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Factor XII (FXII) is a coagulation protein that initiates surface-activation of the coagulation cascade in vitro. The protein's in vivo role, however, remains poorly defined. Factor XII deficiency, or Hageman trait, is a rare hereditary disorder that is not associated with bleeding, and wide variations in FXII activity (FXII:C) exist among healthy people. While FXII-deficient knockout mice appear to be resistant to arterial thrombosis, human F12 polymorphisms that influence FXII:C have not been associated with thrombotic risk in population surveys. Factor XII deficiency is a naturally occurring hereditary trait in domestic cats. We undertook phenotypic and genotypic analyses of FXII-deficient cats for comparative studies with the human disease counterpart. A retrospective review of feline submissions to our laboratory revealed that FXII deficiency is common in domestic cats, and also present in many different breeds. The trait has a geographic bias toward the Midwestern United States. Clinical history, coagulation assays, and samples for F12 sequencing were obtained from 26 FXII deficient cats. None of the cats had experienced abnormal bleeding and their residual FXII:C was related to F12 mutation number and mutation-type. We found 2 high frequency F12 mutations: an exon 13 missense mutation (c.1631G > C) and an exon 11 deletion mutation (c.1321delC), and additional sequence variants throughout the gene. Factor XII deficiency in pet cat populations provides an animal model system to help clarify the biologic actions and clinical relevance of FXII protein., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Thrombosis remains a major cause of morbidity and mortality. Consequently, advances in antithrombotic therapy are needed to reduce the disease burden. This article focuses on 2 such advances. First, the prevention of atherothrombosis in patients with coronary or peripheral artery disease, which has been enhanced by the finding that the combination of low-dose rivaroxaban plus aspirin is superior to aspirin alone for prevention of recurrent ischemic events. However, this benefit comes at the cost of increased bleeding albeit not fatal bleeding. To overcome this problem, the second advance is the identification of factor XI as a target for new anticoagulants that are potentially safer than those currently available.

Bradykinin mediated angioedema (BK-AE) can be associated either with C1Inhibitor deficiency (hereditary and acquired forms), either with normal C1Inh (hereditary form and drug induced AE as angiotensin converting enzyme inhibitors…). In case of high clinical suspicion of BK-AE, C1Inh exploration must be done at first: C1Inh function and antigenemy as well as C4 concentration. C1Inh deficiency is significant if the tests are below 50 % of the normal values and controlled a second time. In case of C1Inh deficiency, you have to identify hereditary from acquired forms. C1q and anti-C1Inh antibody tests are useful for acquired BK-AE. SERPING1 gene screening must be done if a hereditary angioedema is suspected, even if there is no family context (de novo mutation 15 %). If a hereditary BK-AE with normal C1Inh is suspected, F12 and PLG gene screening is suitable., (Copyright © 2018. Published by Elsevier Masson SAS.)

Factor XII (FXII) is a protease that is mainly produced in the liver and circulates in plasma as a single chain zymogen. Following contact with negatively charged surfaces, FXII is converted into the two-chain active form, FXIIa. FXIIa initiates the intrinsic blood coagulation pathway via activation of factor XI. Furthermore, it converts plasma prekallikrein to kallikrein (PK), which reciprocally activates FXII and liberates bradykinin from high molecular weight kininogen. In addition, FXIIa initiates fibrinolysis via PK-mediated urokinase activation and activates the classical complement pathway. Even though the main function of FXII seems to relate to the activation of the intrinsic coagulation pathway and the kallikrein-kinin system, a growing body of evidence suggests that FXII may also directly regulate cellular responses. In this regard, it has been found that FXII/FXIIa induces the expression of inflammatory mediators, promotes cell proliferation, and enhances the migration of neutrophils and lung fibroblasts. In addition, it has been reported that genetic ablation of FXII protects against neuroinflammation, reduces the formation of atherosclerotic lesions in Apoe -/- mice, improves wound healing, and inhibits postnatal angiogenesis. Although the aforementioned effects can be partially explained by the downstream products of FXII activation, the ability of FXII/FXIIa to directly regulate cellular responses has recently emerged as an alternative hypothesis. These direct cellular reactions to FXII/FXIIa will be discussed in the review., (Copyright © 2018 Elsevier Inc. All rights reserved.)

Combinations of proinflammatory and procoagulant reactions are the unifying principle for a variety of disorders affecting the cardiovascular system. Factor XII (FXII, Hageman factor) is a plasma protease that initiates the contact system. The biochemistry of the contact system in vitro is well understood; however, its in vivo functions are just beginning to emerge. The current review concentrates on activators and functions of the FXII-driven contact system in vivo. Elucidating its physiologic activities offers the exciting opportunity to develop strategies for the safe interference with both thrombotic and inflammatory diseases.

Introduction: During the fluid phase of hemostasis, fibrinogen is converted into fibrin, but other hemostatic factors are required. Reference values of hemostatic factors are established by manufacturers producing reagents using individuals with a specific genetic background., Objective: To establish reference values for hemostatic factors in the Mexican indigenous and Mestizo populations., Material and Methods: We carried out a cross-sectional, descriptive study of healthy adult Mexicans. Clotting activity was evaluated using coagulometric assays. Blood donors were informed about the nature of the study and informed consent was obtained prior to blood being drawn. The protocol was approved by the Ethics Committee of our institution., Results: One hundred and twenty samples were assayed (60 females and 60 males). Fibrinogen was higher in mestizos and in females. Reference values for factor XII ranged from 40-170% in indigenous subjects and from 36-159% in mestizos. Factor VIII ranged from 57-160% in indigenous subjects and from 51-209% in mestizo subjects. Reference values for the other hemostatic factors were also clearly different from the commercial reference values. Reference values for hemostatic factors in the Mexican population are different from traditionally used commercial reference values. There were significant differences between indigenous and mestizo Mexicans in the concentration of hemostatic factors with a tendency among mestizos to have higher factor concentrations. Low levels of plasma factor XII are frequent and perhaps may represent a risk factor for thrombotic events. Using these reference values may individualize the reposition of factors in Mexican hemophiliac patients.

