Back To Search Results

Biochemistry, Complement

Editor: Allecia M. Wilson Updated: 8/28/2023 9:40:18 PM


The complement system consists of several complement proteins synthesized by the liver’s Kupffer cells and subsequently found in the body’s blood and tissues. The proteins themselves are both zymogens, meaning they are typically inactive, and are meta-stable when activated, meaning they require a cell surface to remain active. When these complement proteins (mostly named C1 to C9) initiate a cascade that engages both the innate and adaptive immune system, they serve as the first line of defense in response to pathogen attack. One goal of the complement system is the formation of a membrane attack complex (MAC), which compromises the pathogen’s cell wall, causing swelling that ultimately leads to cell death. The complement system is diffusely active within the body, and deficiencies or dysregulation results in immune system deficiencies, autoimmune disorders, or bleeding disorders.

While often considered as part of the innate immune system, this is not entirely the case. One method of complement cascade initiation, the classical activation pathway, involves antibodies and, thus, the adaptive immune system. The other two well-studied pathways are the alternative and lectin activation pathways. Neither requires adaptive immune system activation and, therefore, truly are mechanisms of the innate immune system. Although they differ in mechanisms, the commonly needed step of all pathways is the conversion of C3 to C3a and C3b; the latter is necessary for the formation of the MAC.


Register For Free And Read The Full Article
Get the answers you need instantly with the StatPearls Clinical Decision Support tool. StatPearls spent the last decade developing the largest and most updated Point-of Care resource ever developed. Earn CME/CE by searching and reading articles.
  • Dropdown arrow Search engine and full access to all medical articles
  • Dropdown arrow 10 free questions in your specialty
  • Dropdown arrow Free CME/CE Activities
  • Dropdown arrow Free daily question in your email
  • Dropdown arrow Save favorite articles to your dashboard
  • Dropdown arrow Emails offering discounts

Learn more about a Subscription to StatPearls Point-of-Care


Alternative Activation Pathway

The alternative pathway can begin with the spontaneous conversion of C3 to C3b, microbial surface molecules, complex carbohydrates, or other substances.  It is continually activated at low levels throughout the cell body and must be tightly regulated to ensure immunological homeostasis. For example, if C3b is unable to bind to an amino or hydroxyl group on a pathogenic surface, it is rapidly made inert by the arguably more ubiquitous water molecules in the plasma.[1][2] The stabilization of C3b, therefore, requires a proximal pathogen, and this requirement serves as a form of regulation.

If a pathogenic surface stabilizes C3b, then complement protein B binds to it; complement protein D then cleaves the B attached to C3b to form Bb, thus yielding C3bBb, or C3 convertase. C3 convertase mediates C3's conversion to C3b in proximity to the pathogen, resulting in a "complement cascade."[3] Many C3b proteins will now bind to C3bBb and form C3bBbC3b or C5 convertase. C5 convertase cleaves C5 into C5a and C5b; C5b will structurally contribute to the membrane attack complex with complement proteins 6, 7, and 8. The final addition of complement protein 9 forms a pore into the pathogen. The pathogen swells, bursts, and is now no longer able to reproduce within its host.

Factor H serves an essential regulatory role in the alternative pathway, blocking its effects on host cells. Factor H is also synthesized in the liver and is acquired by host cells as it travels past them in the plasma; once on a host cell, it either inactivates C3b or ensures the decay of C3bBb.[4]

Lectin Activation Pathway

The lectin pathway utilizes complement proteins 2 and 4 to achieve the same result as the alternative pathway. However, this pathway uses mannose-binding lectin (MBL), a glycoprotein also produced in the liver, to bind mannose on the surface of similar pathogens to that of the alternate pathway. MBLs circulate with and help 'dock' mannose associated serine proteases (MASPs) so that MASPs can cleave C2 and C4, forming C2b and C4b; the union of these two products, C2b4b, serves as a C3 convertase.[5]

The resulting cascade carries out identically to the alternative activation pathway. However, MBL exhibits a higher affinity to Candida albicans than molecules of the other pathways.[6][7]

