Immunohematology Lecture 1 PDF

HAEMOSTASIS / COAGULATION Ms. REGINA FRANCIS  Coagulation is a complex network of interactions involving vessels, platelets, and factors. The ability to form and to remove a clot is truly a system dependent on many synergistic forces. Hemostasis depends on a system of checks and bala...

HAEMOSTASIS / COAGULATION Ms. REGINA FRANCIS  Coagulation is a complex network of interactions involving vessels, platelets, and factors. The ability to form and to remove a clot is truly a system dependent on many synergistic forces. Hemostasis depends on a system of checks and balances between thrombosis and hemorrhage that includes both procoagulants and anti- coagulants. This scale needs to be kept in balance.  Thrombosis is an activation of the hemostatic system at an inappropriate time in a vessel. Thrombi formed in this fashion are pathologic and beyond the normal hemostatic mechanism. If physiological anticoagulants are decreased in the circulation there will be a clot..  will If procoagulants tip toward or clotting factors are decreased, the scale bleeding. vessel Hemorrhage or excessive disease, rupture, bleeding may be due to blood platelet abnormalities, and acquired or congenital abnormalities.  enzymatic Hemostasis is comprised of the vascular system, platelets, and a series of reactions of the coagulation factors. The role of hemostasis is to arrest bleeding from a vessel wall defect, while at the same time maintaining fluidity within circulation. Under physiological conditions, fluidity is maintained anticoagulant, profibrinolytic, by the and antiplatelet properties of the normal endothelium.  Coagulation is divided into two major systems: the primary and secondary systems of hemostasis. The primary system comprises platelet function and vasoconstriction. The secondary system involves coagulation proteins and a series of enzymatic reactions. Once the coagulation proteins become involved, fibrin is formed and this reinforces platelet plug formation until healing is complete. The product of the coagulation cascade is the conversion of soluble fibrinogen into an insoluble fibrin clot. This is accomplished by the action of a powerful coagulant, thrombin. Thrombin is formed by a pre- cursor circulating protein, prothrombin. Dissolution of  VASCULAR SYSTEM The vascular system prevents contraction, diversion of bleeding through vessel blood flow from damaged vessels, initiation of contact activation of platelets with aggregation, and contact activation of the coagulation system. The vessel wall contains such as collagen varying amounts of fibrous tissue and elastin, as well as smooth muscle cells and fibroblasts.  Arteries are the vessels that take blood away from the heart and have the thickest walls of the vascular system. Veins return blood to the heart, and are larger with a more irregular lumen than the arteries. Veins, however, are thin walled, with elastic fibers found only in larger veins. Arterioles are a smaller subdivision of arteries, and venules are smaller subdivisions of veins. Capillaries are the thinnest walled and most numerous of the blood vessels. They are composed of only one cell layer of endothelium that permits a rapid rate of transport materials between blood and tissue.  Mechanism of Vasoconstriction The process in which coagulation occurs begins with injury to a vessel. The first response of a cut vessel is vasoconstriction or narrowing of the lumen of the arterioles to minimize the flow of blood from the wound site. The blood is ordinarily exposed to only the endothelial cell lining of the vasculature. When this is invaded, the exposed deeper layers of the blood vessel become targets for cellular and plasma components.  Vasoconstriction occurs immediately and lasts a short period of time. It allows for increased contact between the damaged vessel wall, blood platelets, and coagulation factors. Vasoconstriction is caused by several regulatory molecules including serotonin and thromboxane A2, which interacts with receptors on the surface of cells of the blood vessel wall. These are products of platelet activation and endothelium. Endothelial cells lining the lumen of the blood vessel are the principal elements regulating vascular functions.  Physiologically, the surface of endothelial cells is negatively charged and repels circulating proteins and platelets, which are negatively charged. Vasoconstriction occurs very quickly and is effective in stopping bleeding in small blood vessels but cannot prevent bleeding in larger vessels. Other systems are required for this task The Endothelium The endothelium contains connective tissue such as collagen and elastin. This matrix regulates the permeability of the inner vessel wall and provides the principal stimuli to thrombosis following injury to a blood vessel. Circulating platelets recognize and bind to insoluble sub- endothelial connective tissue molecules. This process is dependent on molecules that are in plasma and on platelets. Two factors, von Willebrand (vWF) and fibrinogen, participate in the formation of the platelet plug and the insoluble protein clot, resulting in the activation of the coagulation proteins. Receptor molecules not only adhere to platelets and damaged vessel components but also allow platelets to use vWF and fibrinogen to bind platelets and form a plug. Blood flows out through the wall and comes in contact with collagen. Collagen is an insoluble fibrous protein that accounts for much of the body’s connective tissue. Vessel injury leads to the stimulation of platelets. Platelets contain more of the contractile protein actomyosin than any cells, other than muscle cells, giving them the ability to contract. Basically platelets adhere to collagen and other platelets adhere to them. A plug is built and the platelets’ ability to further contract compacts the mass. In forming the initial plug, platelets have now built a template on a lipoprotein surface, which in turn activates tissue factor. The balance between coagulation proteins and anticoagulants now leans toward coagulation. This process will accelerate vasoconstriction, platelet plug development, and the formation of cross- linked fibrin clot.  