Background: High levels of activated protein–inhibitor complexes of the intrinsic coagulation proteins are associated with ischemic stroke (IS) but not with myocardial infarction (MI). This study was aimed at determining whether the antigen levels of coagulation factors(factor XII, FXII, and FXI and prekallikrein (PK)are associated with MI and IS, and whether this association is independent of levels of activated protein–inhibitor complexes., Patients and Methods: The RATIO study included young women (< 50 years) with MI (N = 205)and IS (N = 175), and 638 healthy controls. Antigen levels of FXII, FXI and PK were measured and expressed as percentages of of those in pooled normal plasmas. Odds ratios (ORs) and corresponding 99% confidence intervals (CIs) were calculated for high levels (i.e. ≥ 90th percentile of controls) as measures of rate ratios., Results: After adjustment for potential confounders, high levels of FXII antigen were not associated with MI risk or IS risk(OR(MI) 1.18, 99% CI 0.51–2.74; ORIS 1.03, 9% CI 0.41–2.55). High levels of FXI antigen were slightly associated with an increase in MI risk (OR(MI) 1.55, 9% CI 0.74–3.21), whereas there was a substantial association with IS risk (ORIS 2.65, 9% CI 1.27–5.56). PK antigen was slightly associated with MI risk but not with IS risk(ORMI 1.54, 9% CI 0.67–3.52; ORIS 0.90, 9% CI 0.35–2.33). All associations remained similar after adjustment for levels of protein–inhibitor complexes., Conclusion: Increased levels of FXI antigen were associated with an increase in IS risk, whereas they showed only a marginal association with MI risk. FXII antigen and PK antigen levels were not substantially associated with MI risk and IS risk.

The plasma zymogens factor XII (fXII) and factor XI (fXI) contribute to thrombosis in a variety of mouse models. These proteins serve a limited role in hemostasis, suggesting that antithrombotic therapies targeting them may be associated with low bleeding risks. Although there is substantial epidemiologic evidence supporting a role for fXI in human thrombosis, the situation is not as clear for fXII. We generated monoclonal antibodies (9A2 and 15H8) against the human fXII heavy chain that interfere with fXII conversion to the protease factor XIIa (fXIIa). The anti-fXII antibodies were tested in models in which anti-fXI antibodies are known to have antithrombotic effects. Both anti-fXII antibodies reduced fibrin formation in human blood perfused through collagen-coated tubes. fXII-deficient mice are resistant to ferric chloride-induced arterial thrombosis, and this resistance can be reversed by infusion of human fXII. 9A2 partially blocks, and 15H8 completely blocks, the prothrombotic effect of fXII in this model. 15H8 prolonged the activated partial thromboplastin time of baboon and human plasmas. 15H8 reduced fibrin formation in collagen-coated vascular grafts inserted into arteriovenous shunts in baboons, and reduced fibrin and platelet accumulation downstream of the graft. These findings support a role for fXII in thrombus formation in primates.

Objective: To investigate the prevalence and clinical significance of congenital factor XII (FXII) deficiency in the South-European Caucasian (Greek) population in a cohort of women with recurrent spontaneous abortions (RSAs)., Study Design: One hundred women with a history of > or =2 RSAs of unexplained nature were compared to 100 age-matched, healthy controls with no history of thrombotic disease or adverse pregnancy outcomes, regarding FXII activity. Women were included in the RSA group if they had normal coagulation parameters and no congenital or acquired thrombophilia., Results: Fifteen of 100 women with RSA had reduced FXII activity, whereas all controls had normal FXII activity. FXII activity was significantly lower in the RSA than in the control group (median 100.5, range 10-150 vs. median 104.2, range 58.3-143.2, p < 0.016 by Mann-Whitney test). FXII activity was positively correlated with age in both the RSA and the control groups (r = +0.1, p = 0.04 and r = +0.04, p = 0.2, respectively), but this correlation reached statistical significance in the RSA group only. A negative correlation between FXII activity and the number of abortions in the RSA group was found (r = -0.2, p = 0.03)., Conclusion: Congenital FXII deficiency is strongly associated with RSA in the Greek population.

Binding of negatively charged macromolecules to factor XII induces a conformational change such that it becomes a substrate for trace amounts of activated factor present in plasma (less than 0.01%). As activated factor XII (factor XIIa or factor XIIf) forms, it converts prekallikrein (PK) to kallikrein and kallikrein cleaves high molecular weight kininogen (HK) to release bradykinin. A far more rapid activation of the remaining unactivated factor XII occurs by enzymatic cleavage by kallikrein (kallikrein-feedback) and sequential cleavage yields two forms of activated factor XII; namely, factor XIIa followed by factor XII fragment (factor XIIf). PK circulates bound to HK and binding induces a conformational change in PK so that it acquires enzymatic activity and can stoichiometrically cleave HK to produce bradykinin. This reaction is prevented from occurring in plasma by the presence of C1 inhibitor (C1 INH). The same active site leads to autoactivation of the PK-HK complex to generate kallikrein if a phosphate containing buffer is used. Theoretically, formation of kallikrein by this factor XII-independent route can activate surface-bound factor XII to generate factor XIIa resulting in a marked increase in the rate of bradykinin formation as stoichiometric reactions are replaced by Michaelis-Menton, enzyme-substrate, kinetics. Zinc-dependent binding of the constituents of the bradykinin-forming cascade to the surface of endothelial cells is mediated by gC1qR and bimolecular complexes of gC1qR-cytokeratin 1 and cytokeratin 1-u-PAR (urokinase plasminogen activator receptor). Factor XII and HK compete for binding to free gC1qR (present in excess) while cytokeratin 1-u-PAR preferentially binds factor XII and gC1qR-cytokeratin 1 preferentially binds HK. Autoactivation of factor XII can be initiated as a result of binding to gC1qR but is prevented by C1 INH. Yet stoichiometric activation of PK-HK to yield kallikrein in the absence of factor XII can be initiated by heat shock protein 90 (HSP-90) which forms a zinc-dependent trimolecular complex by binding to HK. Thus, endothelial cell-dependent activation can be initiated by activation of factor XII or by activation of PK-HK. Hereditary angioedema (HAE), types I and II, are due to autosomal dominant mutations of the C1 INH gene. In type I disease, the level of C1 INH protein and function is proportionately low, while type II disease has a normal protein level but diminished function. There is trans-inhibition of the one normal gene so that functional levels are 30% or less and severe angioedema affecting peripheral structures, the gastrointestinal tract, and the larynx results. Prolonged incubation of plasma of HAE patients (but not normal controls) leads to bradykinin formation and conversion of PK to kallikrein which is reversed by reconstitution with C1 INH. The disorder can be treated by C1 INH replacement, inhibition of plasma kallikrein, or blockade at the bradykinin B-2 receptor. A recently described HAE with normal C1 INH (based on inhibition of activated C1s) presents similarly; the defect is not yet clear, however one-third of patients have a mutant factor XII gene. We have shown that this HAE has a defect in bradykinin overproduction whether the factor XII mutation is present or not, that patients' C1 INH is capable of inhibiting factor XIIa and kallikrein (and not just activated C1) but the functional level is approximately 40-60% of normal, and that α2 macroglobulin protein levels are normal. In vitro abnormalities can be suppressed by raising C1 INH to twice normal levels. Finally, aggregated proteins have been shown to activate the bradykinin-forming pathway by catalyzing factor XII autoactivation. Those include the amyloid β protein of Alzheimer's disease and cryoglobulins. This may represent a new avenue for kinin-dependent research in human disease. In allergy (anaphylaxis; perhaps other mast cell-dependent reactions), the oversulfated proteoglycan of mast cells, liberated along with histamine, also catalyze factor XII autoactivation., (© 2014 Elsevier Inc. All rights reserved.)

