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Opsonins are extracellular proteins that, when bound to substances or cells, induce
phagocytes Phagocytes are cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells. Their name comes from the Greek ', "to eat" or "devour", and "-cyte", the suffix in biology denoting "cell", from the Greek '' ...
to phagocytose the substances or cells with the opsonins bound. Thus, opsonins act as tags to label things in the body that should be phagocytosed (i.e. eaten) by phagocytes (cells that specialise in
phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...
, i.e. cellular eating). Different types of things ("targets") can be tagged by opsonins for phagocytosis, including: pathogens (such as bacteria), cancer cells, aged cells, dead or dying cells (such as apoptotic cells), excess synapses, or protein aggregates (such as amyloid plaques). Opsonins help clear pathogens, as well as dead, dying and diseased cells. Opsonins were discovered and named "opsonins" in 1904 by Wright and Douglas, who found that incubating bacteria with blood plasma enabled phagocytes to phagocytose (and thereby destroy) the bacteria. They concluded that: “We have here conclusive proof that the blood fluids modify the bacteria in a manner which renders them a ready prey to the phagocytes. We may speak of this as an “opsonic” effect (opsono - I cater for; I prepare victuals for), and we may employ the term “opsonins” to designate the elements in the blood fluids which produce this effect.” Subsequent research found two main types of opsonin in blood that opsonised bacteria:
complement A complement is something that completes something else. Complement may refer specifically to: The arts * Complement (music), an interval that, when added to another, spans an octave ** Aggregate complementation, the separation of pitch-clas ...
proteins and
antibodies An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...
. However, there are now known to be at least 50 proteins that act as opsonins for pathogens or other targets.


Mechanisms

Opsonins induce phagocytosis of targets by binding the targets (e.g. bacteria) and then also binding phagocytic receptors on phagocytes. Thus, opsonins act as bridging molecules between the target and the phagocyte, bringing them into contact, and then usually activating the phagocytic receptor to induce engulfment of the target by the phagocyte. All cell membranes have negative charges (
zeta potential Zeta potential is the electrical potential at the slipping plane. This plane is the interface which separates mobile fluid from fluid that remains attached to the surface. Zeta potential is a scientific term for electrokinetic potential in coll ...
) which makes it difficult for two cells to come close together. When opsonins bind to their targets they boost the kinetics of phagocytosis by favoring interaction between the opsonin and cell surface receptors on immune cells. This overrides the negative charges from cell membranes. It is important that opsonins do not tag healthy, non-pathogenic cells for phagocytosis, as phagocytosis results in digestion and thus destruction of targets. Therefore, Some opsonins (including some complement proteins) have evolved to bind
Pathogen-associated molecular patterns Pathogen-associated molecular patterns (PAMPs) are small molecular motifs conserved within a class of microbes. They are recognized by toll-like receptors (TLRs) and other pattern recognition receptors (PRRs) in both plants and animals. A vast arra ...
, molecules only found on the surface of pathogens, enabling phagocytosis of these pathogens, and thus innate immunity. Antibodies bind to antigens on the pathogen surface, enabling adaptive immunity. Opsonins that opsonise host body cells (e.g.
GAS6 Growth arrest – specific 6, also known as GAS6, is a human gene coding for the GAS6 protein. It is similar to the Protein S with the same domain organization and 43% amino acid identity. It was originally found as a gene upregulated by growth a ...
that opsonises apoptotic cells) bind to "eat-me" signals (such as
phosphatidylserine Phosphatidylserine (abbreviated Ptd-L-Ser or PS) is a phospholipid and is a component of the cell membrane. It plays a key role in cell cycle signaling, specifically in relation to apoptosis. It is a key pathway for viruses to enter cells via ap ...
) exposed by dead, dying or stressed cells.


Types

Opsonins are related to the two types of
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...
s: the
adaptive immune system The adaptive immune system, also known as the acquired immune system, is a subsystem of the immune system that is composed of specialized, systemic cells and processes that eliminate pathogens or prevent their growth. The acquired immune system ...
and the innate immune system.