Classical Activation Pathway

The classical activation pathway requires some level of adaptive response (humoral) to initiate its complement cascade. For this reason, the body produces IgM antibodies as a first-line response to early infection. The pattern in which IgM antibodies form antibody-antigen complexes (with proximal Fc regions) allows for the proximal binding of two C1 complexes to two Fc regions. The proximity between the two C1 complexes (which are specific to the classical activation pathway) results in the release of their inhibitors (C1-inhibitors). The release of C1-inhibitors activates each complex's neighboring C1r and C1s complement proteins, creating a C3 convertase that triggers the complement cascade on the pathogen's surface.

C-reactive protein (CRP), a protein synthesized in the liver in response to acute inflammation, has been found to enhance classical pathway activation weakly.[8] Dysregulation of CRP results in the consumption of C3 and C4, and the generation of C3b.

Issues of Concern

Complement activation pathways are tightly regulated and have multiple points of inhibition to protect normal tissue from complement dysregulation, and include MCP, decay-accelerating factor (DAF), MAC-inhibitory protein (CD59 or protectin), and the previously described Factor H. MCP deactivates C3b while DAF facilitates the destruction of C3bBb (C3 convertase).[9][10][11][12] CD59 removes near-finished MACs located on human cell membranes.[13]


Membrane Attack Complex

As stated in the Fundamentals section, all complement pathways result in the construction of a MAC, which compromises the pathogen cell wall, resulting in its swift malfunction.

Inflammation and Chemotaxis

All forms of C3 convertase yield C3b and C3a. The C3a fragment is an anaphylatoxin that mediates inflammation in two ways: it triggers histamine release from mast cells and increases vascular permeability.

Similarly, all forms of C5 convertase yield both C5b (for the MAC) and C5a. C5a is also an anaphylatoxin, in addition to a neutrophil chemotactic agent.

Opsonization for enhancement of phagocytosis

There are two well-studied mechanisms by which the complement system opsonizes the pathogen for enhanced phagocytosis. We will first discuss the mechanism that all pathways have in common. C3b binds C3bBb forming C5 convertase, but C3b also acts as an opsonin to attract macrophages to the site of inflammation (namely, complement activation) to enhance phagocytosis.

The alternative activation pathway performs the second mechanism of opsonization. This pathway is capable of synthesizing a ‘soluble’ C3 convertase, named iC3Bb. iC3Bb can also attach to the pathogen and, in turn, attract phagocytes, bind them, and ultimately facilitate phagocytosis.


Complement testing is available for many of the complement proteins, including: 

  • C1
  • C1q
  • C2
  • C3
  • C4
  • CH50
  • CH100
  • Total complement 

Specific tests may assist in monitoring disease activity and treatment, such as in hypocomplementemia and systemic lupus erythematosus (both with low C3 and C4).[14][15][16][17]


Testing for 50% hemolytic complement activity of serum (serum CH50) assesses the activity of the classical activation pathway. The normal range is 22 to 40 units/mL, indicating the presence of complement proteins 1 through 9.[18]

Clinical Significance

Factor H

This regulatory plasma glycoprotein can be used by pathogens and cancer cells to evade the immune system.[19][20] Specific pathogens using this mechanism of evasion include Haemophilus influenzae, Streptococcus pneumoniae, Borrelia burgdorferi, Staphylococcus aureus, West Nile Virus, and HIV-1.[21][22][23]

Inherited C3 Deficiency

Inherited C3 deficiency occurs because of a secretory malfunction (as a result of a protein substitution).[24][25][26] This condition presents with recurrent encapsulated bacterial infections or with systemic lupus erythematosus-like symptoms, from the beginning of life onward.[27] This deficiency might also result in higher levels of immune complex deposition, resulting in a build-up of these complexes in the kidney leading to glomerulonephritis.[28] 

Paroxysmal Nocturnal Hemoglobinuria

Deficiencies in molecules that inhibit complement (MCP, decay-accelerating factor (DAF), CD59, and Factor H) results in hemolysis or paroxysmal nocturnal hemoglobinuria. Clinical signs of hemolysis include hemoglobinuria at night, anemia, and increased abdominal pain.[29] Thrombosis in the venous system and bone marrow failure are the leading causes of death.[30] Thrombosis and increased abdominal pain are both results of abnormally high levels of free plasma hemoglobin due to lysis.[31] However, unlike thrombosis, the mechanism for abdominal pain involves NO depletion.