Coagulation factor XII (FXII, Hageman factor, EC = 3.4.21.38) is the zymogen of the serine protease, factor XIIa (FXIIa). FXII is converted to FXIIa through autoactivation induced by "contact" to charged surfaces. FXIIa is of crucial importance for fibrin formation in vitro, but deficiency in the protease is not associated with excessive bleeding. For decades, FXII was considered to have no function for coagulation in vivo. Our laboratory developed the first murine knockout model of FXII. Consistent with their human counterparts, FXII(-/-) mice have a normal hemostatic capacity. However, thrombus formation in FXII(-/-) mice is largely defective, and the animals are protected from experimental cerebral ischemia and pulmonary embolism. This murine model has created new interest in FXII because it raises the possibility for safe anticoagulation, which targets thrombosis without influence on hemostasis. We recently have identified platelet polyphosphate (an inorganic polymer) and mast cell heparin as in vivo FXII activators with implications on the initiation of thrombosis and edema during hypersensitivity reactions. Independent of its protease activity, FXII exerts mitogenic activity with implications for angiogenesis. The goal of this review is to summarize the in vivo functions of FXII, with special focus to its functions in thrombosis and vascular biology.

Background: Particulate matter (PM) is a key component of ambient air pollution and has been associated with an increased risk of thrombotic events and mortality. The underlying mechanisms remain unclear., Objectives:  To study the mechanisms of PM-driven procoagulant activity in human plasma and to investigate mainly, the coagulation driven by ultrafine particles (UFPs; < 0.1 μm) in genetically modified mice., Methods: Thrombin generation in response to PM of different sizes was assessed in normal human platelet-poor plasma, as well as in plasmas deficient in the intrinsic pathway proteases factors XII (FXII) or XI (FXI). In addition, UFPs were intratracheally instilled in wild-type (WT) and FXII-deficient (FXII(-/-) ) mice and plasma thrombin generation was analyzed in plasma from treated mice at 4 and 20 h post-exposure., Results: In normal human plasma, thrombin generation was enhanced in the presence of PM, whereas PM-driven thrombin formation was completely abolished in FXII- and FXI-deficient plasma. UFPs induced a transient increase in tissue factor (TF)-driven thrombin formation at 4 h post-instillation in WT mice compared with saline instillation. Intratracheal instillation of UFPs resulted in a procoagulant response in WT mice plasma at 20 h, whereas it was entirely suppressed in FXII(-/-) mice., Conclusions:  Overall, the data suggest that PM promotes its early procoagulant actions mostly through the TF-driven extrinsic pathway of coagulation, whereas PM-driven long lasting thrombogenic effects are predominantly mediated via formation of activated FXII. Hence, FXII-driven thrombin formation may be relevant to an enhanced thrombotic susceptibility upon chronic exposure to PM in humans., (© 2011 International Society on Thrombosis and Haemostasis.)

The contact system is a protease cascade that is initiated by factor XII activation on cardiovascular cells. The system starts procoagulant and proinflammatory reactions, via the intrinsic pathway of coagulation and the kallikrein-kinin system, respectively. The biochemistry of the contact system in vitro is well understood. However, activators of the system in vivo and their contributions to disease states have remained enigmatic. Recent experimental and clinical data have identified misfolded proteins, collagens, and polyphosphates as the long-sought activators of the contact system in vivo. Here we present an overview about contact system activators and their contributions to health and pathology., (© Thieme Medical Publishers.)

Activated mast cells trigger edema in allergic and inflammatory disease. We report a paracrine mechanism by which mast cell-released heparin increases vascular permeability in vivo. Heparin activated the protease factor XII, which initiates bradykinin formation in plasma. Targeting factor XII or kinin B2 receptors abolished heparin-triggered leukocyte-endothelium adhesion and interfered with a mast cell-driven drop in blood pressure in rodents. Intravital laser scanning microscopy and tracer measurements showed heparin-driven fluid extravasation in mouse skin microvessels. Ablation of factor XII or kinin B2 receptors abolished heparin-induced skin edema and protected mice from allergen-activated mast cell-driven leakage. In contrast, heparin and activated mast cells induced excessive edema in mice deficient in the major inhibitor of factor XII, C1 esterase inhibitor. Allergen exposure triggered edema attacks in hereditary angioedema patients, lacking C1 esterase inhibitor. The data indicate that heparin-initiated bradykinin formation plays a fundamental role in mast cell-mediated diseases., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Background: Classically, intrinsic coagulation proteins are thought to have a minor role in hemostasis. Recently, these proteins, especially FXII, were implicated as possible key players in the pathogenesis of arterial thrombosis. This study aims to determine the risks of arterial thrombosis conferred by increased activation of intrinsic coagulation proteins in young women and the effect of oral contraceptive use on this association., Methods and Results: The Risk of Arterial Thrombosis In relation to Oral contraceptives (RATIO) study is a population-based case-control study including young women (age 18 to 50 years) with myocardial infarction (n=205) and ischemic stroke (n=175) and 638 healthy controls. Intrinsic coagulation protein activation was determined by measuring activated protein-inhibitor complexes. This complex is with C1 esterase inhibitor (FXIIa-C1-INH, FXIa-C1-INH, Kallikrein-C1-INH) or antitrypsin inhibitor (FXIa-AT-INH). Odds ratios (ORs) and corresponding confidence intervals (95% CIs) were calculated with logistic regression. High levels of protein activation (>90th percentile of controls) showed an increased risk of ischemic stroke: FXIIa-C1-INH (OR, 2.1; 95% CI, 1.3 to 3.5), FXIa-C1-INH (OR, 2.8; 95% CI, 1.6 to 4.7), FXIa-AT-INH (OR, 2.3; 95% CI, 1.4 to 4.0), and Kallikrein-C1 (OR, 4.3; 95% CI, 2.6 to 7.2). If anything, myocardial infarction risk was only increased by Kallikrein-C1-INH (OR, 1.5; 95% CI, 0.9 to 2.5). Oral contraceptive use further increased the risks., Conclusions: High levels of activated proteins of the intrinsic coagulation system are associated with arterial thrombosis, whereas the strength of these associations differs for myocardial infarction and ischemic stroke. This contradicts similar analyses among men in the Northwick Park Heart Study. Together with the finding that oral contraceptive use further increases the risks, the question of whether the role of intrinsic coagulation proteins in the pathogenesis of arterial thrombosis is sex-specific is raised.

