A group of serum proteins involved in the control of inflammation, the activation of phagocytes, and the lytic attack on cell membranes. The system can be activated by interaction with antigen‐antibody complexes or by bacterial substances.
A term originally used to refer to the heat-labile factor in serum that causes immune cytolysis, the lysis of antibody-coated cells, and now referring to the entire functionally related system comprising at least 20 distinct serum proteins that is the effector not only of immune cytolysis but also of other biologic functions. Complement activation occurs by two different sequences, the classic and alternative pathways. The proteins of the classic pathway are termed ‘components of complement’ and are designated by the symbols C1 through C9. C1 is a calcium-dependent complex of three distinct proteins C1q, C1r and C1s. The proteins of the alternative pathway (collectively referred to as the properdin system) and complement regulatory proteins are known by semisystematic or trivial names. Fragments resulting from proteolytic cleavage of complement proteins are designated with lower-case letter suffixes, e.g., C3a. Inactivated fragments may be designated with the suffix ‘i’, e.g. C3bi. Activated components or complexes with biological activity are designated by a bar over the symbol e.g. C1 or C4b,2a. The classic pathway is activated by the binding of C1 to classic pathway activators, primarily antigen-antibody complexes containing IgM, IgG1, IgG3; C1q binds to a single IgM molecule or two adjacent IgG molecules. 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 classic pathway triggers an enzymatic cascade involving C1, C4, C2 and C3; activation of the alternative pathway triggers a cascade involving C3 and factors B, D and P. Both result in the cleavage of C5 and the formation of the membrane attack complex. Complement activation also results in the formation of many biologically active complement fragments that act as anaphylatoxins, opsonins, or chemotactic factors.
The fourth component to attach in the complement reaction sequence. It is a beta-globulin with a sedimentation coefficient of 5.5, a molecular weight of 185,000 and a serum concentration of 1.3 micrograms/ml. Its fragments have anaphylatoxic, chemotactic, and histaminic action and affect smooth muscle.
A group of more than 15 soluble proteins found in blood serum that interacts in a sequential fashion, in which a precurser molecule is converted into an active enzyme. Each enzyme uses the next molecule in the system as a substrate and converts it into its active (enzyme) form. This cascade of events and reactions leads ultimately to the formation of an attack complex that forms a transmembrane channel in the cell membrane. It is the presence of the channel that leads to lysis (rupturing) of the cell.
In biology, a normal component of mammalian serum which must be present in order for some lysins to function.
A substance which forms part of blood plasma and is essential to the work of antibodies and antigens.
Thermolabile, non-specific constituent of serum which is an essential link in the action of a specific serum antibody (bacteriolysin, bactericidin, or haemolysin etc) against its homologous antigen (bacteria, red cells etc.). Complement thus used is said to be ‘fixed’.
One of a series of enzymes, part of the immune-response mechanism in the blood serum, that works to break down invading microorganisms.
A blood protein capable of killing bacteria and other cells or organisms in the presence of an antigen-antibody complex.
A substance in the blood, consisting of a group of nine different fractions, that aids the body’s defenses when antibodies combine with invading antigens. Complement is involved with the breaking up (lysis), agglutination, and ‘opsonization of foreign cells. Following antibody antigen reaction it may also attract scavenging cells (‘phagocytes) to the area of conflict.
Complement is a normal constituent of blood serum which plays an important part in the antibody-antigen reaction, which is the basis of many immunity processes.
A group of proteins in the blood that play a vital role in the body’s immune defenses through a cascade of interactions. Components of complement are labeled Cl through C9.
A substance that is sensitive to heat and found in the blood serum of higher animals has the capability to join with the complex formed by the binding of an antigen to an antibody. Once this binding occurs, the complement is eliminated from the serum. This process of removing the complement from the serum, also known as complement fixation, is utilized as the foundation for a delicate test which can detect antigen-antibody reactions.
One among numerous proteins present in the blood that collaborates with other proteins to aid in the eradication of bacteria.
A cluster of proteins present in the plasma, the liquid component of blood, which plays a crucial role in eliminating foreign cells and serves as a vital component of the immune system.
A complex material present in blood plasma that, aided by antibodies, eliminates bacteria and other foreign substances.