s, and Sputnik V and the first attempt to immunize against a coronavirus. Which, if any, of these new frontiers might be linked to serious side effects? Which, if any, of the other vaccines could be drawn into this story, too? How can a tiny but disturbing risk be mitigated as we fight our way out of this pandemic? And what might be the implications for vaccine design in the years to come?

To answer these questions, scientists will have to figure out the biology behind this rare blood condition: what exactly causes it when and why it happens. This is not an easy task. While the evidence available so far is fairly limited, some useful theories have emerged. The notions listed below are not all in competition with one another: Some are overlapping—or even mutually reinforcing—in important ways. And their details matter quite a bit. A better understanding of the cause of this condition may allow us to predict its reach.

Theory 1: Platelet Problems

The leading theory behind the blood abnormalities associated with the AstraZeneca vaccine traces to a case from late February. A 49-year-old nurse in Austria who had received the vaccine developed clotting in her veins and arteries, along with a low platelet count. That’s an odd combination: Platelets are like the bricks of the clotting system, held together with a mortar of molecules called coagulation factors when your body has a shortage of these bricks, it typically has a hard time building clots.

There are a couple of exceptions, though, as the science journalists Kai Kupferschmidt and Gretchen Vogel explained in March. In a condition called disseminated intravascular coagulation, so much clotting occurs that circulating platelets get used up. A rare reaction to a blood-thinning drug called heparin, which is safely prescribed to millions of people in the U.S. every year, can also produce the same signal. When that occurs, heparin attaches itself to a molecule called platelet factor 4, which is released by damaged platelets. The body mistakenly treats this complex as a pathogen before going on to attack the platelet cells directly, which sometimes leads to clots.

A hematologist named Sabine Eichinger first treated the nurse in Austria. She had an inkling that the problem resembled the heparin reaction, so she reached out to Andreas Greinacher at the University of Greifswald in Germany, who has published extensively on that topic over the past three decades. Other, similar cases following immunization with the AstraZeneca vaccine had emerged, and Greinacher rallied his team. They put in 18-hour days in the lab to analyze the patients’ blood samples.

Last Friday, Greinacher and his team published a paper on their findings in the New England Journal of Medicine. In a press briefing, he said they’d analyzed blood from several dozen people who had experienced blood abnormalities after exposure to the AstraZeneca vaccine, and that every single person tested positive for antibodies against platelet factor 4, and against platelet factor 4 joined with another molecule.

On the same day, a separate group in Norway published similar findings from five patients there who had received the AstraZeneca vaccine. Then, in a meeting this week of the Advisory Committee on Immunization Practices, which helps the CDC make vaccine recommendations, it was reported that five of the six American patients who developed this same blood condition after receiving the Johnson & Johnson shot had been tested for antibodies to platelet factor 4—and all were positive. “It is, in my opinion, absolutely clear that there’s a causal relationship” between the presence of these antibodies and the abnormal clotting, Greinacher had said at last Friday’s briefing. “There’s no doubt about this.”

Greinacher has given the blood condition the tongue-twisting moniker of “vaccine-induced immune thrombotic thrombocytopenia” or VITT. A lot remains to be confirmed, but if this theory is borne out, scientists will have to determine whether the AstraZeneca vaccine is the only one—or whether the AstraZeneca and Johnson & Johnson vaccines are the only two—that can generate this haywire autoimmune response. Greinacher’s theory has a vital, near-term implication too: Patients who present with blood clots are often given heparin as treatment if what they have is really VITT, then this standard treatment may only make things worse.

Theory 2: The Spike’s the Problem

If the theory above is correct, then some unknown component of a COVID-19 vaccine can, in very rare situations, spur an autoimmune reaction against platelet factor 4. One potential culprit is the all-important spike protein—the one used by coronaviruses to infect cells, and involved in every available COVID-19 vaccine as a training target for the immune system. As it happens, the structure of the platelet factor 4 molecule has some similarities, in its curves and crevices, with that of the spike protein. In other words, training up the body to go after the spike protein could yield antibodies that stick to clotting-related proteins too.