Platelet and coagulation factor-dependent thrombus formation is critical to limit posttraumatic blood loss at sites of vascular injury. However, under pathological conditions like rupture of an atherosclerotic plaque, it may also lead to vessel occlusion causing myocardial infarction or stroke. Therefore, antithrombotic treatment is the prime therapeutic option in the prophylaxis and treatment of ischaemic cardio- and cerebrovascular diseases. The use of existing antithrombotic agents is, however, limited by their inherent effect on primary haemostasis. In recent years, major advances have been made in understanding the mechanisms of thrombus formation in haemostasis and thrombosis and some studies raised the interesting possibility that occlusive thrombus formation and haemostasis may involve partially different mechanisms. This review briefly summarizes these developments and highlights newly identified mechanisms involved in platelet adhesion and activation, intracellular calcium signaling, integrin activation and initiation of coagulation. The suitability of these pathways as novel targets for antithrombotic therapy is discussed.

Background: Laminin is the most abundant non-collagenous protein in the basement membrane. Recent studies have shown that laminin supports platelet adhesion, activation and aggregation under flow conditions, highlighting a possible role for laminin in hemostasis., Objective: To investigate the ability of laminin to promote coagulation and support thrombus formation under shear., Results and Methods: Soluble laminin accelerated factor (F) XII activation in a purified system, and shortened the clotting time of recalcified plasma in a FXI- and FXII-dependent manner. Laminin promoted phosphatidylserine exposure on platelets and supported platelet adhesion and fibrin formation in recalcified blood under shear flow conditions. Fibrin formation in laminin-coated capillaries was abrogated by an antibody that interferes with FXI activation by activated FXII, or an antibody that blocks activated FXI activation of FIX., Conclusion: This study identifies a role for laminin in the initiation of coagulation and the formation of platelet-rich thrombi under shear conditions in a FXII-dependent manner.

Factor XII (FXII) is a coagulation protein that is essential for surface-activated blood coagulation tests but whose deficiency is not associated with bleeding. For over forty years, investigators in hemostasis have not considered FXII important because its deficiency is not associated with bleeding. It is because there is a dichotomy between abnormal laboratory assay findings due to FXII deficiency and clinical hemostasis that investigators sought explanations for physiologic hemostasis independent of FXII. FXII is a multidomain protein that contains two fibronectin binding consensual sequences, two epidermal growth factor regions, a kringle region, a proline-rich domain, and a catalytic domain that when proteolyzed turns into a plasma serine protease. Recent investigations with FXII deleted mice that are protected from thrombosis indicate that it contributes to the extent of developing thrombus in the intravascular compartment. These findings suggest that it has a role in thrombus formation without influencing hemostasis. Last, FXII has been newly appreciated to be a growth factor that may influence tissue injury repair and angiogenesis. These combined studies suggest that FXII may become a pharmacologic target to reduce arterial thrombosis risk and promote cell repair after injury, without influencing hemostasis., ((c) 2009 Elsevier Ltd. All rights reserved.)

In vivo mouse models have indicated that the intrinsic coagulation pathway, initiated by factor XII, contributes to thrombus formation in response to major vascular damage. Here, we show that fibrillar type I collagen provoked a dose-dependent shortening of the clotting time of human plasma via activation of factor XII. This activation was mediated by factor XII binding to collagen. Factor XII activation also contributed to the stimulating effect of collagen on thrombin generation in plasma, and increased the effect of platelets via glycoprotein VI activation. Furthermore, in flow-dependent thrombus formation under coagulant conditions, collagen promoted the appearance of phosphatidylserine-exposing platelets and the formation of fibrin. Defective glycoprotein VI signaling (with platelets deficient in LAT or phospholipase Cgamma2) delayed and suppressed phosphatidylserine exposure and thrombus formation. Markedly, these processes were also suppressed by absence of factor XII or XI, whereas blocking of tissue factor/factor VIIa was of little effect. Together, these results point to a dual role of collagen in thrombus formation: stimulation of glycoprotein VI signaling via LAT and PLCgamma2 to form procoagulant platelets; and activation of factor XII to stimulate thrombin generation and potentiate the formation of platelet-fibrin thrombi.

Physiologic hemostasis upon injury involves many plasma proteins in a well-regulated cascade of proteolytic reactions to form a clot. Deficiency of blood coagulation Factors VIII, IX, or XI is associated with hemophilia. Factor XII (FXII) autoactivates by contact with a variety of artificial or biologic negatively charged surfaces (contact activation), resulting in blood coagulation and activation of the inflammatory kallikrein-kinin and complement systems. However, surprisingly, individuals deficient in FXII rarely suffer from bleeding disorders. Most biologic surfaces that activate FXII become expressed in disease states. Investigators have long searched for physiologic activators of FXII and its role in vivo. In this issue of the JCI, Maas et al. show that misfolded protein aggregates produced during systemic amyloidosis allow for plasma FXIIa and prekallikrein activation and increased formation of kallikrein-C1 inhibitor complexes, without Factor XIa activation and coagulation (see the related article beginning on page 3208). This study describes a novel biologic surface for FXII activation and activity, which initiates inflammatory events independent of hemostasis.

