Complement system 543131 225289369 2008-07-12T22:23:08Z DHN-bot 575307 robot Adding: [[vi:Bổ thể]] [[Image:complement_death.PNG|thumb|right|A [[Complement membrane attack complex|complement protein]] attacking an invader.]] {{this|an aspect of the immune system|complementation}} The '''complement system''' is a [[biochemical cascade]] which 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|innate immune system]]. However, it can be recruited and brought into action by the [[adaptive immune system]]. The complement system consists of a number of small proteins found in the blood, normally circulating as inactive [[zymogens]]. When stimulated by one of several triggers, [[proteases]] in the system cleave specific proteins to release [[cytokines]] and initiate an amplifying cascade of further cleavages. The end result of this activation cascade is massive amplification of the response and activation of the cell-killing [[membrane attack complex]]. Over 20 proteins and protein fragments make up the complement system, including [[blood plasma|serum]] proteins, [[serosal proteins]], and cell membrane receptors. These proteins are synthesized mainly in the [[liver]], and they account for about 5% of the [[serum globulins|globulin]] fraction of blood serum. Three [[Metabolic pathway|biochemical pathway]]s activate the complement system: the [[classical complement pathway]], the [[alternative complement pathway]], and the [[mannose-binding lectin pathway]].<ref name=Janeway_2001>{{cite book | author = Janeway CA Jr., Travers P, Walport M, Shlomchik MJ | title = Immunobiology. | edition = 5th ed. | publisher = Garland Publishing | year = 2001 | id = [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=imm.section.161 (via NCBI Bookshelf)] ISBN 0-8153-3642-X }}</ref> ==History== In the late 19th century, blood serum was found to contain a "factor" or "principle" which was capable of killing bacteria. In 1896, [[Jules Bordet]], a young Belgian scientist in Paris at the Pasteur Institute, demonstrated that this principle could be analyzed into two components: a heat-stable and a heat-labile component. (Heat-labile meaning that it lost its effectiveness if the serum was heated.) The heat-stable component was found to confer immunity against specific microorganisms, while the heat-labile component was found to be responsible for the non-specific antimicrobial activity conferred by all normal serum. This heat-labile component is what we now call "complement". The term "complement" was introduced by [[Paul Ehrlich]] in the late 1890s, as part of his larger theory of the immune system. According to this theory, the immune system consists of cells which have specific receptors on their surface to recognize antigens. Upon immunization with an antigen, more of these receptors are formed, and they are then shed from the cells to circulate in the blood. These receptors, which we now call "antibodies", were called by Ehrlich "amboceptors" to emphasize their bifunctional binding capacity: they recognize and bind to a specific antigen, but they also recognize and bind to the heat-labile antimicrobial component of fresh serum. Ehrlich therefore named this heat-labile component "complement", because it is something in the blood which "complements" the cells of the immune system. Ehrlich believed that each antigen-specific amboceptor had its own specific complement, while Bordet believed that there is only one type of complement. In the early 20th century, this controversy was resolved when it was understood that complement can act in combination with specific antibodies, or on its own in a non-specific way. ==Overview== [[Image:c1protein.png|thumb|200px|left|The C1 protein, showing subunits C1r, C1s, and the C1q tails.]] The three pathways all generate homologous variants of the [[protease]] [[C3-convertase]]. The classical complement pathway typically requires antibodies for activation (specific immune response), while the alternative and mannose-binding lectin pathways can be activated by C3 hydrolysis or antigens without the presence of antibodies (non-specific immune response). In all three pathways, a C3-convertase cleaves and activates component [[C3 (complement)|C3]], creating C3a and C3b and causing a cascade of further cleavage and activation events. C3b binds to the surface of pathogens leading to greater internalization by [[phagocyte|phagocytic cells]] by [[opsonization]]. [[C5a]] is an important [[chemokine|chemotactic protein]], helping recruit inflammatory cells. Both C3a and C5a have [[anaphylatoxin]] activity, directly triggering [[degranulation]] of [[mast cell]]s as well as increasing vascular permeability and [[smooth muscle]] contraction. C5b initiates the [[Complement membrane attack complex|membrane attack pathway]], which results in the [[Complement membrane attack complex|membrane attack complex]] (MAC), consisting of C5b, [[Complement component 6|C6]], [[Complement component 7|C7]], [[C8 complex|C8]], and polymeric [[Complement component 9|C9]].