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A substance in blood serum that renders bacteria more susceptible to phagocytosis by leukocytes.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a type of antibody. Opsonins are immunoglobulins of the G class (IgG) and are largely responsible for the body’s resistance to bacteria, viruses, and tumors (seeIMMUNOGLOBULINS).

The British scientists A. Wright and S. Douglas introduced the term “opsonin” in 1903 to denote humoral factors in the blood that facilitate and stimulate the phagocytosis of bacteria by leukocytes. The cytophil portion of the opsonin molecule has an affinity for the plasma membrane of phagocytes. When opsonins combine with bacterial or viral antigens or with foreign macromolecules, the cytophil portion becomes exposed and attached to the surface of a phagocyte. An opsonin promotes the adhesion reaction and the absorption and destruction of a particle by reducing the energy of the surface interaction between the leukocyte and the object of phagocytosis. This reduction of energy can be accomplished, for example, by decreasing the repulsive electrostatic forces between the leukocyte and the foreign body.

Immunoglobulins of the M class (IgM) can also act as opsonins in the presence of complement (seeCOMPLEMENT). The first five components of complement greatly enhance the ability of IgG to act as an opsonin. In addition to humoral antibodies, cytophil antibodies that are fixed on certain phagocytes are opsonins. Like complement, fibrin and a polypeptide that is released by lymphocytes upon contact with certain antigens act as nonspecific opsonins. Although insects do not have immunoglobulins or phagocytes with IgG receptors, the hemolymph of insects does contain specific opsonin proteins.

Opsonins are responsible for selectivity, an important property of the phagocytic reaction. Because of opsonins, a phagocyte recognizes and attacks only foreign substances, not its own macromolecules. Some bacterial substances, for example, polysaccharides of pneumococci and meningococci and proteins of streptococci, are able to suppress the phagocytic activity of leukocytes. Antibodies to these suppressor substances perform the function of opsonins. Virulent strains of staphylococci and Escherichia coli release a specific protein that can block the cytophil portion of an opsonin, thereby inhibiting phagocytosis. Along with complement, thrombocytes, and phagocytes, opsonins neutralize foreign substances and microbes.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
Van Voorhis, "Opsonic potential, protective capacity, and sequence conservation of the Treponema pallidum subspecies pallidum Tp92," Journal of Infectious Diseases, vol.
Johannisson, "Opsonic capacity of foal serum for the two neonatal pathogens Escherichia coli and Actinobacillus equuli," Equine Veterinary Journal, vol.
Improved circulation time can be mainly attributable to the prohibited biorecognition of the adsorbed opsonic proteins by the macrophages.
falciparum infection has been attributed to the impaired opsonic phagocytic clearance of malaria parasites, especially in multigravid women (33,46), and reduced levels of specific antibodies associated with protection against malaria in pregnancy.
Neutrophyl function, serum opsonic activity, and delayed hypersensitivity in surgical patients.
Phagocytes can easily recognize and ingest non-self cells and particles (Smith, 1970; Anderson, 1971; Fuke and Fukumoto, 1993; Ballarin et at., 1994; Ohtake et at., 1994; Dan-Sohkawa et at., 1995; Cima et at., 1996) and are able to synthesize and release opsonic agglutinins (Coombe et at., 1984; Kelly et at., 1992; Ballarmn et at., 1999).
When bacteriologist Almroth Wright claimed a successful vaccine for typhoid, based on an "opsonic index" of blood antibodies, Greenwood showed in 1908 that the frequency distributions were skewed.
Avery and Gordon, [21] studied the opsonic components of breast milk responsible for phagocytosis of surface-adherent Staphylococcus aureus by human polymorphonuclear leukocytes.
Flagellin was found to be important for protective immunity by opsonic activity, resulting in the clearance of C.
There are various factors which attribute for high association of diabetes with TB like low opsonic index, decreased bactericidal activity, decreased synthesis of collagen, impaired defensive function of reticuloendothelial cell, increased availability of glycerol in tissue that act as substrate for growth of Mycobacterium tuberculosis bacilli, lack of adequate substrate for antibody formation due to disturbed protein metabolism and lowered production of IL-1 beta and TNF alpha by peripheral blood monocytes.
Tabona et al., "Molecular basis of opsonic defect in immunodeficient children," The Lancet, vol.
Generally, [greater than or equal to]50% opsonic killing activity of immune serum is considered biologically significant [10].