Purpose of Review: Blood coagulation is a tightly regulated process, involving vascular endothelium, platelets, and plasma coagulation factors. Formation of fibrin involves a series of sequential proteolytic reactions, initiated by the 'extrinsic' and 'intrinsic' pathway of coagulation. As hereditary deficiency of factor XII, the protease that triggers the intrinsic pathway and the kallikrein-kinin system, is not associated with a bleeding disorder or other disease states, the physiological role of factor XII is unknown., Recent Findings: Patient studies, genetically altered mouse models, and plasma assays analyzed functions of the factor XII-driven contact activation system for coagulation and inflammation. This review focuses on articles, which report phenotypization of animals deficient in the contact system proteins factor XII, factor XI and high-molecular-weight kininogen, as well as novel links between factor XII and edema formation, discovery of new in-vivo activators of factor XII, and functions of the factor XII downstream protease factor XI., Summary: Recent studies improved understanding of the factor XII-driven contact system in hemostasis, thrombosis, and inflammation. Studies in mouse models revealed that deficiency in contact system proteins protects from arterial thrombus formation, but does not affect hemostasis. Targeting contact system proteins offers new opportunities for safe anticoagulation associated with minimal bleeding risk. Furthermore, targeting factor XII activity provides an opportunity to treat edema formation.

The plasma kallikrein/kinin system that consists of the proteins factor XII, prekallikrein, and high molecular weight kininogen was first recognized as a surface-activated coagulation system arising when blood or plasma interacts with artificial surfaces. Although surface-activated contact activation occurs in vivo when various negatively charged surfaces become exposed, including a developing platelet thrombus, a physiologic, non-injury mechanism for activation, regulation, and function of this system has been elusive. Recent investigations have shown that there is a physiologic pathway for assembly and activation of this system independent of factor XII. Gene deficient mice of the bradykinin B2 receptor and factor XII have been recognized to have reduced risk for arterial thrombosis. This plasma proteolytic system influences arterial thrombosis independent of influencing hemostasis. Thus, the plasma kallikrein/kinin system has two mechanisms for its activation: one that is dependent and another independent of factor XII. Better understanding of this system may lead to insight into mechanisms for arterial thrombosis, independent of hemostasis.

Background: Whether factor XII (FXII) activity, its 46C>T polymorphism and activated FXII (FXIIa) are associated with coronary heart disease (CHD) remains to be determined., Methods: FXII, FXIIa and the FXII 46C>T polymorphism were determined in a hospital-based cohort of 2615 patients undergoing coronary angiography., Results: Fifty-seven per cent of the participants were identified as wild-type (46CC), 38% as heterozygous (46CT) and 5% as homozygous (46TT) for FXII 46C>T. FXII and FXIIa levels were significantly lower in carriers of the T-allele: 132 (97-151) U dL(-1) FXII in 46CC, 87 (77-99) U dL(-1) FXII in 46CT and 53 (42-67) U dL(-1) FXII in 46TT carriers (P < 0.001), and 2.8 (2.3-3.5) microg L(-1) FXIIa in CC, 2.1 (1.6-2.6) microg L(-1) FXIIa in CT and 1.2 (0.9-1.5) microg L(-1) FXIIa in TT carriers (P < 0.001; medians, lower and upper quartiles). Patients with stable CHD (n = 935), a history of myocardial infarction (n = 785) or who were suffering from acute coronary syndromes (ACS; n = 323) had significantly lower FXII levels than controls (n = 572). The differences remained statistically significant after adjustments for age, sex, diabetes mellitus, smoking, hypercholesterolemia and hypertension. Significantly reduced FXIIa levels in ACS patients lost significance once adjusted for covariates. FXII genotype was not associated with any clinical phenotype., Conclusion: Lower FXII activity represents an independent risk for CHD and ACS. This is not the case for FXIIa levels or the FXII 46C>T variation.

The plasma coagulation system reacts quickly to limit blood loss from injury sites but also contributes to vascular thrombosis. In current models of hemostatic balance, normal coagulation and thrombosis represent two sides of the same coin, however, recent data from gene-deleted murine models have challenged this dogma. Deficiency of coagulation Factor XII (Hageman factor), a serine protease that initiates the intrinsic pathway of coagulation, severely impairs arterial thrombus formation but is not associated with excessive bleeding. These findings suggest that fibrin-generating mechanisms that operate during pathologic thrombus formation involve pathways distinct from those that are active during normal hemostasis. As Factor XII selectively contributes to thrombus formation in occlusive disease, but not to normal hemostasis, inhibition of this protease may offer a novel treatment strategy for prevention of arterial thrombosis with minimal or no risk of bleeding.

The classic intrinsic pathway of coagulation is triggered by contact activation of the plasma protease factor (F)XII, followed by sequential proteolytic activation of FX1 and FIX. While a key mechanism for initiating coagulation in some clinically useful in vitro assays, the absence of abnormal bleeding associated with congenital FXII deficiency indicates that the intrinsic pathway is not important for normal blood coagulation in vivo. However, recent work with mice lacking FXII or FXI suggest that these proteases make important contributions to formation of pathologic intravascular thrombi. In models of arterial injury, FXII or FXI null mice are protected from formation of platelet rich occlusive thrombi to a degree similar to that seen in FIX deficient mice (a model for the severe bleeding disorder hemophilia B) or to wild type mice treated with high dose heparin. FXII or FXI deficiency does not appear to prevent the initiation of thrombus formation in these models, but instead causes significant thrombus instability that prevents occlusion of the vessel. These findings raise the possibility that a pathway similar or identical to the intrinsic pathway may operate in vivo under some circumstances. Furthermore, the disproportionate importance of FXII and FXI to occlusive thrombus formation compared to normal hemostasis makes these proteases attractive candidates for therapeutic inhibitors to treat or prevent thromboembolic disorders.

Blood coagulation is a highly regulated process involving interactions between platelets, plasma coagulation factors, and the vessel wall. During coagulation in vivo, fibrin formation is thought to be initiated when plasma factor VIIa forms a complex with the membrane protein tissue factor. Coagulation factor XII (FXII, Hageman factor) is required for some in vitro coagulation systems; however, FXII deficiency is not associated with hemorrhage, leading to the conclusion that it is not necessary for hemostasis. We generated FXII-deficient mice to study the contributions of FXII to thrombosis and hemostasis in arterial injury models and in models of acute arterial occlusion. FXII-deficient mice do not experience excessive injury-related bleeding; however, intravital fluorescence microscopy and blood flow measurements in three separate arterial beds revealed a severe defect in formation and stabilization of platelet-rich occlusive thrombi induced by different methods of injuries. Similar findings were observed for mice deficient in factor XI, a substrate of activated FXII. Infusion of human FXII into FXII null mice restored thrombus formation. These findings demonstrate that FXII-mediated fibrin formation is crucial for pathological arterial thrombosis but not for hemostasis and suggest that FXII could be an ideal target for safe anticoagulation.