HAEMOSTASIS The process by which the body stops bleeding upon injury and maintains blood in the fluid state in the vascular compartment. This process is rapid and localized.  FOUR MAJOR SYSTEMS IN HAEMOSTATIC MECHANISM PRIMARY HEMOSTASIS 1) Vascular System (constriction of the blood vessels). 2) Platelets (formation of a temporary “platelet plug").  FOUR MAJOR SYSTEMS IN THE HEMOSTATIC MECHANISM SECONDARY HEMOSTASIS 3) Activation of the coagulation cascade. 4) Formation of “fibrin plug” or clot finally fibrinolysis.  VASCULAR SYSTEM The vascular system, also called the circulatory system, is made up of the vessels that carry blood and lymph through the body. The arteries and veins carry blood throughout the body, delivering oxygen and nutrients to the body tissues and taking away tissue waste matter.  The vessels of the blood circulatory system are: Arteries. Blood vessels that carry oxygenated blood away from the heart to the body. Veins. Blood vessels that carry blood from the body back into the heart. Capillaries. Tiny blood vessels between arteries and veins that distribute oxygen-rich blood to the body.  PLATELET SYSTEM What Are Platelets? Platelets are tiny blood cells that help your body form clots to stop bleeding. If one of your blood vessels gets damaged, it sends out signals to the platelets. The platelets then rush to the site of damage and form a plug (clot) to fix the damage.  PLATELET SYSTEM The process of spreading across the surface of a damaged blood vessel to stop bleeding is called adhesion. This is because when platelets get to the site of the injury, they grow sticky tentacles that help them stick (adhere) to one another. They also send out chemical signals to attract more platelets. The additional platelets pile onto the clot in a process called aggregation.  FACTS ABOUT PLATELETS Platelets are non-nucleated disc-like cells created from megakaryocytes that arise from the bone marrow. Your bone marrow is the spongy center inside your bones. Another name for platelets is thrombocytes and a clot is called a thrombus. Once platelets are made and circulated into your bloodstream, they live for 8 to 10 days.  FACTS ABOUT PLATELETS They are about 2 to 3 microns in size. Some of their unique structural elements include plasma membrane, open canalicular system, spectrin and actin cytoskeleton, microtubules, mitochondria, lysosomes, granules, and peroxisomes. These cells release proteins involved in clotting and platelet aggregation.  DURING INJURY TO BLOOD VESSELS In a healthy blood vessel, and under normal blood flow, platelets do not adhere to surfaces or aggregate with each other. However, in the event of injury platelets are exposed to subendothelial matrix, and adhesion and activation of platelets begins. As soon as a blood vessel wall is damaged, a series of reactions activates platelets so that they stick to the injured area. The “glue” that holds platelets to the blood vessel wall is von Willebrand factor, a large protein produced by the cells of the vessel wall. Multiple receptors on the surface of platelets are involved in these adhesive interactions These receptors are targeted by multiple adhesive proteins. The key for all of these receptors is that the adhesive interaction only takes place in the event of an injury to the blood vessel.  PLATELET ADHESION The extracellular matrix (ECM) releases cytokines and inflammatory markers that lead to adhesion of the platelets and their aggregation at that site which leads to the formation of a platelet plug and sealing of the defect. The platelet adhesion is a complex process mediated by interactions between various receptors and proteins including tyrosine kinase receptors, glycoprotein receptors, other G- protein receptors as well as the von Willebrand Factor (vWF). The von Willebrand Factor functions via binding to the Gp 1b-9 within the platelets  PLATELET ACTIVATION The platelets that have adhered undergo very specific changes. They release their cytoplasmic granules that include ADP, thromboxane A2, serotonin, and multiple other activation factors. They also undergo a transformation of their shape into a pseudopodal shape which in-turn leads to release reactions of various chemokines. P2Y1 receptors help in the conformational changes in platelets.  The platelets change shape from round to spiny, and they release proteins and other substances that entrap more platelets and clotting proteins in the enlarging plug that becomes a blood clot. The proteins collagen and thrombin act at the site of the injury to induce platelets to stick together. As platelets accumulate at the site, they form a mesh that plugs the injury. With the mechanisms mentioned above, various platelets are activated, adhered to each other and the damaged endothelial surface leading to the formation of a primary or temporary platelet plug.  Primary hemostasis; the platelet response. Platelet aggregation at the site of injury is mediated by platelet receptors, platelet-derived agonists, platelet-derived adhesive proteins and plasma-derived adhesive proteins. Fibrin deposition around the resulting platelet plug is generated by the coagulation cascade also know as secondary hemostasis. Abbreviations: ADP = adenosine diphosphate, TXA2 = thromboxane A2 and VWF = von Willebrand factor. MORPHOLOGICAL CHANGES IN PLATELETS Platelet Activation  Filopodia (singular filopodium) are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. Lamellipodia are flattened extensions of a cell, by which it moves over or adheres to a surface  Physiological and pathological platelet morphology. (A) Normal platelets (B) and (C) agranular (no granules) and hypo-granular (low granules) platelets in a patient with a myelodysplastic syndrome. (group of cancers in which immature blood cells in the bone marrow do not mature or become healthy blood cells) (D) giant platelet (E) platelet anisocytosis, large platelets and platelets with abnormal granulation in a patient with primary myelofibrosis (an uncommon type of cancer that disrupts the body’s normal production of red blood cells, (F) platelet anisocytosis, a giant platelet and granulation anomalies in a patient with essential thrombocythemia. (platelet count that is not caused by another health condition)