<ref name=Baron>{{cite book | author = Goldman AS, Prabhakar BS | title = The Complement System. ''in:'' Baron's Medical Microbiology ''(Baron S ''et al'', eds.)| edition = 4th ed. | publisher = Univ of Texas Medical Branch | year = 1996 | id = [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.section.238 (via NCBI Bookshelf)] ISBN 0-9631172-1-1 }}</ref> MAC is the cytolytic endproduct of the complement cascade; it forms a transmembrane channel, which causes [[osmosis|osmotic]] lysis of the target cell. [[Kupffer cells]] and other macrophage cell types help clear complement-coated pathogens. As part of the innate immune system, elements of the complement cascade can be found in species earlier than vertebrates; most recently in the protostome horseshoe crab species, putting the origins of the system back further than was previously thought. ===Classical pathway=== {{Main|Classical complement pathway}} [[Image:Complement pathway.png|thumb|right|270px|The classical and alternative complement pathways.]] The [[classical complement pathway|classical pathway]] is triggered by activation of the C1-complex (C1q, C1r, and C1s), which occurs when C1q binds to [[IgM]] or [[IgG]] complexed with [[antigen]]s (a single IgM can initiate the pathway, while multiple IgG's are needed), or when [[C1Q complex|C1q]] binds directly to the surface of the pathogen. Such binding leads to conformational changes in the C1q molecule, which leads to the activation of two C1r (a serine protease) molecules. They then cleave [[Complement component 1S|C1s]] (another serine protease). The C1-complex now binds to and splits [[Complement component 4|C4]] and then [[Complement component 2|C2]], producing C2a and C4b. The inhibition of C1r and C1s is controlled by [[C1-inhibitor]]. C4b and C2a bind to form the classical pathway C3-convertase (C4b2a complex), which promotes cleavage of C3 into C3a and C3b; the latter joins with C2a and C4b (the C3 convertase) to make C5 convertase. C3-convertase can be inhibited by [[Decay Accelerating Factor]] (DAF), which is bound to erythrocyte plasma membranes via a [[Glycophosphatidylinositol|GPI]] anchor. ===Alternative pathway=== {{Main|Alternative complement pathway}} The [[alternate complement pathway|alternative pathway]] is triggered by C3 hydrolysis directly on the surface of a pathogen. It does not rely on a pathogen-binding protein like the other pathways.<ref name=Janeway_2001 /> In the alternative pathway, the protein C3 is produced in the liver, and is then cleaved into C3a and C3b by enzymes in the blood. If there is no pathogen in the blood, the C3a and C3b protein fragments will be deactivated. However, if there is a nearby pathogen, some of the C3b is bound to the plasma membrane of the pathogen. Then, it will bind to [[complement factor B|factor B]]. This complex will then be cleaved by [[factor D]] into Ba and the alternative pathway C3-convertase, Bb. The C3bBb complex, which is "hooked" onto the surface of the pathogen, will then act like a "chain saw", catalyzing the hydrolysis of C3 in the blood into C3a and C3b, which positively affects the number of C3bBb hooked onto a pathogen. After hydrolysis of C3, C3b complexes to become C3bBbC3b, which cleaves C5 into C5a and C5b. C5a and C3a are known to trigger [[mast cell]] degranulation. C5b with C6, C7, C8, and C9 (C5b6789) complex to form the [[membrane attack complex]], also known as MAC, which is inserted into the cell membrane, "punches a hole", and initiates cells lysis. ===Lectin pathway (MBL - MASP)=== {{Main|Mannose-binding lectin pathway}} The [[lectin]] pathway is homologous to the classical pathway, but with the opsonin, mannose-binding lectin (MBL) and ficolins, instead of C1q. This pathway is activated by binding mannose-binding lectin to mannose residues on the pathogen surface, which activates the MBL-associated serine proteases, MASP-1 and MASP-2 (very similar to C1r and C1s, respectively),which can then split C4 into