Blood coagulation factor XII (FXII, Hageman factor) is a plasma serine protease which is autoactivated following contact with negatively charged surfaces in a reaction involving plasma kallikrein and high-molecular-weight kininogen (contact phase activation). Active FXII has the ability to initiate blood clotting via the intrinsic pathway of coagulation and inflammatory reactions via the kallikrein-kinin system. Here we have determined FXII-mediated bradykinin formation and clotting in plasma. Western blotting analysis with specific antibodies against various parts of the contact factors revealed that limited activation of FXII is sufficient to promote plasma kallikrein activation, resulting in the conversion of high-molecular-weight kininogen and bradykinin generation. The presence of platelets significantly promoted FXII-initiated bradykinin formation. Similarly, in vitro clotting assays revealed that platelets critically promoted FXII-driven thrombin and fibrin formation. In summary, our data suggest that FXII-initiated protease cascades may proceed on platelet surfaces, with implications for inflammation and clotting.

Factor XII, plasma prekallikrein and high molecular weight kininogen were first identified as coagulation proteins in the intrinsic pathway because patients deficient in these proteins had marked prolongation of in vitro surface-activated coagulation time. However, deficiencies of these proteins are not associated with clinical bleeding. Paradoxically, studies suggest that these proteins have anticoagulant and profibrinolytic activities. In fact, association between deficiencies of these proteins and thrombosis has been reported. Also, deficiencies of these proteins, auto-antibodies to these proteins and anti-phospholipid antibodies are frequent hemostatis-related abnormalities found in unexplained recurrent aborters. Recently, evidence has accumulated for the presence of the kallikrein-kininogen-kinin system in the fetoplacental unit. Since contact proteins or kallikrein-kininogen-kinin system may play an important role in pregnancy especially in the fetoplacental unit, deficiencies of these proteins and/or auto-antibodies to these proteins may be associated with pregnancy losses. These possibilities will be reviewed, the functions of the individual components will be summarized, and their role in blood coagulation and pregnancy discussed.

Potentiation of the intrinsic pathway of human blood plasma coagulation in vitro by contact with a solid procoagulant surface leads to bolus release of thrombin (FIIa) in concentration proportion to the intensity of activation as measured by procoagulant surface area or energy (water wettability). This rather remarkable finding is confirmed using two different assays: one triggering coagulation substantially through the intrinsic pathway alone and the second triggering coagulation through the intrinsic pathway in the presence of exogenous FIIa spikes. Similarity of experimental outcomes of these assays strongly suggests that endogenous FIIa production through the intrinsic pathway is independent of the absolute amount of FIIa present in plasma. Furthermore, we corroborate previous work indicating that procoagulant surfaces remain activating after repeated use and are not poisoned or denatured in the process of activating plasma coagulation. It is concluded that the sharp control mechanism that gives rise to bolus-production of FIIa from the intrinsic pathway must occur between surface activation of FXII and the FII --> FIIa step, is not related to inhibition by FIIa, and does not involve deactivation of procoagulant surfaces.

To analyze the biological role of factor XII (FXII, Hageman Factor) in vivo, we generated mice deficient for FXII using a gene targeting approach on two distinct genetic backgrounds, i.e. mixed C57Bl/6J X 129X1/SvJ and inbred 129X1/SvJ. Homozygous FXII knockout (FXII(-)/(-)) mice showed no FXII plasma activity and had a markedly prolonged activated partial thromboplastin time (aPTT). In contrast, coagulation factors XI, VIII, IX, X,VII, V, II and fibrinogen did not differ between FXII(-/-) mice and their wild-type littermates. Heterozygous matings segregated according to the Mendelian inheritance indicating that FXII deficiency does not increase fetal loss. Furthermore, matings of FXII(-/-) males and FXII(-/-) females resulted in normal litter sizes demonstrating that total FXII deficiency in FXII(-/-) females does not affect pregnancy outcome. Also, gross and histological anatomy of FXII(-/-) mice was indistinguishable from that of their wild-type littermates on both genetic backgrounds. Thus it appears that deficiency of murine FXII does not cause thrombophilia or impaired fibrinolysis in vivo. These results indicate that FXII deficiency does not affect hemostasis in vivo and we anticipate that the FXII(-/-) mice will be helpful to elucidate the biological role(s) of FXII in health and disease.

Background: Human plasma new pressor protein (NPP) increases blood pressure, heart rate and plasma adrenal medullary catecholamines in bioassay rats - all potentiated by angiotensin-converting enzyme (ACE) inhibition. High plasma NPP activity has been found in haemodialysis patients with hypertension., Objective: To investigate whether the bradykinin B2-receptor mediates the effects of NPP., Methods and Results: Male Wistar bioassay rats were anaesthetized with inactin, ganglion blocked with pentolinium and injected intravenously (i.v.) with human NPP (20 microl plasma equivalent) or bradykinin (100 ng/kg). Both NPP and bradykinin increased systolic (SBP) and diastolic (DBP) blood pressures, heart rate and plasma adrenaline and noradrenaline concentrations. All these responses were potentiated by the ACE inhibitor, captopril (10 mg/kg i.v.), but not by antagonism of the angiotensin II type 1 receptor with losartan (10 mg/kg i.v.). Administration of the bradykinin B2-receptor antagonist, HOE-140 (20 microg/kg i.v.), significantly attenuated the peak NPP responses as follows: SBP from 58 +/- 3 to 40 +/- 4 mmHg (decrease of 30%; P < 0.05); DBP from 22 +/- 4 to 10 +/- 1 mmHg (decrease of 55%; P < 0.05); heart rate from 124 +/- 8 to 28 +/-6 beats/min (decrease of 77%; P < 0.05); plasma adrenaline from 14297 +/- 2477 to 3318 +/- 1105 pg/ml (decrease of 77%; P < 0.05) and noradrenaline from 505 +/- 66 to 77 +/-29 pg/ml (decrease of 85%; P < 0.05)., Conclusions: The haemodynamic and sympathoadrenal effects of NPP involve substantial mediation by the bradykinin B2-receptor, in addition to other mechanisms.