Hereditary Angioedema

Hereditary angioedema is an autosomal dominant disorder due to a C1 inhibitor protein deficiency.[32][33] This condition is characterized by episodes of edema and swelling, especially of the gastrointestinal tract and throat, which can lead to potentially fatal upper airway swelling. Low C4 levels confirm the diagnosis due to its overconsumption (secondary to the aforementioned C1 inhibitor protein deficiency).



Sahu A, Pangburn MK. Covalent attachment of human complement C3 to IgG. Identification of the amino acid residue involved in ester linkage formation. The Journal of biological chemistry. 1994 Nov 18:269(46):28997-9002     [PubMed PMID: 7961863]

Level 3 (low-level) evidence


Sahu A, Pangburn MK. Tyrosine is a potential site for covalent attachment of activated complement component C3. Molecular immunology. 1995 Jul:32(10):711-6     [PubMed PMID: 7659097]


Müller-Eberhard HJ, Götze O. C3 proactivator convertase and its mode of action. The Journal of experimental medicine. 1972 Apr 1:135(4):1003-8     [PubMed PMID: 4111773]


Ferreira VP, Pangburn MK, Cortés C. Complement control protein factor H: the good, the bad, and the inadequate. Molecular immunology. 2010 Aug:47(13):2187-97. doi: 10.1016/j.molimm.2010.05.007. Epub     [PubMed PMID: 20580090]

Level 3 (low-level) evidence


Choteau L, Vasseur F, Lepretre F, Figeac M, Gower-Rousseau C, Dubuquoy L, Poulain D, Colombel JF, Sendid B, Jawhara S. Polymorphisms in the Mannose-Binding Lectin Gene are Associated with Defective Mannose-Binding Lectin Functional Activity in Crohn's Disease Patients. Scientific reports. 2016 Jul 12:6():29636. doi: 10.1038/srep29636. Epub 2016 Jul 12     [PubMed PMID: 27404661]


Hammad NM, El Badawy NE, Ghramh HA, Al Kady LM. Mannose-Binding Lectin: A Potential Therapeutic Candidate against Candida Infection. BioMed research international. 2018:2018():2813737. doi: 10.1155/2018/2813737. Epub 2018 May 2     [PubMed PMID: 29854737]


Hammad NM, El Badawy NE, Nasr AM, Ghramh HA, Al Kady LM. Mannose-Binding Lectin Gene Polymorphism and Its Association with Susceptibility to Recurrent Vulvovaginal Candidiasis. BioMed research international. 2018:2018():7648152. doi: 10.1155/2018/7648152. Epub 2018 Apr 4     [PubMed PMID: 29850562]


Bíró A, Rovó Z, Papp D, Cervenak L, Varga L, Füst G, Thielens NM, Arlaud GJ, Prohászka Z. Studies on the interactions between C-reactive protein and complement proteins. Immunology. 2007 May:121(1):40-50     [PubMed PMID: 17244159]


Hakulinen J, Junnikkala S, Sorsa T, Meri S. Complement inhibitor membrane cofactor protein (MCP; CD46) is constitutively shed from cancer cell membranes in vesicles and converted by a metalloproteinase to a functionally active soluble form. European journal of immunology. 2004 Sep:34(9):2620-9     [PubMed PMID: 15307194]


Kinoshita T. Congenital Defects in the Expression of the Glycosylphosphatidylinositol-Anchored Complement Regulatory Proteins CD59 and Decay-Accelerating Factor. Seminars in hematology. 2018 Jul:55(3):136-140. doi: 10.1053/j.seminhematol.2018.04.004. Epub 2018 Apr 16     [PubMed PMID: 30032750]


