| extinction factor | <microscopy> In a polarizing or Differential Interference Contrast microscope, the ratio of the amount of light that is transmitted with the axes of the polars parallel over the amount transmitted with their axes crossed. Unless the lenses are rectified, the extinction factor drops exponentially as the numerical aperture is increased. Acronym: EF (26 Mar 1998) |
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| extrinsic factor | Dietary vitamin B12. (05 Mar 2000) |
| extrinsic factor deficiency | <disease> An inherited disorder that causes abnormal blood clotting due to the congenital absence of one of the 20 different plasma proteins involved in the coagulation process. Symptoms include bleeding of the gums, nosebleeds, easy bruising, bleeding in muscles or joints and excessive menstrual bleeding. Treatment includes the administration of plasma concentrates of factor VII (extrinsic factor). (27 Sep 1997) |
| factor | <pharmacology> Any of several substances or activities that are necessary to produce a result, for example a coagulation factor. Often, use of the term factor indicates that the chemical nature of the substance or its mechanism of action is unknown, as in endocrinology, where factors are renamed as hormones when their chemical nature is determined. (18 Nov 1997) |
| factor 3 | Operational name given to an incompletely characterised selenium-containing natural product which, in minute amounts, prevents liver damage in rats due to deficiency of vitamin E, factor III in the vitamin B12 series, 5-hydroxybenzimidazole, analogue of the usual B12 nucleotide components. (05 Mar 2000) |
| factor A | A component of the properdin system; a hydrazine-sensitive b1-globulin (mw about 180,000), now known to be C3 (third component of complement). (05 Mar 2000) |
| factor analysis, statistical | A set of statistical methods for analyzing the correlations among several variables in order to estimate the number of fundamental dimensions that underlie the observed data and to describe and measure those dimensions. It is used frequently in the development of scoring systems for rating scales and questionnaires. (12 Dec 1998) |
| factor B | <enzyme> A glycine-rich, heat-labile beta-glycoprotein found in blood. It is a proactivator of complement 3 in the alternate pathway of complement activation. Factor b is converted by factor d to c3 convertase. Registry number: EC 3.4.21.47 (12 Dec 1998) |
| factor D | <enzyme> A serum protein which during the alternate pathway of complement activation converts the inactive properdin factor b to c3 convertase. Registry number: EC 3.4.21.46 (12 Dec 1998) |
| factor E | A serum protein (mw 160,000) required for activation of C3 (third component of complement) by cobra venom factor. See: properdin system. (05 Mar 2000) |
| factor Gm | A factor that determines certain of the allotypes of human immunoglobulins; found only on the g chains of IgG (gamma-globulin). (05 Mar 2000) |
| factor H | Former designation for biotin, vitamin B12 analogue or precursor, a glycoprotein that regulates the activity of complement factor C3b; a deficiency results in the lack of inhibition of the alternative haemolytic pathway leading to continuous activation and consumption of factor C3 (haemolytic uraemic syndrome). (05 Mar 2000) |
| factor I | <haematology> A protein which is synthesised by the liver. Fibrinogen is converted to fibrin, in the formation of a blood clot, via the enzymatic action of thrombin. A fibrinogen assay measures the concentration of fibrinogen in the blood and may be used to evaluate abnormal blood clotting. A lack of fibrinogen may be congenital or acquired. A condition known as DIC results in the excessive utilisation and depletion of fibrinogen. Synonym: fibrinogen. (15 Nov 1997) |
| factor II | <haematology> A protein which is synthesised by the liver. Fibrinogen is converted to fibrin, in the formation of a blood clot, via the enzymatic action of thrombin. A fibrinogen assay measures the concentration of fibrinogen in the blood and may be used to evaluate abnormal blood clotting. A lack of fibrinogen may be congenital or acquired. A condition known as DIC results in the excessive utilisation and depletion of fibrinogen. Synonym: thrombin. (15 Nov 1997) |
| factor IIa | <enzyme> Protease (34 kD) generated in blood clotting that acts on fibrinogen to produce fibrin. Consists of two chains, A and B, linked by a disulphide bond. B chain has sequence homology with pancreatic serine proteases: cleaves at Arg Gly. Thrombin is produced from prothrombin by the action either of the extrinsic system (tissue factor + phospholipid) or, more importantly, the intrinsic system (contact of blood with a foreign surface or connective tissue). Both extrinsic and intrinsic systems activate plasma factor X to form factor Xa which then, in conjunction with phospholipid (tissue derived or platelet factor 3) and factor V, catalyses the conversion. (18 Nov 1997) |