Human plasma kallikrein, a serine proteinase, plays a key role in intrinsic blood clotting, in the kallikrein-kinin system, and in fibrinolysis. The proteolytic enzymes involved in these processes are usually controlled by specific inhibitors and may be influenced by several factors including glycosaminoglycans, as recently demonstrated by our group. The aim of the present study was to investigate the effect of glycosaminoglycans (30 to 250 micro/ml) on kallikrein activity on plasminogen and factor XII and on the inhibition of kallikrein by the plasma proteins C1-inhibitor and antithrombin. Almost all available glycosaminoglycans (heparin, heparan sulfate, bovine and tuna dermatan sulfate, chondroitin 4- and 6-sulfates) reduced (1.2 to 3.0 times) the catalytic efficiency of kallikrein (in a nanomolar range) on the hydrolysis of plasminogen (0.3 to 1.8 microM) and increased (1.9 to 7.7 times) the enzyme efficiency in factor XII (0.1 to 10 microM) activation. On the other hand, heparin, heparan sulfate, and bovine and tuna dermatan sulfate improved (1.2 to 3.4 times) kallikrein inhibition by antithrombin (1.4 microM), while chondroitin 4- and 6-sulfates reduced it (1.3 times). Heparin and heparan sulfate increased (1.4 times) the enzyme inhibition by the C1-inhibitor (150 nM).

There are few reports of the use of Thrombelastgraph (TEG) in patients with isolated or inherited bleeding disorders. Three surgical patients are presented with von Willebrand's disease, factor XII deficiency, and hemophilia A with dysfibrinogenemia in which the abnormalities detected by the TEG were similar to the expected coagulation abnormalities based on preoperative laboratory studies. This initial report suggests that this potential application of the TEG be further investigated.

Cellulose is one of the hemostyptic biomaterials, which are able to initiate or accelerate blood coagulation at the site of their application. It belongs to surgical sealants. The mechanism of its action is not clearly understood. We studied the participation of blood platelets in this mechanism. As a marker of platelet activation we used serotonin release reaction. Serotonin release in platelet rich plasma incubated with various concentrations of oxidized cellulose (0.5%-2.0%) started in about 20 min. Washed platelets were not directly activated by oxidized cellulose within one hour. Washed platelets reconstituted in plasma obtained from two patients with coagulation factor XII deficiency were activated by oxidized cellulose with a prolonged lag phase. Our results demonstrate the significant influence of factor XII on blood platelets activation by oxidized cellulose.

Understanding the importance and physiologic activity of the plasma kallikrein/kinin system (KKS) has been thwarted by the absence of an inclusive theory for its assembly and activation. The contact activation hypothesis describes the assembly and activation of this system in test tubes and disease states, but not under physiologic circumstances. Recent investigations have indicated a new cohesive hypothesis for understanding physiologic activation of this system. Prekallikrein (PK) and factor XI (FXI) through high molecular weight kininogen (HK) assemble on a co-localized, multiprotein receptor complex on endothelial cells that consists of at least cytokeratin 1 (CKI), gClqR, and urokinase plasminogen activator receptor (muPAR). When assembled on these proteins, prekallikrein becomes activated to kallikrein by the membrane-expressed enzyme prolylcarboxypeptidase (PRCP). Formed kallikrein then activates factor XII (FXII) for amplification of its activation and single chain urokinase. The plasma kallikrein/kinin system may serve as a physiologic counterbalance to the plasma renin angiotensin system (RAS) by lowering blood pressure and preventing thrombosis. Insights into the integrated role of these two systems may afford the development of novel therapeutic drugs to manage hypertension and thrombosis.

Objectives: To review the literature for conditions, diseases, and disorders that affect activity of the contact factors, and further to review the literature for evidence that less than normal activity of any of the contact factors may be associated with thrombophilia., Data Sources: MEDLINE search for English-language articles published from 1988 to 2001 and pertinent references contained therein, as well as search of references in recent relevant articles and reviews., Study Selection: Relevant clinical and laboratory information was extracted from selected articles. Meta-analysis was not feasible because of heterogeneity of reports., Data Extraction and Synthesis: Evidence for association of altered levels of the contact factors and thrombophilia was sought. A wide variety of disorders is associated with decreased activity of the contact factors; chief among these disorders are liver disease, hepatic immaturity of newborns, the antiphospholipid syndrome, and, for factor XII, being of Asian descent. These disorders are more common than homozygous deficiency. The few series and case reports of thrombophilic events in patients homozygous for deficiency of contact factors are not persuasive enough to support causality. The apparent association between levels consistent with heterozygosity (40%-60% of normal) of any of the contact factors (but especially factor XII) in persons with antiphospholipid antibodies appears to be due to falsely decreased in vitro activity levels of these factors, which are normal on antigenic testing. The apparent association with thrombosis is better explained by the antiphospholipid syndrome than by the modest reduction of the levels of contact factors., Conclusions: Presently, it is not recommended to measure activity of contact factors during routine evaluation of patients who have suffered venous or arterial thromboembolism or acute coronary syndromes.

Objective: To investigate the major cardiovascular effects of human plasma "new pressor protein" (NPP) and how the adrenal medulla contributes to these effects., Methods: NPP was injected into bioassay rats intravenously, and the effects on blood pressure and cardiac function were investigated. Acute adrenal medullectomy (2MDX), alpha- and beta-adrenergic blockade and plasma catecholamine levels were also used to evaluate the role of the sympathoadrenal system in mediating the NPP effects., Results: NPP significantly raised systolic blood pressure (SBP) and mean arterial pressure but not diastolic blood pressure (DBP), with no significant change in total peripheral resistance. Heart rate, cardiac output and stroke volume rose by 16%, 53% and 36%, respectively. Plasma catecholamines increased massively, notably adrenaline, raising the adrenaline to noradrenaline ratio from about 4:1 to 18:1. 2MDX attenuated the increments of SBP and heart rate by more than 90% and more than 70%, respectively, implicating the adrenal medulla. Beta-adrenergic blockade (propranolol) potentiated the NPP-induced increase of SBP and DBP, but not that of heart rate. Combined alpha- and beta-adrenergic blockade (phentolamine and propranolol) blocked the rise in SBP, DBP and heart rate., Conclusions: NPP's hypertensive action is attributable mainly to increases in systolic blood pressure, heart rate and cardiac output (an increase in heart rate and stroke volume) with massive release of adrenal medullary catecholamines. Such effects suggest a novel axis between coagulation factor XII and the sympathoadrenal system, the cardiovascular effects of which are controlled by combined alpha- and beta-adrenergic blockade, but not by angiotensin-converting enzyme inhibition. Clinical relevance depends on whether NPP is formed in vivo in thrombotic states.

