Opsonin


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Related to Opsonin: opsonization, Cytokines

opsonin

[′äp·sə·nən]
(immunology)
A substance in blood serum that renders bacteria more susceptible to phagocytosis by leukocytes.

Opsonin

 

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.

A. N. MATS

References in periodicals archive ?
He knows that Ridgeon's dilemma is false, because Ridgeon has already explained to him at the beginning of act 1 that his discovery is no more than a simple test at the hospital that takes fifteen minutes to indicate the patient's opsonin level.
12) Although most antibodies function as opsonins and accelerate platelet clearance by phagocytic cells, occasional antibodies bind to epitopes on crucial regions of these GPs and impair platelet function.
Further, injection of decoys of nanoparticle can be used to eliminate plasma opsonins and reduce uptake of the nanoparticles.
However as the hydrophobicity of the surface reduces the affinity of the protein to the surface also reduces, and bigger opsonins will attach to the surface.
The classical complement pathway and heat-stable opsonins are required for maximal opsonic activity against the major serotypes of CBS.
It is hypothesized that the polymer-lipid in the bilayer prolongs liposome circulation due to reduced adsorption of opsonins.
In some cases plant lectins have been shown to be able to act as opsonins in invertebrate phagocyte systems, and concanavalin A receptors have been described on oyster hemocytes (Sami et al.