Complement activation (Homo sapiens)

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1Prevents Complement Attackantigen/antibodycomplexPathogenic Bacterial CellMembraneAttack Complexaggregated IgG,IgM-antigen complex,polyanions, RNA, DNAlipopolysaccharidesProteolytic Cleavage & ActivationC2AC3C1QBC1SDAFMASP2C1RH2OC1QGC5C4AC8BC6C8AC4BC9C1QAC7C2C4BC3BC3 convertaseC2AC4BC5 convertaseC5BC1QBC1QGC1QAC1RC1SC1 complexC8GH2OOsmotic lysisMASP1C3BCFBbClassical pathwayLectin pathwayAlternative pathwayC3C3CFBC3 convertaseCFDC3BHydrolysisCFPC3BCFBbCFPC5 convertaseC5


The complement system is a biochemical cascade that helps, or complements, the ability of antibodies to clear pathogens from an organism. It is part of the immune system called the innate immune system that is not adaptable and does not change over the course of an individual's lifetime. However, it can be recruited and brought into action by the adaptive immune system. The Classical pathway of activation of the complement system is a group of blood proteins that mediate the specific antibody response. [source: Wikipedia]

The Classical pathway begins with circulating C1Q binding to an antigen on the surface of a pathogen, which goes on to active and recruit 2 copies of each C1R and C1S, forming a C1 complex. The activated C1 complex cleaves C2 and C4. Activated cleavage products C2A and C4B combine to form C3 convertase, which cleaves C3. The cleavage product C3B joins the complex to form C5 convertase, which cleaves C5. The cleavage product C5B joins C6, C7, C8 and multiple copies of C9 to form the Membrane Attack Complex, which forms a channel for water to flood into the target cell, leading to osmotic lysis. The Decay accelerating factor (DAF) inhibits C3 convertase.

The Lectin pathway involves mannose-binding lectin (MBL) binding the surface of the pathogen instead of C1Q. MBL-associated serine proteases MASP1 and MASP1 can cleave C2 and C4 in place of the C1 complex, leading to the formation of C3 convertase and the subsequent cascade.

The Alternative pathway relies on the spontaneous hydrolysis of C3 and the cleavage of factor B (CFB) by factor D (CFD), which form an alternative C3 convertase stabilized by factor P (CFP). Additional copies of the cleavage product C3B are recruited to the complex, resulting in an alternative C5 convertase, which cleaves C5 and contributes C5B to the formation of the Membrane Attack Complex.

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Ontology Terms



  1. Brook E, Herbert AP, Jenkins HT, Soares DC, Barlow PN; ''Opportunities for new therapies based on the natural regulators of complement activation.''; Ann N Y Acad Sci, 2005 PubMed Europe PMC Scholia


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116815view10:12, 14 May 2021EweitzModified title
106816view13:29, 17 September 2019MaintBotHMDB identifier normalization
106364view23:04, 22 August 2019KhanspersModified description
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89636view06:10, 22 September 2016EgonwConnected lines.
87260view14:39, 20 July 2016MaintBotmissing graphids
82142view01:23, 9 September 2015AlexanderPicofixed C1Q complex
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82139view01:13, 9 September 2015AlexanderPicoAdded other pathways
82136view00:08, 9 September 2015AlexanderPicoAdded MASP2
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82132view23:58, 8 September 2015AlexanderPicoModified description
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82130view23:46, 8 September 2015AlexanderPicoresolved curved interaction
82129view23:42, 8 September 2015AlexanderPicoadjusting inhibition tbar
82128view23:41, 8 September 2015AlexanderPicoadjusting inhibition tbar
82127view23:39, 8 September 2015AlexanderPicoFinished update of layout and components
82126view22:37, 8 September 2015AlexanderPicoin progress
82125view21:59, 8 September 2015AlexanderPicoUpdating layout and component details (in progress)
72062view17:25, 24 October 2013Mkutmonupdated annotation for C1R
69756view10:50, 11 July 2013Mkutmonreplace annotation lines with graphical lines (instead of interactions)
67122view10:09, 26 June 2013Christine ChichesterOntology Term : 'classical complement pathway' added !
63169view20:26, 8 May 2013MaintBotUpdating gpml version
41148view23:40, 1 March 2011MaintBotRemoved redundant pathway information and comments
37346view17:20, 28 May 2010KhanspersConnectors
35499view13:11, 12 February 2010MichielModified description
35496view13:09, 12 February 2010MichielAdded ref
32462view17:26, 15 August 2009MaintBotFixed text labels
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21826view11:32, 14 November 2008MaintBot[[Pathway:Homo sapiens:Complement Activation Classical]] moved to [[Pathway:WP545]]: Moved to stable identifier
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12932view10:41, 17 May 2008MaintBotSticky edges patch by Sjoerd
7966view13:46, 7 January 2008MaintBot[[Pathway:Human:Complement Activation Classical]] moved to [[Pathway:Homo sapiens:Complement Activation Classical]]: Renaming species
7607view16:03, 18 December 2007MaintBotfixed category names
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6304view22:17, 22 May 2007Nsalomonisgpml file for [[Human:Complement_Activation_Classical]]

External references


View all...
NameTypeDatabase referenceComment
C1QAGeneProduct712 (Entrez Gene) update: C1QA HUMAN
C1QBGeneProduct713 (Entrez Gene)
C1QGGeneProduct714 (Entrez Gene) update C1QC HUMAN
C1RGeneProductENSG00000159403 (Ensembl)
C1SGeneProduct716 (Entrez Gene)
C2GeneProduct717 (Entrez Gene)
C2AGeneProduct717 (Entrez Gene)
C3GeneProduct718 (Entrez Gene)
C3BGeneProduct718 (Entrez Gene)
C4AGeneProduct720 (Entrez Gene)
C4BGeneProduct721 (Entrez Gene)
C5GeneProduct727 (Entrez Gene)
C5BGeneProduct727 (Entrez Gene)
C6GeneProduct729 (Entrez Gene)
C7GeneProduct730 (Entrez Gene)
C8AGeneProduct731 (Entrez Gene)
C8BGeneProduct732 (Entrez Gene)
C8GGeneProduct733 (Entrez Gene)
C9GeneProduct735 (Entrez Gene)
CFBGeneProduct629 (Entrez Gene)
CFBbGeneProduct629 (Entrez Gene)
CFDGeneProduct1675 (Entrez Gene)
CFPGeneProduct5199 (Entrez Gene)
DAFGeneProduct1604 (Entrez Gene)
H2OMetaboliteHMDB02111 (HMDB)
MASP1GeneProduct5648 (Entrez Gene)
MASP2GeneProduct10747 (Entrez Gene)

Annotated Interactions

No annotated interactions

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