Platelet Adhesiveness: The process whereby PLATELETS adhere to something other than platelets, e.g., COLLAGEN; BASEMENT MEMBRANE; MICROFIBRILS; or other "foreign" surfaces.Blood Platelets: Non-nucleated disk-shaped cells formed in the megakaryocyte and found in the blood of all mammals. They are mainly involved in blood coagulation.Platelet Aggregation: The attachment of PLATELETS to one another. This clumping together can be induced by a number of agents (e.g., THROMBIN; COLLAGEN) and is part of the mechanism leading to the formation of a THROMBUS.Blood Coagulation Tests: Laboratory tests for evaluating the individual's clotting mechanism.Blood Cell Count: The number of LEUKOCYTES and ERYTHROCYTES per unit volume in a sample of venous BLOOD. A complete blood count (CBC) also includes measurement of the HEMOGLOBIN; HEMATOCRIT; and ERYTHROCYTE INDICES.Adenine NucleotidesFibrinolysis: The natural enzymatic dissolution of FIBRIN.Fibrinogen: Plasma glycoprotein clotted by thrombin, composed of a dimer of three non-identical pairs of polypeptide chains (alpha, beta, gamma) held together by disulfide bonds. Fibrinogen clotting is a sol-gel change involving complex molecular arrangements: whereas fibrinogen is cleaved by thrombin to form polypeptides A and B, the proteolytic action of other enzymes yields different fibrinogen degradation products.Adhesiveness: A property of the surface of an object that makes it stick to another surface.Platelet Count: The number of PLATELETS per unit volume in a sample of venous BLOOD.Cell Adhesion: Adherence of cells to surfaces or to other cells.Platelet Membrane Glycoproteins: Surface glycoproteins on platelets which have a key role in hemostasis and thrombosis such as platelet adhesion and aggregation. Many of these are receptors.Platelet Factor 4: A CXC chemokine that is found in the alpha granules of PLATELETS. The protein has a molecular size of 7800 kDa and can occur as a monomer, a dimer or a tetramer depending upon its concentration in solution. Platelet factor 4 has a high affinity for HEPARIN and is often found complexed with GLYCOPROTEINS such as PROTEIN C.Platelet Transfusion: The transfer of blood platelets from a donor to a recipient or reinfusion to the donor.Platelet Activating Factor: A phospholipid derivative formed by PLATELETS; BASOPHILS; NEUTROPHILS; MONOCYTES; and MACROPHAGES. It is a potent platelet aggregating agent and inducer of systemic anaphylactic symptoms, including HYPOTENSION; THROMBOCYTOPENIA; NEUTROPENIA; and BRONCHOCONSTRICTION.Platelet Function Tests: Laboratory examination used to monitor and evaluate platelet function in a patient's blood.Platelet Glycoprotein GPIIb-IIIa Complex: Platelet membrane glycoprotein complex important for platelet adhesion and aggregation. It is an integrin complex containing INTEGRIN ALPHAIIB and INTEGRIN BETA3 which recognizes the arginine-glycine-aspartic acid (RGD) sequence present on several adhesive proteins. As such, it is a receptor for FIBRINOGEN; VON WILLEBRAND FACTOR; FIBRONECTIN; VITRONECTIN; and THROMBOSPONDINS. A deficiency of GPIIb-IIIa results in GLANZMANN THROMBASTHENIA.Cell Aggregation: The phenomenon by which dissociated cells intermixed in vitro tend to group themselves with cells of their own type.Platelet Glycoprotein GPIb-IX Complex: Platelet membrane glycoprotein complex essential for normal platelet adhesion and clot formation at sites of vascular injury. It is composed of three polypeptides, GPIb alpha, GPIb beta, and GPIX. Glycoprotein Ib functions as a receptor for von Willebrand factor and for thrombin. Congenital deficiency of the GPIb-IX complex results in Bernard-Soulier syndrome. The platelet glycoprotein GPV associates with GPIb-IX and is also absent in Bernard-Soulier syndrome.Thrombin: An enzyme formed from PROTHROMBIN that converts FIBRINOGEN to FIBRIN.Adenosine Diphosphate: Adenosine 5'-(trihydrogen diphosphate). An adenine nucleotide containing two phosphate groups esterified to the sugar moiety at the 5'-position.P-Selectin: Cell adhesion molecule and CD antigen that mediates the adhesion of neutrophils and monocytes to activated platelets and endothelial cells.Antigens, Human Platelet: Human alloantigens expressed only on platelets, specifically on platelet membrane glycoproteins. These platelet-specific antigens are immunogenic and can result in pathological reactions to transfusion therapy.Thrombocytopenia: A subnormal level of BLOOD PLATELETS.Endothelium, Vascular: Single pavement layer of cells which line the luminal surface of the entire vascular system and regulate the transport of macromolecules and blood components.Cell Adhesion Molecules: Surface ligands, usually glycoproteins, that mediate cell-to-cell adhesion. Their functions include the assembly and interconnection of various vertebrate systems, as well as maintenance of tissue integration, wound healing, morphogenic movements, cellular migrations, and metastasis.Neutrophils: