complement fixation

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a term originally used to refer to the heat-labile factor in serum that causes immune cytolysis (lysis of antibody-coated cells). It is now used to refer to the entire functionally related system comprising at least 20 distinct serum proteins, their cellular receptors, and related regulatory proteins; this system is the effector not only of immune cytolysis but also of other biologic functions including anaphylaxis, phagocytosis, opsonization, and hemolysis.

Complement activation occurs by two different sequences, the classical pathway and the alternative pathway. All of the “components of complement,” designated C1 through C9 (C1 being composed of three distinct proteins, C1q, C1r, and C1s), participate in the classical pathway; the alternative pathway lacks components C1, C2, and C4 but adds factor b, factor d, and properdin. Regulatory proteins include factor h, factor i, clusterin, C3 nephritic factor, decay accelerating factor, homologous restriction factor, C1 inhibitor, C4 binding protein, membrane cofactor protein, protectin, and vitronectin.

The classical pathway is primarily activated by the binding of C1 to antigen-antibody complexes containing the immunoglobulins IgM or IgG. The alternative pathway can be activated by IgA immune complexes and also by nonimmunologic materials including bacterial endotoxins, microbial polysaccharides, and cell walls. Activation of the classical pathway triggers an enzymatic cascade involving C1, C4, C2, and C3; activation of the alternative pathway triggers a cascade involving C3 and factors B and D and properdin. Both pathways result in cleavage of C5 and formation of the membrane attack complex, which in its final state creates a pore in the cell wall and causes cell lysis. Complement activation also results in the formation of many biologically active complement fragments that act as anaphylatoxins, opsonins, or chemotactic factors. Fragments resulting from proteolytic cleavage of complement proteins are designated with lower-case-letter suffixes, e.g., C3a.
 Complement activation. Activation of the classical and alternative pathways leads to a common terminal pathway from C5 to C9. These complement components form the final membrane attack complex (MAC). Other intermediate complexes and fragments are also biologically active: opsonins facilitate phagocytosis, anaphylatoxins act on mast cells and mediate a release of histamine which acts on blood vessels, and chemotactic fragments and intermediate complexes attract leukocytes to the site of inflammation. Redrawn from Damjanov, 2000.
complement fixation the combining of complement with the antigen-antibody complex, rendering the complement inactive, or fixed. Its presence or absence as free, active complement can be shown by adding sensitized blood cells to the mixture. If free complement is present, hemolysis occurs; if not, no hemolysis is observed. This reaction is the basis of many serologic tests for infection, including the wassermann test for syphilis, and reactions for gonococcus infection, glanders, typhoid fever, tuberculosis, and amebiasis. Called also Bordet-Gengou phenomenon. See also immunity.
complement fixation tests tests that use antigen-antibody reaction and result in hemolysis to determine the presence of various organisms in the blood; see also complement fixation.


1. the act or operation of holding, suturing, or fastening in a fixed position.
2. the condition of being held in a fixed position.
3. in psychiatry, a term with two related but distinct meanings: (a) arrest of development at a particular stage (if this is temporary it is a normal reaction to difficulties, but if continued it is a cause of emotional problems); and (b) a close and suffocating attachment to some person, especially a childhood figure such as a parent.
4. in microscopy, the treatment of material so that its structure can be examined in greater detail with minimal alteration of the normal state, and also to provide information concerning the chemical properties (as of cell constituents) by interpretation of fixation reactions.
5. in chemistry, the process whereby a substance is removed from the gaseous or solution phase and localized, as in carbon dioxide or nitrogen fixation.
6. in ophthalmology, direction of the gaze so that the visual image of the object falls on the fovea centralis.
7. in film processing, the chemical removal of all unexposed and undeveloped silver compounds of the film emulsion, as on x-ray films.
complement fixation see complement fixation.
intermaxillary fixation (IMF) a technique used to stabilize a fractured jaw; the teeth are wired or banded together. Extreme caution must be exercised to insure that oral secretions and vomitus are not aspirated as the patient is unable to expectorate any fluids. Antiemetics are often administered to prevent vomiting. Wire cutters should be kept with the patient at all times.
Teeth wired in intermaxillary fixation. From Ignatavicius et al., 1995.

com·ple·ment fix·a·tion

1. antigen-antibody interactions cause a conformational shift in immunoglobulin structure promoting binding of C1 and subsequent activation of the complement cascade, that is, complement fixation.
See also: Bordet-Gengou phenomenon, Wassermann test.
2. an immunoassay using reference antibodies with complement fixation and lysis of red blood cells as an indicator of detectable antigen. The degree of hemolysis affects the optic density of the sample, which can be measured and compared with a standard reference concentration curve.
See also: Bordet-Gengou phenomenon, Wassermann test.

complement fixation

The binding of active serum complement to an antigen-antibody pair. It is the basis for various diagnostic tests to detect the presence of a specific antigen or antibody.

complement fixation

an immunological reaction in which an antigen combines with an antibody and its complement, causing the complement factor to become inactive or fixed. The complement-fixation reaction can be tested in the laboratory by exposing the patient's serum to antigen, complement, and specially sensitized red blood cells. Complement-fixation tests can be used to detect antibodies for infectious diseases, especially syphilis and viral illnesses. They are rarely used in clinical practice today. See also anticomplement, complement, immune system, immunity, Wassermann blood test.
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Complement fixation

com·ple·ment fix·a·tion

(kom'plĕ-mĕnt fik-sā'shŭn)
A process in serum whereby an antigen-antibody combination is rendered unavailable to complete a reaction in a second antigen-antibody combination for which complement is necessary; the second system usually serves as an indicator (red blood cells plus specific hemolysin); if complement is fixed with the first antigen-antibody union, hemolysis does not occur, but, if complement is not so removed, it causes hemolysis in the second system.

complement fixation

The taking up of COMPLEMENT by antigen-antibody complexes. This process forms the basis of a number of tests to confirm infection with particular organisms.


Jules, Belgian bacteriologist and Nobel laureate, 1870-1961.
Bordet amboceptor
Bordetella - a genus of strictly aerobic bacteria that are pathogens of the mammalian respiratory tract.
Bordet-Gengou bacillus - a species that causes whooping cough. Synonym(s): Bordetella pertussis
Bordet-Gengou phenomenon - the phenomenon of complement fixation.
Bordet-Gengou potato blood agar - glycerin-potato agar with 25% of blood, used for the isolation of Bordetella pertussis.
Bordet-Gengou reaction - Synonym(s): complement fixation


Octave, French bacteriologist, 1875-1957.
Bordet-Gengou bacillus - see under Bordet
Bordet-Gengou phenomenon - see under Bordet
Bordet-Gengou potato blood agar - see under Bordet
Bordet-Gengou reaction - Synonym(s): complement fixation
Gengou phenomenon - noncellular antigens.

complement fixation,

n an immune reaction in which an antigen-antibody complex inactivates or fixes the complement of the antibody. Complement fixation tests are used to determine antibodies for infectious diseases, such as syphilis and certain viral infections.
Enlarge picture
Complement fixation.

com·ple·ment fix·a·tion

(kom'plĕ-mĕnt fik-sā'shŭn)
A process in serum whereby an antigen-antibody combination is rendered unavailable to complete a reaction in a second antigen-antibody combination for which complement is necessary.