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An opsonin from the Greek opsōneîn, to prepare for eating is any molecule that enhances phagocytosis by marking an antigen for an immune response or marking dead cells for recycling ie, causes the phagocyte to "relish" the marked cell Opson in ancient Greece referred to the delicious side-dish of any meal, versus the sitos, or the staple of the meal

Opsonization also, opsonisation is the molecular mechanism whereby molecules, microbes, or apoptotic cells are chemically modified to have stronger interactions with – to be more "delicious" to – cell surface receptors on phagocytes and NK cells With the antigen coated in opsonins, binding to immune cells is greatly enhanced Opsonization also mediates phagocytosis via signal cascades from cell surface receptors

Opsonins aid the immune system in a number of ways In a healthy individual, they mark dead and dying self cells for clearance by macrophages and neutrophils, activate complement proteins, and target cells for destruction through the action of natural killer NK cells


  • 1 Mechanism
  • 2 Varieties
    • 21 Antibodies
    • 22 Complement proteins
    • 23 Circulating proteins
  • 3 Targets
    • 31 Apoptotic cells
    • 32 Pathogens
  • 4 See also
  • 5 References
  • 6 External links


All cell membranes have negative charges Zeta potential 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 This principle holds true for clearance of pathogens as well as dead or dying self cells


Different opsonins perform different functions Opsonin molecules include:


Antibodies are part of the adaptive immune response and are generated by B cells in response to antigen exposure The Fab region of the antibody binds to the antigen, whereas the Fc region of the antibody binds to an Fc receptor on the phagocyte, facilitating phagocytosis The antigen-antibody complex can also activate complement through the classical complement pathway Phagocytic cells do not have an Fc receptor for immunoglobulin M IgM, making IgM ineffective in assisting phagocytosis alone However, IgM is extremely efficient at activating complement and is, therefore, considered an opsonin IgG antibodies are also capable of binding immune effector cells via their Fc domain, triggering a release of lysis products from the bound immune effector cell monocytes, neutrophils, eosinophils, and natural killer cells This process, called antibody-dependent cellular cytotoxicity, can cause inflammation of surrounding tissues and damage to healthy cells

Complement proteins

The complement system is a part of the innate immune response C3b, C4b, and C1q are important complement molecules that serve as opsonins As a part of the alternative complement pathway, the spontaneous activation of a complement cascade converts C3 to C3b, a component that can serve as an opsonin when bound to an antigen's surface Antibodies can also activate complement via the classical pathway, resulting in deposition of C3b and C4b onto the antigen surface After C3b has bound to the surface of an antigen, it can be recognized by phagocyte receptors that signal for phagocytosis Complement receptor 1 is expressed on all phagocytes and recognizes a number of complement opsonins, including C3b and C4b which are both parts of C3-convertase C1q, a member of the C1 complex, is able to interact with the Fc region of antibodies

Circulating proteins

Pentraxins, collectins, and ficolins are all circulating proteins that are capable of serving as opsonins They are secreted Pattern recognition receptors PRRs These molecules coat the microbes as opsonins and enhance neutrophil reactivity against them through a number of mechanisms


Apoptotic cells

Apoptosis is related to low tissue inflammation A number of opsonins play a role in marking apoptotic cells for phagocytosis without a pro-inflammatory response

Members of the pentraxin family can bind to apoptotic cell membrane components like phosphatidylcholine PC and phosphatidylethanolamine PE IgM antibodies also bind to PC Collectin molecules such as mannose-binding lectin MBL, surfactant protein A 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 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 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


As part of the late stage adaptive immune response, pathogens and other particles are marked by 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 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


  1. ^ Barret, James 1980 Basic Immunology and its Medical Application 2 ed StLouis:The CV Mosby Company ISBN 0-8016-0495-8 
  2. ^ a b Zhang, Youxin; Hoppe, Adam D; Swanson, Joel A 2010-11-09 "Coordination of Fc receptor signaling regulates cellular commitment to phagocytosis" Proceedings of the National Academy of Sciences 107 45:19332–19337 doi:101073/pnas1008248107 ISSN 0027-8424 PMC 2984174  PMID 20974965 
  3. ^ a b c d Roos, Anja; Xu, Wei; Castellano, Giuseppe; Nauta, Alma J; Garred, Peter; Daha, Mohamed R; van Kooten, Cees 2004-04-01 "Mini-review:A pivotal role for innate immunity in the clearance of apoptotic cells" European Journal of Immunology 34 4:921–929 doi:101002/eji200424904 ISSN 1521-4141 PMID 15048702 
  4. ^ Parham, P 2005 The Immune System," Garland Science Publishing, New York, NY
  5. ^ Shima, Hideaki; Hiroyuki, Takatsu 2009 "Identification of TOSO/FAIM3 as an Fc receptor for IgM" International Immunology Oxford Journals 22 3:149–56 doi:101093/intimm/dxp121 Retrieved 30 November 2014 
  6. ^ Shulman, MJ; Pennel, N 1986 "Activation of complement by immunoglobulin M is impaired by the substitution serine-406----asparagine in the immunoglobulin mu heavy chain" PDF Proceedings of the National Academy of Sciences of the United States of America 83 20:7678–82 doi:101073/pnas83207678 PMC 386784  PMID 3094013 Retrieved 30 November 2014 
  7. ^ Williams, MR; Hill, AW 1982 "A role for IgM in the in vitro opsonisation of Staphylococcus aureus and Escherichia coli by bovine polymorphonuclear leucocytes" Research in Veterinary Science Elsevier 33 1:47–53 PMID 6753075 Retrieved 30 November 2014 
  8. ^ Tosi, Michael F 2005-08-01 "Innate immune responses to infection" Journal of Allergy and Clinical Immunology 116 2:241–249 doi:101016/jjaci200505036 
  9. ^ a b c d Sarma, J Vidya; Ward, Peter A 2010-09-14 "The complement system" Cell and Tissue Research 343 1:227–235 doi:101007/s00441-010-1034-0 ISSN 0302-766X PMC 3097465  PMID 20838815 
  10. ^ a b c Litvack, Michael L; Palaniyar, Nades 2010-06-01 "Review:Soluble innate immune pattern-recognition proteins for clearing dying cells and cellular components:implications on exacerbating or resolving inflammation" Innate Immunity 16 3:191–200 doi:101177/1753425910369271 ISSN 1753-4259 PMID 20529971 

External links

  • Opsonins at the US National Library of Medicine Medical Subject Headings MeSH

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