Granular leukocytes having a nucleus with three to five lobes connected by slender threads of chromatin, and cytoplasm containing fine inconspicuous granules and stainable by neutral dyes.Collagen: A polypeptide substance comprising about one third of the total protein in mammalian organisms. It is the main constituent of SKIN; CONNECTIVE TISSUE; and the organic substance of bones (BONE AND BONES) and teeth (TOOTH).Megakaryocytes: Very large BONE MARROW CELLS which release mature BLOOD PLATELETS.Bleeding Time: Duration of blood flow after skin puncture. This test is used as a measure of capillary and platelet function.Cells, Cultured: Cells propagated in vitro in special media conducive to their growth. Cultured cells are used to study developmental, morphologic, metabolic, physiologic, and genetic processes, among others.Leukocytes: White blood cells. These include granular leukocytes (BASOPHILS; EOSINOPHILS; and NEUTROPHILS) as well as non-granular leukocytes (LYMPHOCYTES and MONOCYTES).Blood Coagulation: The process of the interaction of BLOOD COAGULATION FACTORS that results in an insoluble FIBRIN clot.von Willebrand Factor: A high-molecular-weight plasma protein, produced by endothelial cells and megakaryocytes, that is part of the factor VIII/von Willebrand factor complex. The von Willebrand factor has receptors for collagen, platelets, and ristocetin activity as well as the immunologically distinct antigenic determinants. It functions in adhesion of platelets to collagen and hemostatic plug formation. The prolonged bleeding time in VON WILLEBRAND DISEASES is due to the deficiency of this factor.Thrombosis: Formation and development of a thrombus or blood clot in the blood vessel.Platelet Membrane Glycoprotein IIb: Platelet membrane glycoprotein IIb is an integrin alpha subunit that heterodimerizes with INTEGRIN BETA3 to form PLATELET GLYCOPROTEIN GPIIB-IIIA COMPLEX. It is synthesized as a single polypeptide chain which is then postranslationally cleaved and processed into two disulfide-linked subunits of approximately 18 and 110 kDa in size.Intercellular Adhesion Molecule-1: A cell-surface ligand involved in leukocyte adhesion and inflammation. Its production is induced by gamma-interferon and it is required for neutrophil migration into inflamed tissue.Platelet Factor 3: A phospholipid from the platelet membrane that contributes to the blood clotting cascade by forming a phospholipid-protein complex (THROMBOPLASTIN) which serves as a cofactor with FACTOR VIIA to activate FACTOR X in the extrinsic pathway of BLOOD COAGULATION.Bacterial Adhesion: Physicochemical property of fimbriated (FIMBRIAE, BACTERIAL) and non-fimbriated bacteria of attaching to cells, tissue, and nonbiological surfaces. It is a factor in bacterial colonization and pathogenicity.Vascular Cell Adhesion Molecule-1: Cytokine-induced cell adhesion molecule present on activated endothelial cells, tissue macrophages, dendritic cells, bone marrow fibroblasts, myoblasts, and myotubes. It is important for the recruitment of leukocytes to sites of inflammation. (From Pigott & Power, The Adhesion Molecule FactsBook, 1993, p154)Microscopy, Electron, Scanning: Microscopy in which the object is examined directly by an electron beam scanning the specimen point-by-point. The image is constructed by detecting the products of specimen interactions that are projected above the plane of the sample, such as backscattered electrons. Although SCANNING TRANSMISSION ELECTRON MICROSCOPY also scans the specimen point by point with the electron beam, the image is constructed by detecting the electrons, or their interaction products that are transmitted through the sample plane, so that is a form of TRANSMISSION ELECTRON MICROSCOPY.Integrins: A family of transmembrane glycoproteins (MEMBRANE GLYCOPROTEINS) consisting of noncovalent heterodimers. They interact with a wide variety of ligands including EXTRACELLULAR MATRIX PROTEINS; COMPLEMENT, and other cells, while their intracellular domains interact with the CYTOSKELETON. The integrins consist of at least three identified families: the cytoadhesin receptors(RECEPTORS, CYTOADHESIN), the leukocyte adhesion receptors (RECEPTORS, LEUKOCYTE ADHESION), and the VERY LATE ANTIGEN RECEPTORS. Each family contains a common beta-subunit (INTEGRIN BETA CHAINS) combined with one or more distinct alpha-subunits (INTEGRIN ALPHA CHAINS). These receptors participate in cell-matrix and cell-cell adhesion in many physiologically important processes, including embryological development; HEMOSTASIS; THROMBOSIS; WOUND HEALING; immune and nonimmune defense mechanisms; and oncogenic transformation.Thromboxane A2: An unstable intermediate between the prostaglandin endoperoxides and thromboxane B2. The compound has a bicyclic oxaneoxetane structure. It is a potent inducer of platelet aggregation and causes vasoconstriction. It is the principal component of rabbit aorta contracting substance (RCS).