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Opsonin
 
 
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An opsonin is any molecule that acts as a binding enhancer
Enhancer

Enhancer can mean one of the following:* In genetics, an Enhancer is a short region of DNA that can bind proteins called an activator , binding of activators to this enhancer region can initiate the transcription of a gene that may be some distance away from the enhancer, or can even be on a different chromosome....
 for the process of phagocytosis
Phagocytosis

File:Phagocytosis in three steps.pngPhagocytosis is the cell process of Phagocytes and Protists of engulfing solid particles by the cell membrane to form an internal phagosome, which is a food vacuole, or pteroid....
, for example, by coating the negatively-charged molecules on the membrane.

the membrane of a phagocytosing cell, as well as its target, have a negative charge (zeta-potential), making it difficult for the two cells to come close together. During the process of opsonization, antigen
Antigen

An antigen is a substance that prompts the generation of antibodies and can cause an immune response. The word originated from the notion that they can stimulate antibody generation....
s are bound by antibody
Antibody

Antibodies are gamma globulin proteins that are found in blood or other bodily fluids of vertebrates, and are used by the immune system to identify and neutralize foreign objects, such as bacterium and viruses....
 and/or complement
Complement system

The complement system is a biochemical cascade that helps clear pathogens from an organism. It is part of the larger immune system that is not adaptable and does not change over the course of an individual's lifetime; as such it belongs to the innate immunity....
 molecules.






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An opsonin is any molecule that acts as a binding enhancer
Enhancer

Enhancer can mean one of the following:* In genetics, an Enhancer is a short region of DNA that can bind proteins called an activator , binding of activators to this enhancer region can initiate the transcription of a gene that may be some distance away from the enhancer, or can even be on a different chromosome....
 for the process of phagocytosis
Phagocytosis

File:Phagocytosis in three steps.pngPhagocytosis is the cell process of Phagocytes and Protists of engulfing solid particles by the cell membrane to form an internal phagosome, which is a food vacuole, or pteroid....
, for example, by coating the negatively-charged molecules on the membrane.

Mechanism

Both the membrane of a phagocytosing cell, as well as its target, have a negative charge (zeta-potential), making it difficult for the two cells to come close together. During the process of opsonization, antigen
Antigen

An antigen is a substance that prompts the generation of antibodies and can cause an immune response. The word originated from the notion that they can stimulate antibody generation....
s are bound by antibody
Antibody

Antibodies are gamma globulin proteins that are found in blood or other bodily fluids of vertebrates, and are used by the immune system to identify and neutralize foreign objects, such as bacterium and viruses....
 and/or complement
Complement system

The complement system is a biochemical cascade that helps clear pathogens from an organism. It is part of the larger immune system that is not adaptable and does not change over the course of an individual's lifetime; as such it belongs to the innate immunity....
 molecules. Phagocytic cells express receptors, CR1 and Fc receptors, that bind opsonin molecules, C3b and antibody, respectively. With the antigen coated in these molecules, binding of the antigen to the phagocyte is greatly enhanced. Most phagocytic binding cannot occur without opsonization of the antigen.

Furthermore, opsonization of the antigen and subsequent binding to an activated phagocyte will cause increased expression of complement receptors on neighboring phagocytes.

Examples

Examples of opsonin molecules include:
  • antibodies: IgG and IgA
    IGA

    IGA may stand for:Acronyms* Islamic Golden Age, also known as the Islamic Renaissance* Irish Games Association* International Gamers Award...
  • components of the complement system: C3b
    C3b

    C3b is a one of the elements formed by the cleavage of complement component 3.C3b may bind to microbe cell surfaces within an organism's body....
    , C4b, and iC3b
    IC3b

    iC3b is the proteolyticly inactive product of the complement cleavage fragment C3b that still opsonizes microbes, but cannot associate with factor B....
  • Surfactant
    Surfactant

    Surfactants are wetting agents that lower the surface tension of a liquid, allowing easier spreading, and lower the interfacial tension between two liquids....
  • Mannose-binding lectin (initiates the formation of C3b)


The most important are IgG and C3b.

See also

Antibody opsonization
Antibody opsonization

Antibody opsonization is the process by which a pathogen is marked for ingestion and destruction by a phagocyte. Opsonization involves the binding of an opsonin, i.e., antibody, to a receptor on the pathogen's cell membrane....
.

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