Toomey CB, Cauvi DM, Pollard KM. The role of decay accelerating factor in environmentally induced and idiopathic systemic autoimmune disease. Autoimmune diseases. 2014:2014():452853. doi: 10.1155/2014/452853. Epub 2014 Jan 27     [PubMed PMID: 24592327]


Auret J, Abrahams A, Prince S, Heckmann JM. The effects of prednisone and steroid-sparing agents on decay accelerating factor (CD55) expression: implications in myasthenia gravis. Neuromuscular disorders : NMD. 2014 Jun:24(6):499-508. doi: 10.1016/j.nmd.2014.02.010. Epub 2014 Mar 6     [PubMed PMID: 24703255]

Level 3 (low-level) evidence


Wei Y, Ji Y, Guo H, Zhi X, Han S, Zhang Y, Gao Y, Chang Y, Yan D, Li K, Liu DX, Sun S. CD59 association with infectious bronchitis virus particles protects against antibody-dependent complement-mediated lysis. The Journal of general virology. 2017 Nov:98(11):2725-2730. doi: 10.1099/jgv.0.000962. Epub 2017 Oct 25     [PubMed PMID: 29068273]


Li H, Lin S, Yang S, Chen L, Zheng X. Diagnostic value of serum complement C3 and C4 levels in Chinese patients with systemic lupus erythematosus. Clinical rheumatology. 2015 Mar:34(3):471-7. doi: 10.1007/s10067-014-2843-4. Epub 2015 Jan 20     [PubMed PMID: 25597615]

Level 2 (mid-level) evidence


Garabet L, Gilboe IM, Mowinckel MC, Jacobsen AF, Mollnes TE, Sandset PM, Jacobsen EM. Antiphospholipid Antibodies are Associated with Low Levels of Complement C3 and C4 in Patients with Systemic Lupus Erythematosus. Scandinavian journal of immunology. 2016 Aug:84(2):95-9. doi: 10.1111/sji.12445. Epub     [PubMed PMID: 27135178]


Birmingham DJ, Irshaid F, Nagaraja HN, Zou X, Tsao BP, Wu H, Yu CY, Hebert LA, Rovin BH. The complex nature of serum C3 and C4 as biomarkers of lupus renal flare. Lupus. 2010 Oct:19(11):1272-80. doi: 10.1177/0961203310371154. Epub 2010 Jul 6     [PubMed PMID: 20605879]


Ricker DM, Hebert LA, Rohde R, Sedmak DD, Lewis EJ, Clough JD. Serum C3 levels are diagnostically more sensitive and specific for systemic lupus erythematosus activity than are serum C4 levels. The Lupus Nephritis Collaborative Study Group. American journal of kidney diseases : the official journal of the National Kidney Foundation. 1991 Dec:18(6):678-85     [PubMed PMID: 1962653]


Costabile M. Measuring the 50% haemolytic complement (CH50) activity of serum. Journal of visualized experiments : JoVE. 2010 Mar 29:(37):. pii: 1923. doi: 10.3791/1923. Epub 2010 Mar 29     [PubMed PMID: 20351687]

Level 3 (low-level) evidence


Wilczek E, Rzepko R, Nowis D, Legat M, Golab J, Glab M, Gorlewicz A, Konopacki F, Mazurkiewicz M, Sladowski D, Gornicka B, Wasiutynski A, Wilczynski GM. The possible role of factor H in colon cancer resistance to complement attack. International journal of cancer. 2008 May 1:122(9):2030-7. doi: 10.1002/ijc.23238. Epub     [PubMed PMID: 18183578]


Junnikkala S, Hakulinen J, Jarva H, Manuelian T, Bjørge L, Bützow R, Zipfel PF, Meri S. Secretion of soluble complement inhibitors factor H and factor H-like protein (FHL-1) by ovarian tumour cells. British journal of cancer. 2002 Nov 4:87(10):1119-27     [PubMed PMID: 12402151]


Hallström T, Zipfel PF, Blom AM, Lauer N, Forsgren A, Riesbeck K. Haemophilus influenzae interacts with the human complement inhibitor factor H. Journal of immunology (Baltimore, Md. : 1950). 2008 Jul 1:181(1):537-45     [PubMed PMID: 18566420]


