homogeneous population of cells but consist of three major groups: the granulocytes, the monocytes(to form macrophages in tissue), and the lymphocytes( for specific immune responses) These cell types, are distinguished on the basis of morphology, function, and site of origin According to the staining properties of the granules, the granulocytes are classified as neutropils (for phagocytosis), basophils(for anaphylactic reaction), and eosinophils(to attack parasites and appose anaphylactic reaction). All leukocytes are capable of amoeboid movement, which permits them to emigrate through the walls of blood vessels(this process is also called diapedesis ). They are also attracted(chemotaxis) by bacterial toxins, the products of decomposition of bacteria or body cells, and antigen-antibody complexes; they can surround foreign bodies and take them into the cytoplasm(phagocytosis) Healthy adults are found to have 100-300x10/L platelets in their blood. They are produced in the bone marrow by the shedding of cytoplasmic buds of megakaryocytes. Platelets formation is regulated mainly by a glycoprotein hormone, erythropoietin (EPO). The physiological characteristics of platelets include adhesion, aggregation, secretion reactions, absorption contraction and repair. The main function of platelets is hemostasis hemo The stoppage of bleeding is known as hemostasis Whenever a vessel is severed or ruptured, stasis is achieved by several mechanisms: (1)vascular spasm, (2) formation of a platelet plug, and (3)formation of a blood clot as a result of blood coagulation. From the laboratory viewpoint the coagulation can be brought about by an extrinsic( tissue-based) pathway or intrinsic (plasma-based) pathway, each of which is made up of many steps involving clotting factors. The result of either extrinsic pathway or intrinsic pathway is the formation of a complex of activated lbstances collectively called prothrombin activator, which catalyzes the conversion of prothrombin into thrombin. The thrombin acts as an enzyme to convert fibrinogen into fibrin fibers that enmesh blood cells and plasma to form the clot. There are four plasma anti-clotting abstances that oppose clot formation to limit this process and prevent it from excessively. They are serine protease inhibitor, heparin, protein C system and tissue factor pathway inhibitor. A fibrin clot is a transitory device until permanent repair of the vessel occurs The fibrinolytic system is the principal effecter of clot removal. It constitutes a plasma proenzyme, plasminogen, which can be activated to the active enzyme plasmin by plasminogen activators Once formed, plasmin digests fibrin, thereby dissolving the clot Agglutination would occur in the circulatory system following blood transfusion, if two compatible types of blood came into contact. The cause of agglutination is an antigen-antibody reaction. The erythrocyte membrane includes specific glycolipids which are called agglutinogens The specific antibodies that react with these agglutinogens of the erythrocyte membrane are dissolved in the plasma and called agglutinins. The ABO and Rh systems are of the greatest significance in clinical medicine. In ABO system, group o blood, although containing no agglutinogens, does contain both anti-A and anti- B agglutinins. Group a blood only contains ty A agglutinogens and anti-B agglutinins. Group b blood only contains type B agglutinogens and anti-A agglutinins. Group ab blood contains both A and b agglutinogens but no agglutinins. Two of the three alleles A, B, O(H) are found in the diploid chromosome complement of each ndividual (genotype); together they determine the blood-group phenotype. Blood containing D antigen erythrocytes are called Rh-positive, and those lacking the D antigen property are called Rh-negative. One difference between the Rh and the ABo systems is that the agglutinins of the ABO system are always present after the first few months of life, whereas anti-D antibodies do nothomogeneous population of cells but consist of three major groups: the granulocytes, the monocytes (to form macrophages in tissue), and the lymphocytes (for specific immune responses). These cell types, are distinguished on the basis of morphology, function,and site of origin. According to the staining properties of the granules, the granulocytes are classified as neutropils (for phagocytosis), basophils (for anaphylactic reaction), and eosinophils (to attack parasites and appose anaphylactic reaction). All leukocytes are capable of amoeboid movement, which permits them to emigrate through the walls of blood vessels (this process is also called diapedesis). They are also attracted (chemotaxis) by bacterial toxins, the products of decomposition of bacteria or body cells, and antigen-antibody complexes; they can surround foreign bodies and take them into the cytoplasm (phagocytosis). Healthy adults are found to have 100-300×109 /L platelets in their blood. They are produced in the bone marrow by the shedding of cytoplasmic buds of megakaryocytes. Platelets formation is regulated mainly by a glycoprotein hormone, erythropoietin (EPO). The physiological characteristics of platelets include adhesion, aggregation, secretion reactions, absorption, contraction and repair. The main function of platelets is hemostasis. The stoppage of bleeding is known as hemostasis Whenever a vessel is severed or ruptured, hemostasis is achieved by several mechanisms: (1) vascular spasm, (2) formation of a platelet plug, and (3) formation of a blood clot as a result of blood coagulation. From the laboratory viewpoint, the coagulation can be brought about by an extrinsic (tissue-based) pathway or intrinsic (plasma-based) pathway, each of which is made up of many steps involving clotting factors. The result of either extrinsic pathway or intrinsic pathway is the formation of a complex of activated substances collectively called prothrombin activator, which catalyzes the conversion of prothrombin into thrombin. The thrombin acts as an enzyme to convert fibrinogen into fibrin fibers that enmesh blood cells and plasma to form the clot. There are four plasma anti-clotting substances that oppose clot formation to limit this process and prevent it from spreading excessively. They are serine protease inhibitor, heparin , protein C system and tissue factor pathway inhibitor. A fibrin clot is a transitory device until permanent repair of the vessel occurs. The fibrinolytic system is the principal effecter of clot removal. It constitutes a plasma proenzyme, plasminogen, which can be activated to the active enzyme plasmin by plasminogen activators. Once formed, plasmin digests fibrin, thereby dissolving the clot. Agglutination would occur in the circulatory system following blood transfusion, if two incompatible types of blood came into contact. The cause of agglutination is an antigen-antibody reaction. The erythrocyte membrane includes specific glycolipids which are called agglutinogens. The specific antibodies that react with these agglutinogens of the erythrocyte membrane are dissolved in the plasma and called agglutinins. The ABO and Rh systems are of the greatest significance in clinical medicine. In ABO system, group O blood, although containing no agglutinogens, does contain both anti-A and anti-B agglutinins. Group A blood only contains type A agglutinogens and anti-B agglutinins. Group B blood only contains type B agglutinogens and anti-A agglutinins. Group AB blood contains both A and B agglutinogens but no agglutinins. Two of the three alleles A, B, O (H) are found in the diploid chromosome complement of each individual (genotype); together they determine the blood-group phenotype. Blood containing D antigen erythrocytes are called Rh-positive, and those lacking the D antigen property are called Rh-negative. One difference between the Rh and the ABO systems is that the agglutinins of the ABO system are always present after the first few months of life, whereas anti-D antibodies do not 12