Adaptive

Antibodies are synthesized by B cells and are secreted in response to recognition of specific antigenic
epitope An epitope, also known as antigenic determinant, is the part of an antigen that is recognized by the immune system, specifically by antibodies, B cells, or T cells. The epitope is the specific piece of the antigen to which an antibody binds. The p ...
s, and bind only to specific epitopes (regions) on an antigen. They comprise the adaptive opsonization pathway, and are composed of two fragments: the antigen binding region (Fab region) and the fragment crystallizable region (Fc region). The Fab region is able to bind to a specific epitope on an antigen, such as a specific region of a bacterial surface protein. The Fc region of
IgG Immunoglobulin G (Ig G) is a type of antibody. Representing approximately 75% of serum antibodies in humans, IgG is the most common type of antibody found in blood circulation. IgG molecules are created and released by plasma B cells. Each IgG ...
is recognized by the Fc Receptor (FcR) on
natural killer cell Natural killer cells, also known as NK cells or large granular lymphocytes (LGL), are a type of cytotoxic lymphocyte critical to the innate immune system that belong to the rapidly expanding family of known innate lymphoid cells (ILC) and repres ...
s and other
effector cell In cell biology, an effector cell is any of various types of cell that actively responds to a stimulus and effects some change (brings it about). Examples of effector cells include: * The muscle, gland or organ cell capable of responding to ...
s; the binding of IgG to antigen causes a conformational change that allows FcR to bind the Fc region and initiate attack on the pathogen through the release of lytic products. Antibody may also tag tumor cells or virally infected cells, with NK cells responding via the FcR; this process is known as
antibody-dependent cellular cytotoxicity Antibody-dependent cellular cytotoxicity (ADCC), also referred to as antibody-dependent cell-mediated cytotoxicity, is a mechanism of cell-mediated immune defense whereby an effector cell of the immune system actively lyses a target cell, whose ...
(ADCC). Both
IgM Immunoglobulin M (IgM) is one of several isotypes of antibody (also known as immunoglobulin) that are produced by vertebrates. IgM is the largest antibody, and it is the first antibody to appear in the response to initial exposure to an antig ...
and IgG undergo conformational change upon binding antigen that allows complement protein
C1q The complement component 1q (or simply C1q) is a protein complex involved in the complement system, which is part of the innate immune system. C1q together with C1r and C1s form the C1 complex. Antibodies of the adaptive immune system can bi ...
to associate with the Fc region of the antibody. C1q association eventually leads to the recruitment of complement C4b and
C3b C3b is the larger of two elements formed by the cleavage of complement component 3, and is considered an important part of the innate immune system. C3b is potent in opsonization: tagging pathogens, immune complexes (antigen-antibody), and apopto ...
, both of which are recognized by
complement receptor A complement receptor is a membrane-bound receptor belonging to the complement system, which is part of the innate immune system. Complement receptors bind effector protein fragments that are produced in response to antigen-antibody complexes or ...
1, 3, and 4 ( CR1, CR3, CR4), which are present on most phagocytes. In this way, the complement system participates in the adaptive immune response. C3d, a cleavage product of C3, recognizes pathogen-associated molecular patterns (
PAMPs Pathogen-associated molecular patterns (PAMPs) are small molecular motifs conserved within a class of microbes. They are recognized by toll-like receptors (TLRs) and other pattern recognition receptors (PRRs) in both plants and animals. A vast arra ...
) and can opsonize molecules to the CR2 receptor on B cells. This lowers the threshold of interaction required for B cell activation via the
B cell receptor The B cell receptor (BCR) is a transmembrane protein on the surface of a B cell. A B cell receptor is composed of a membrane-bound immunoglobulin molecule and a signal transduction moiety. The former forms a type 1 transmembrane receptor protein, ...
, and aids in the activation of the adaptive response.


Innate

The complement system, independently of the adaptive immune response, is able to opsonize pathogen before adaptive immunity may even be required. Complement proteins involved in innate opsonization include C4b, C3b and
iC3b iC3b is a protein fragment that is part of the complement system, a component of the vertebrate immune system. iC3b is produced when complement factor I cleaves C3b.Robbins Basic Pathology 8th ed 2007. R Cotran, S Robbins, V Kumar, J Perkins. W.B ...
. In the alternative pathway of complement activation, circulating C3b is deposited directly onto antigens with particular PAMPs, such as
lipopolysaccharide Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O-antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the outer m ...
s on
gram-negative bacteria Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...
. C3b is recognized by CR1 on phagocytes. iC3b attaches to apoptotic cells and bodies and facilitates clearance of dead cells and remnants without initiating inflammatory pathways, through interaction with CR3 and CR4 on phagocytes. Mannose-binding lectins, or ficolins, along with
pentraxins Pentraxins (PTX), also known as pentaxins, are an evolutionary conserved family of proteins characterised by containing a pentraxin protein domain. Proteins of the pentraxin family are involved in acute immunological responses. They are a clas ...
and
collectin Collectins (collagen-containing C-type lectins) are a part of the innate immune system. They form a family of collagenous Ca2+-dependent defense lectins, which are found in animals. Collectins are soluble pattern recognition receptors (PRRs). Their ...
s are able to recognize certain types of
carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ma ...
s that are expressed on the cell membranes of
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
,
fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
,
virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky's 1 ...
es, and
parasites Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted structurally to this way of lif ...
, and can act as opsonin by activating the complement system and phagocytic cells.