Hammerschmidt S, Agarwal V, Kunert A, Haelbich S, Skerka C, Zipfel PF. The host immune regulator factor H interacts via two contact sites with the PspC protein of Streptococcus pneumoniae and mediates adhesion to host epithelial cells. Journal of immunology (Baltimore, Md. : 1950). 2007 May 1:178(9):5848-58     [PubMed PMID: 17442969]


Chung KM, Liszewski MK, Nybakken G, Davis AE, Townsend RR, Fremont DH, Atkinson JP, Diamond MS. West Nile virus nonstructural protein NS1 inhibits complement activation by binding the regulatory protein factor H. Proceedings of the National Academy of Sciences of the United States of America. 2006 Dec 12:103(50):19111-6     [PubMed PMID: 17132743]

Level 3 (low-level) evidence


Singer L, Whitehead WT, Akama H, Katz Y, Fishelson Z, Wetsel RA. Inherited human complement C3 deficiency. An amino acid substitution in the beta-chain (ASP549 to ASN) impairs C3 secretion. The Journal of biological chemistry. 1994 Nov 11:269(45):28494-9     [PubMed PMID: 7961791]

Level 3 (low-level) evidence


Katz Y, Singer L, Wetsel RA, Schlesinger M, Fishelson Z. Inherited complement C3 deficiency: a defect in C3 secretion. European journal of immunology. 1994 Jul:24(7):1517-22     [PubMed PMID: 8026514]


Matsuyama W, Nakagawa M, Takashima H, Muranaga F, Sano Y, Osame M. Molecular analysis of hereditary deficiency of the third component of complement (C3) in two sisters. Internal medicine (Tokyo, Japan). 2001 Dec:40(12):1254-8     [PubMed PMID: 11813855]

Level 3 (low-level) evidence


Sano Y, Nishimukai H, Kitamura H, Nagaki K, Inai S, Hamasaki Y, Maruyama I, Igata A. Hereditary deficiency of the third component of complement in two sisters with systemic lupus erythematosus-like symptoms. Arthritis and rheumatism. 1981 Oct:24(10):1255-60     [PubMed PMID: 7306227]

Level 3 (low-level) evidence


Borzy MS, Houghton D. Mixed-pattern immune deposit glomerulonephritis in a child with inherited deficiency of the third component of complement. American journal of kidney diseases : the official journal of the National Kidney Foundation. 1985 Jan:5(1):54-9     [PubMed PMID: 3155591]

Level 3 (low-level) evidence


Brodsky RA. Paroxysmal nocturnal hemoglobinuria. Blood. 2014 Oct 30:124(18):2804-11. doi: 10.1182/blood-2014-02-522128. Epub 2014 Sep 18     [PubMed PMID: 25237200]


Parker CJ. Update on the diagnosis and management of paroxysmal nocturnal hemoglobinuria. Hematology. American Society of Hematology. Education Program. 2016 Dec 2:2016(1):208-216     [PubMed PMID: 27913482]


Hill A, Kelly RJ, Hillmen P. Thrombosis in paroxysmal nocturnal hemoglobinuria. Blood. 2013 Jun 20:121(25):4985-96; quiz 5105. doi: 10.1182/blood-2012-09-311381. Epub 2013 Apr 22     [PubMed PMID: 23610373]


Morelli C, Formica V, Pellicori S, Menghi A, Guarino MD, Perricone R, Roselli M. Chemotherapy in Patients with Hereditary Angioedema. Anticancer research. 2018 Dec:38(12):6801-6807. doi: 10.21873/anticanres.13052. Epub     [PubMed PMID: 30504393]


Zeerleder S, Levi M. Hereditary and acquired C1-inhibitor-dependent angioedema: from pathophysiology to treatment. Annals of medicine. 2016:48(4):256-67. doi: 10.3109/07853890.2016.1162909. Epub 2016 Mar 26     [PubMed PMID: 27018196]