| C3a | activated third component of complement |
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| C4 | fourth cervical nerve; fourth cervical vertebra; fourth component of complement |
| C4a | activated fourth component of complement |
| C5 | fifth cervical nerve; fifth cervical vertebra; fifth component of complement |
| C6 | sixth cervical nerve; sixth cervical vertebra; sixth component of complement |
| complement-fixation test | An immunological test for determining the presence of a particular antigen or antibody when one of the two is known to be present, based on the fact that complement is "fixed" in the presence of antigen and its specific antibody. See: Bordet-Gengou phenomenon. (05 Mar 2000) |
|---|---|
| complement fixation tests | Serologic tests based on inactivation of complement by the antigen-antibody complex (stage 1). Binding of free complement can be visualised by addition of a second antigen-antibody system such as red cells and appropriate red cell antibody (haemolysin) requiring complement for its completion (stage 2). Failure of the red cells to lyse indicates that a specific antigen-antibody reaction has taken place in stage 1. If red cells lyse, free complement is present indicating no antigen-antibody reaction occurred in stage 1. (12 Dec 1998) |
| complement-fixing antibody | Antibody that combines with and sensitises antigen leading to the activation of complement, which may result in cell lysis. Synonym: CF antibody, sensitizing substance. (05 Mar 2000) |
| complement haemolytic activity assay | Usual screening assay for complement. Dilutions of the serum to be tested are added to antibody-coated erythrocytes and the percentage of lysis is measured. The values are expressed by ch50, haemolytic complement units per milliliter, which is the dilution of serum required to lyse 50 percent of the erythrocytes in the assay. (12 Dec 1998) |
| complement inactivators | Serum proteins which act at key sites in the complement sequence to modulate or prevent the progression of the reaction. Absence of these factors leads to uncontrolled activation of the complement system with accompanying disease. (12 Dec 1998) |
| complement membrane attack complex | The assembly of complement plasma glycoproteins c5b, c6, c7, c8, and polymeric c9 as a group on biological membranes. The complex forms transmembrane channels which displace lipid molecules and other constituents, thus disrupting the phospholipid bilayer of target cells leading to cell lysis by osmotic leakage. The formation of the membrane attack complex is the terminal step in the complement cascade. (12 Dec 1998) |
| complement pathway, alternative | The complement activation sequence initiated by the activation of complement factor c3, which is triggered by the interaction of microbial polysaccharides and properdin without participation of an antigen-antibody reaction. (12 Dec 1998) |
| complement pathway, classical | The sequential activation of complement, initiated by antigen-antibody complex and the binding of complement factor c1q to the fc region of the antibody. (12 Dec 1998) |
| complement system | A group of more than 20 serum proteins, some of which can be serially activated and participate in a cascade resulting in cell lysis. (05 Mar 2000) |
| complement unit | The smallest amount (highest dilution) of complement that will cause haemolysis of a unit of red blood cells in the presence of a haemolysin unit. Synonym: alexin unit. (05 Mar 2000) |
| component of complement | Any one of the nine distinct protein units (designated C1 through C9 and distributed in the a, b, and g electrophoretic partitions of normal serum) that effect the immunological activities long associated with complement. C1 is a complex of three subunits: C1q, C1r, and C1s. C1q (overbar indicates "active form") activates proenzyme C1r to C1r which activates C1s to C1s (also known as C1 esterase), which converts proenzyme C2 to C2b and produces C4b from C4. C2b combines with C4b to form "classical-complement-pathway C3/C5 convertase" (also known as C3 convertase, C5 convertase, and C42). This enzyme cleaves C3 to C3a and C3b, and C5 to yield C5a and C5b, as does "alternative-complement-pathway C3/C5 convertase" (also known as proenzyme factor B, properdin factor B, C3 proactivator, and heat-labile factor). Complement factor I (also known as C3b or C3b/C4b inactivator) inactivates C3b and C4b by a different proteolytic cleavage. Several autosomal recessive disorders have been identified in which one or more of the complement components have been deficient or completely absent. (05 Mar 2000) |
| heparin complement | The protein component of heparin in blood. (05 Mar 2000) |
| thyrotoxic complement-fixation factor | A form of thyrotoxin; an antigen found most readily in thyroid tissue from thyrotoxic individuals; known to be chemically and immunologically distinct from thyroglobulin, and fixes complement when combined with antibody related to the gamma-globulin fraction of serum. With the exception of extremely small concentrations, the antigen is rarely found in normal glands or in diseased glands that are not associated with thyrotoxicosis; it is probably an intracellular substance (possibly a constituent of the "microsomal fraction"), and does not contain iodine in significant quantity. Not related to the complement-fixation reaction occurring with serum in Hashimoto's disease, in which the antigen is thyroglobulin. (05 Mar 2000) |