Objective: To investigate whether a factor XII genetic polymorphism is associated with first-trimester embryonal loss., Design: Prospective case-control study. SETTING; Nagoya City University Hospital., Patient(s): Eighty-three patients with a history of two or more unexplained first-trimester recurrent miscarriages and 67 controls with no obstetric complications or history of miscarriage., Main Outcome Measure(s): Plasma factor XII activity, a genetic polymorphism (46 C-->T) of factor XII, lupus anticoagulant, and beta(2)glycoprotein I dependent anticardiolipin antibodies., Result(s): Ten of the 83 patients and 1 of the 67 controls had decreased factor XII activity; the difference in frequency was statistically significant. Wild-type (CC), heterozygote (CT), and homozygote (TT) allele patterns were observed in 8, 36, and 39 patients, respectively, compared with 11, 20, and 36 of the patients and controls, respectively. The mean (+/- SD) corresponding factor XII activity was 154.8 +/- 44.8%, 112.7 +/- 30.2%, and 66.2 +/- 29.2% in patients and 164.6 +/- 26.7%, 114.3 +/- 28.1%, and 70.4 +/- 18.1% in controls. The two groups did not differ in the frequency of the T allele or categories of factor XII activity., Conclusion(s): Factor XII activity overall, but not the 46C/T common genetic polymorphism, is associated with recurrent miscarriage.

In thrombus formation associated with hemostasis or thrombotic disease, blood platelets first undergo a rapid transition from a circulating state to an adherent state, followed by activation and aggregation. Under flow conditions in the bloodstream, this process potentially involves platelet-platelet, platelet-endothelium, platelet-subendothelial matrix, and platelet-leukocyte interactions. Specific adhesion receptors on platelets mediate these interactions, by engaging counter-receptors on other cells, or noncellular ligands in the plasma or matrix. The glycoprotein (GP) Ib-IX-V complex on platelets initiates adhesion at high shear stress by binding the adhesive ligand, von Willebrand Factor (vWF). GP Ib-IX-V may also mediate platelet-endothelium or platelet-leukocyte adhesion, by recognition of P-selectin or Mac-1, respectively. Other membrane glycoproteins, such as the collagen receptor GP VI, may trigger platelet activation at low shear rates. Engagement of GP Ib-IX-V or GP VI leads ultimately to platelet aggregation mediated by the integrin, alphaIIbbeta3 (GP IIb-IIIa). This review will focus on recent advances in understanding structure-activity relationships of GP Ib-IX-V, its role in initiating thrombus formation, and its emerging relationships with other vascular cell adhesion receptors.

This paper reports the results of an investigation into the blood response of polymers in vitro, using non-anticoagulated and heparinised blood and plasma. The materials studied were regenerated cellulose, (Cuprophan), an acrylonitrile-allyl sulphonate copolymer (AN69S), and medical grade polyvinyl chloride plasticised with di-2-ethyl-hexyl-phthalate (PVC/DEHP). Blood-material or plasma-material contact was achieved using a parallel plate flow cell, and C3a generation and FXII-like activity measured. The results of the study with non-anticoagulated human blood show that PVC/DEHP is a high complement activator. C3a concentration in the blood was higher after contact with PVC/DEHP than after contact with regenerated cellulose. The introduction of heparin in the blood induced complex alterations in the blood response. C3a generation could be elevated, decreased, or remain the same, depending on the material. The FXII-like activity on the surface of the PVC/DEHP after contact with plasma was also higher than the other two polymers. The introduction of heparin could increase or decrease FXII-like activity, depending on material. The patterns of response obtained with non-anticoagulated blood in vitro for AN69S and Cuprophan bore a strong resemblance with patterns of response obtained in the clinic, whereas those obtained with heparinised blood in vitro did not.

Current concept of blood coagulation is divided into two stages: an "initiation" stage which is handled by tissue factor pathway, and an "augmentation" stage handled by intrinsic pathway beginning in factor XI. Recent studies have demonstrated that the contact system is a modulator for vascular biology with vascular tone regulation, anticoagulant, profibrinolytic, antiadhesive and proinflammatory functions. Changes of contact system are associated with sepsis, thrombosis, etc.

For more than three decades, it has been known that the plasma kallikrein/kinin system becomes activated when exposed to artificial, negatively charged surfaces. The existence of an encompassing in vivo, negatively charged surface capable of activation of the plasma kallikrein/kinin system has, however, never been convincingly demonstrated. In this report, we describe current knowledge on how the proteins of the plasma kallikrein/kinin system assemble to become activated on cell membranes. On endothelial cells, the activation of the plasma kallikrein/kinin system is not initiated by factor XII autoactivation as seen on artificial surfaces. On endothelial cells, prekallikrein is activated by an antipain sensitive protease. Prekallikrein activation is dependent on the presence of high molecular weight kininogen and an optimal free Zn2+ concentration. Kallikrein generated on the surface of endothelial cell is capable of activating factor XII. Further, kallikrein formed on endothelial cell membranes is capable of cleaving its receptor and native substrate, high molecular weight kininogen, liberating bradykinin and the HK PK complex from the endothelial cell surface. Endothelial cell-associated kallikrein also is capable of kinetically favorable pro-urokinase and, subsequent, plasminogen activation.

Prof. Tage Astrup first elaborated the notion that blood fluidity involved a balance between the tendency of blood to clot and for such clots to lyse. It would seem that, at that time, this haemostatic balance involved the notion that forming fibrin orchestrated its own destruction by stimulating fibrinolytic activity. In this review, we have clarified the detail of this balance and developed the thesis that Astrup's far-sighted balance notions involve a variety of control mechanisms. These involve the notion that thrombin, being at first sight a procoagulant, can also, in conjunction with thrombomodulin, act as a stimulus of anticoagulant activity by the generation of activated protein C. The thrombin-activatable fibrinolytic inhibitor (TAFI) is also involved in this balance since the generation of thrombin provokes the neutralisation of fibrinolysis by the TAFI pathway. The kallikrein/factor XII/urokinase pathway is discussed indicating yet another aspect of balance between the generation of coagulation and fibrinolysis. The overall theme of this review, apart from an insight into various aspects of the haemostatic balance, is that blood has a strong tendency to clot when tissue is damaged, and the intact vasculature requires major anticoagulant systems to prevent clots adhering to and stabilising in the vasculature.