Greinacher does not believe this to be true. In an article posted online, which has not yet been published in a scientific journal, he and others found no evidence that the troubling antibodies found in people with VITT were also binding to the spike protein. That’s “fantastic news,” he said in last Friday’s press briefing, because “otherwise there would have been the risk that many other vaccines might have caused the same problem.”

But others think the role of the spike protein warrants further examination. Michel Goldman, an immunologist at the Free University of Brussels and a former director of Europe’s Innovative Medicines Initiative, told me that unvaccinated people with severe COVID-19 have also experienced strange clotting events that mimic what happens in the heparin-related autoimmune disorder. He speculates that the spike protein could be the source of this problem—but not because it looks a bit like platelet factor 4.

It’s possible, he says, that the spike protein interacts with platelets directly and causes them to secrete platelet factor 4. One mouse study published last year by scientists in China found that the spike protein could indeed activate platelets that had been engineered to carry a cell receptor for the coronavirus. Meanwhile, another study from December, not yet published in a journal, suggests that the spike protein can damage the cells that line our blood vessels. Goldman noted that these cells, when perturbed, are known to release molecules that bind to platelet factor 4, which might in turn become a target for rogue antibodies. All of this could potentially contribute to the symptoms seen in VITT.

If the spike protein is involved in the blood disorders that are now under investigation—and that is a gigantic “if”—that isn’t necessarily bad news for all COVID-19 vaccines. While it’s true that each one deploys the coronavirus spike protein, they don’t all use the same version. The Pfizer, Moderna, and Johnson & Johnson vaccines, for example, have a modification in the spike protein that keeps it stabilized and elongated. “The only EMA-approved vaccine that includes a nonstabilized form of the spike is the AstraZeneca vaccine,” Goldman said.

Theory 3: A Suspect Sequence

The AstraZeneca vaccine is built from a non-replicating version of a virus that is thought to be harmless to people, and has been engineered to produce the coronavirus spike protein. That’s the “adenovirus vector” approach, also used in the Johnson & Johnson vaccine, and in Russia’s Sputnik V. But AstraZeneca’s differs from the others’ in that its engineered virus has also been fitted with a small portion of the genetic sequence for a molecule called tissue plasminogen activator, or tPA. When naturally produced in the body, tPA is thought to have a role in regulating normal blood-clotting processes. A manufactured form of the same molecule is often given as a clot-busting drug to people who have suffered strokes. The AstraZeneca vaccine uses a snippet of the gene for tPA that boosts production of the spike protein, with the aim of producing a stronger immune response. *

Some scientists in Europe have wondered whether AstraZeneca’s inclusion of a partial tPA sequence could throw off normal clotting. The European Medicines Agency said in March that there is insufficient evidence to support this theory. And if tPA were somehow involved in the strange blood disorder, there’s no reason to expect that the same problem would show up among people who received vaccines that don’t include the sequence, such as Johnson & Johnson’s.

It’s possible that some other aspect of the adenovirus technology might be in play, which would explain why the only two vaccines to raise major red flags so far both use that delivery system. A couple of past laboratory studies of blood cells have found evidence that adenoviruses can trigger platelets to secrete clotting-related molecules. Still, it’s important to remember that regulators have not yet confirmed a causal link between the unusual blood disorder and the Johnson & Johnson vaccine.

Theory 4: The Mixed Bag

It could be that lots of things are going on at once. We shouldn’t assume that the same thing is happening in every affected patient, says David Lee, an emergency-medicine doctor at NYU Langone Health. “When you just say, ‘Hey, everybody who had a clotting issue, yeah, let’s pile up all those cases,’ you’re mixing a bag,” he told me. Lee and others have shown that COVID-19 patients produce many different antibodies that go after the body’s own proteins, including one that gloms onto a clotting molecule called annexin A2. If platelet factor 4 is involved in some cases of abnormal clotting after vaccination, Lee said, then annexin A2 or other factors might play important roles in others. He pointed, for example, to another molecule involved in clotting, called fibrinogen. The molecule shows up at abnormal levels both in people with severe COVID-19 and some of those who have suffered rare blood disorders following the AstraZeneca vaccine.