Targets


Apoptotic cells

A number of opsonins play a role in marking apoptotic cells for phagocytosis without a pro-inflammatory response. Members of the
pentraxin Pentraxins (PTX), also known as pentaxins, are an evolutionary conserved family of proteins characterised by containing a pentraxin protein domain. Proteins of the pentraxin family are involved in acute immunological responses. They are a clas ...
family can bind to apoptotic cell membrane components like
phosphatidylcholine Phosphatidylcholines (PC) are a class of phospholipids that incorporate choline as a headgroup. They are a major component of biological membranes and can be easily obtained from a variety of readily available sources, such as egg yolk or soyb ...
(PC) and
phosphatidylethanolamine Phosphatidylethanolamine (PE) is a class of phospholipids found in biological membranes. They are synthesized by the addition of cytidine diphosphate- ethanolamine to diglycerides, releasing cytidine monophosphate. ''S''-Adenosyl methionine can ...
(PE).
IgM Immunoglobulin M (IgM) is one of several isotypes of antibody (also known as immunoglobulin) that are produced by vertebrates. IgM is the largest antibody, and it is the first antibody to appear in the response to initial exposure to an antig ...
antibodies also bind to PC.
Collectin Collectins (collagen-containing C-type lectins) are a part of the innate immune system. They form a family of collagenous Ca2+-dependent defense lectins, which are found in animals. Collectins are soluble pattern recognition receptors (PRRs). Their ...
molecules such as mannose-binding lectin (MBL),
surfactant protein A Surfactant protein A is an innate immune system collectin. It is water-soluble and has collagen-like domains similar to SP-D. It is part of the innate immune system and is used to opsonize bacterial cells in the alveoli marking them for phagocyt ...
(SP-A), and SP-D interact with unknown ligands on apoptotic cell membranes. When bound to the appropriate ligand these molecules interact with phagocyte receptors, enhancing phagocytosis of the marked cell.
C1q The complement component 1q (or simply C1q) is a protein complex involved in the complement system, which is part of the innate immune system. C1q together with C1r and C1s form the C1 complex. Antibodies of the adaptive immune system can bi ...
is capable of binding directly to apoptotic cells. It can also indirectly bind to apoptotic cells via intermediates like IgM autoantibodies, MBL, and pentraxins. In both cases C1q activates complement, resulting in the cells being marked for phagocytosis by
C3b C3b is the larger of two elements formed by the cleavage of complement component 3, and is considered an important part of the innate immune system. C3b is potent in opsonization: tagging pathogens, immune complexes (antigen-antibody), and apopto ...
and C4b. C1q is an important contributor to the clearance of apoptotic cells and debris. This process usually occurs in late apoptotic cells. Opsonization of apoptotic cells occurs by different mechanisms in a tissue-dependent pattern. For example, while C1q is necessary for proper apoptotic cell clearance in the peritoneal cavity, it is not important in the lungs where SP-D plays an important role.


Pathogens

As part of the late stage adaptive immune response, pathogens and other particles are marked by
IgG Immunoglobulin G (Ig G) is a type of antibody. Representing approximately 75% of serum antibodies in humans, IgG is the most common type of antibody found in blood circulation. IgG molecules are created and released by plasma B cells. Each IgG ...
antibodies. These antibodies interact with Fc receptors on macrophages and neutrophils resulting in phagocytosis. The C1 complement complex can also interact with the
Fc region The fragment crystallizable region (Fc region) is the tail region of an antibody that interacts with cell surface receptors called Fc receptors and some proteins of the complement system. This property allows antibodies to activate the immune s ...
of IgG and IgM immune complexes activating the classical complement pathway and marking the antigen with C3b. C3b can spontaneously bind to pathogen surfaces through the alternative complement pathway. Furthermore, pentraxins can directly bind to C1q from the C1 complex. SP-A opsonizes a number of bacterial and viral pathogens for clearance by lung alveolar macrophages.


See also

*
Antibody opsonization Antibody opsonization is a process by which a pathogen is marked for phagocytosis. Given normal inflammatory circumstances, microbial pathogen-associated molecular patterns (PAMPs) bind with the endocytic pattern recognition receptors (PRRs) o ...


References


External links

* {{Use dmy dates, date=December 2020 Immune system