C4a and C4b and C2 into C2a and C2b. C4b and C2a then bind together to form the C3-convertase, as in the classical pathway. Ficolins are homologous to MBL and function via MASP in a similar way. In invertebrates without an adaptive immune system, ficolins are expanded and their binding specificities diversified to compensate for the lack of pathogen-specific recognition molecules. ==Regulation of the complement system== The complement system has the potential to be extremely damaging to host tissues meaning its activation must be tightly regulated. The complement system is regulated by [[complement control protein]]s, which are present at a higher concentration in the blood plasma than the complement proteins themselves. Some complement control proteins are present on the membranes of self-cells preventing them from being targeted by complement. One example is [[CD59]], which inhibits C9 polymerisation during the formation of the [[membrane attack complex]]. ==Role in disease== {{Main|Complement deficiency}} It is thought that the complement system might play a role in many diseases with an immune component, such as [[Barraquer-Simons Syndrome]], [[asthma]], [[lupus erythematosus]], [[glomerulonephritis]], various forms of [[arthritis]], [[autoimmune heart disease]], [[multiple sclerosis]], inflammatory bowel disease, and ischemia-reperfusion injuries. The complement system is also becoming increasingly implicated in diseases of the central nervous system such as [[Alzheimer's disease]], and other neurodegenerative conditions. Deficiencies of the terminal pathway predispose to both [[autoimmune disease]] and [[infection]]s (particularly [[Neisseria_meningitidis|Neisseria]] [[meningitis]], due to the role that the C56789 complex plays in attacking [[Gram negative]] bacteria). Mutations in the complement regulators [[factor H]] and [[membrane cofactor protein]] have been associated with atypical haemolytic uraemic syndrome.<ref name="pmid16189652">{{cite journal |author=Dragon-Durey MA, Frémeaux-Bacchi V |title=Atypical haemolytic uraemic syndrome and mutations in complement regulator genes |journal=Springer Semin. Immunopathol. |volume=27 |issue=3 |pages=359–74 |year=2005 |pmid=16189652 |doi=10.1007/s00281-005-0003-2}}</ref><ref name="pmid16575689">{{cite journal |author=Zipfel PF, Misselwitz J, Licht C, Skerka C |title=The role of defective complement control in hemolytic uremic syndrome |journal=Semin. Thromb. Hemost. |volume=32 |issue=2 |pages=146–54 |year=2006 |pmid=16575689 |doi=10.1055/s-2006-939770}}</ref> Moreover a common [[single nucleotide polymorphism]] in factor H (Y402H) has been associated with the common eye disease [[age related macular degeneration]].<ref name="pmid17869048">{{cite journal |author=Mooijaart SP, Koeijvoets KM, Sijbrands EJ, Daha MR, Westendorp RG |title=Complement Factor H polymorphism Y402H associates with inflammation, visual acuity, and cardiovascular mortality in the elderly population at large |journal= Experimental Gerontology|volume= 42|issue= |pages=1116|year=2007 |pmid=17869048 |doi=10.1016/j.exger.2007.08.001}}</ref> Both of these disorders are currently thought to be due to aberrant complement activation on host surfaces. ==Modulation by infections== Recent research has suggested that the complement system is manipulated during [[Human Immunodeficiency Virus|HIV]]/[[Acquired Immunodeficiency Syndrome|AIDS]] to further damage the body.<ref>Bolger MS, Ross DS, Jiang H, Frank MM, Ghio AJ, Schwartz DA, Wright JR, ''Complement Levels and Activity in the Normal and LPS-Injured Lung'', American Journal of Physiology: Lung Cellular and Molecular Physiology. 2006 Oct 27; PMID 17071722</ref><ref>Datta PK, Rappaport J, HIV and Complement: Hijacking an immune defence, Biomedicine and Pharmacotherapy, 2006 Nov; 60(9):561-568 PMID 16978830</ref> ==Additional images== <gallery> Image:Droga klasyczna.png|Classical Image:Droga alternatywna.png|Alternative Image:Formowanie MAC.svg Image:Complemento C3 convertasi via classica.gif|Classical </gallery> ==References== {{Reflist|2}} {{Complement_system}} {{Immune system}} [[Category:Complement system|*]] [[cs:Komplement (biologie)]] [[de:Komplementsystem]] [[es:Sistema del complemento]] [[fr:Système du complément]] [[it:Sistema del complemento]] [[he:מערכת המשלים]] [[nl:Complementsysteem]] [[ja:補体]] [[pl:Układ dopełniacza]] [[pt:Sistema complemento]] [[ru:Система комплемента]] [[fi:Komplementti (biologia)]] [[sv:Komplement (biologi)]] [[vi:Bổ thể]] [[zh:补体系统]]