Lee is not alone in thinking that this story might have many different plots. Saskia Middeldorp, a vascular internist at Radboud University Medical Center in the Netherlands, says she was initially convinced by the VITT theory. “I was really, really excited because I figured, ‘Wow, now we’ve got a case definition. It’s not great, of course, that this is occurring, but we know better what to do.’” But when she and her colleagues analyzed blood samples from one Dutch patient who developed strange blood issues after receiving the AstraZeneca vaccine, they did not find antibodies against platelet factor 4. (Greinacher says there might be differences in the laboratory assays used to detect the antibodies, which could explain the different results.)

At any rate, it’s clear that multiple mechanisms could lead to the formation of clots when people get sick with COVID-19. That’s one reason it’s so much safer to develop immunity from vaccination than from being naturally infected, according to David Kaslow, the vice president for essential medicines at PATH, a Seattle nonprofit. When your body is presented with one specific portion of a virus—the spike protein, for example—there may be some tiny risk of immune dysfunction. But this is far, far better than the alternative, where the body is exposed to a full pathogen. Natural infections may lead to all sorts of unexpected effects, including platelet damage and abnormal blood clots. A vaccine that whittles down the virus to a single subcomponent is a huge benefit, Kaslow says, because it provides fewer opportunities for something to go wrong.

Setting aside the question of whether it was good policy to pause the use of the Johnson & Johnson vaccine in the U.S., or to limit the use of AstraZeneca in Europe, more information is likely to emerge in the coming weeks.

The Vaccine Safety Datalink, a collaboration between the CDC and nine large health-care organizations tracking data from anonymized medical records of millions of patients, was already watching for some clotting events, according to Ed Belongia, an investigator with the program and the director of the Center for Clinical Epidemiology and Population Health at Marshfield Clinic Research Institute, in Wisconsin. He told me the Vaccine Safety Datalink had not identified a risk from any of the three vaccines now authorized for emergency use in the U.S. The program is in the process of adding an alert for cerebral venous sinus thrombosis, the form of clotting that turned up in the six Americans who received the Johnson & Johnson vaccine.

The pandemic is still raging around the globe, and there is a real urgency to solve the riddle of the rare blood abnormalities linked to the AstraZeneca shot, and to figure out whether other vaccines carry the same risk. But recent history suggests that health officials might have to decide whether to restrict or resume vaccinations before scientists reach agreement. During the swine-flu pandemic of 2009, for example, a vaccine known as Pandemrix was given in Europe, and then some evidence emerged in Finland that it might be linked to narcolepsy. (The vaccine never became available in the U.S., and it’s no longer manufactured.) Although many scientists consider the narcolepsy risk well established, there is still no consensus on this point.

The wrath of COVID-19 will likely stretch into the years ahead, and it’s plausible that the debate over risks related to COVID-19 vaccines will do the same. The 2009 swine-flu strain never really went away, and many scientists now believe that this coronavirus is here to stay as well. But the current pandemic is unlike any public-health crisis in our lifetimes, and it has led to an unprecedented pace of scientific progress. This could lead us to a quicker, more definitive resolution of the vaccine-related clotting issue. If that happens, what’s been learned will help us continue to navigate the promises and pitfalls of new vaccine technologies, and it might even shed light on how clotting goes awry because of COVID-19 itself.

* This article previously misstated that the snippet of the gene for tPA included in the AstraZeneca vaccine is noncoding. In fact, that snippet is translated into protein, but the resulting protein segment doesn’t have the biological activity of tPA itself and is not thought to appear in the final version of the spike protein.


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