The complement system

1) Introduction:

Complement system is a group of more than 23 proteins that interacts with each other to opsonize the pathogen and induces a series of inflammatory responses that helps to generate immune response against that pathogen.  The principal participants in this system are 11 proteins designated C1 through C9, B, and D. Term complement means to augment the effect of other complements of immune system. Many of these proteins are proteases that are themselves activated by proteolytic cleavage. In the following sections we will try to understand the functioning of complement system, its regulation and disorders related to its function and regulation.

2) Historical aspect:

In 1895 Bordet discovered that heat-labile activity in serum was responsible for the lysis of bacteria. Vibrio Cholerae were lysed within minutes when serum from an immunized animal was added to the bactria. However, when this serum was incubated at 56°C for few minutes, the lytic activity was lost even though the anti-body activity remained. Untreated serum when added to the reaction restored the lytic activity. Therefore, the lysis of bacteria required both the anti-body and this heat labile substance. Ehrlich named this heat labile substance as Complement.

Bordet described the antibody dependent arm of complement system i.e. classical pathway.  An anti-body independent mechanism of complement system was discovered by Pillemer in 1954 and he named it “properdin system”. All components of the classical pathway and the membrane-attack complex are designated by the letter C followed by a number. The components were numbered in the order of their discovery rather than the sequence of reactions, which is C1, C4, C2, C3, C5, C6, C7, C8 and C9. Their cleavage fragments are designated by adding lower-case letters, the larger fragment being designated as b and the smaller as a.

Activation of complement system:

Components of complement systems are pro-enzymes. These components are cleaved to form active enzymes. Complement system can be activated by immune complexes and immunologic molecules like endotoxins. It occurs by 3 mechanisms:

  1. Classical pathway.
  2. Alternate pathway.
  3. Lectin pathway.

3) Pathways of complement system:

A) Classical pathway:

The binding of antibody to its antigen triggers the complement system through the so-called classical pathway. It can occur in solution or when the antibodies have bound to antigens on a cell surface. Only IgM and IgG can activate complement. One molecule of IgM can activate complement but activation of IgG is with the help of cross linking 2 molecules. C1 binds to the region located in the FC region of heavy chain. Out of IgG’s only IgG1,IgG2, and IgG3 can fix the complement not IgG4.

Components of classical complement pathway:

C1: It exists in blood serum as a molecular complex containing:

  • 6 molecules of C1q
  • 2 molecules of C1r
  • 2 molecules of C1s

C4: It is cleaved into two fragments:

  • C4b, which is the larger one, binds covalently to sugar residues on cell-surface glycoproteins.
  • C4a, which diffuses away.

C2: It is cleaved into:

  • C2b, which binds non-covalently to a site on C4b, leaving a smaller, inactive, fragment of
  • C2a which diffuses away.

NOTE: The complex of C4b•2b is called “C3 convertase” because it catalyzes the cleavage of C3.

C3: It is the most abundant protein of the complement system (~1.3 mg/ml). Because of its abundance and its ability to activate itself, it greatly magnifies the response.

  • C4b•2b cuts C3 into two major fragments:
  • C3b, which binds covalently to glycoproteins scattered across the cell surface. Macrophages and neutrophils have receptors for C3b and can bind the C3b-coated cell or particle preparatory to phagocytosis. This effect qualifies C3b as an opsonin.
  • C3a this small fragment is released into the surrounding fluids. It can bind to receptors on basophils and mast cells triggering them to release their vasoactive contents (e.g., histamine). Because of the role of these materials in anaphylaxis, C3a is called an anaphylatoxin.

C5: Cleavage of C5 by the C3/C5 convertase initiates the assembly of a set of complement proteins that make up the membrane attack complex.

Step by step activation of classical pathway:

C1 activation:
C1, binds to the Fc region of IgG and IgM antibody molecules that have interacted with antigen.  C1 binding does not occur to antibodies that have not complexed with antigen and binding requires calcium and magnesium ions (In some cases C1 can bind to aggregated immunoglobulin [e.g. aggregated IgG] or to certain pathogen surfaces in the absence of antibody).  The binding of C1 to antibody is via C1q and C1q must cross link at least two antibody molecules before it is firmly fixed.  The binding of C1q results in the activation of C1r which in turn activates C1s.  The result is the formation of an activated “C1qrs”, which is an enzyme that cleaves C4 into two fragments C4a and C4b.

C4 and C2 activation (generation of C3 convertase):
The C4b fragment binds to the membrane and the C4a fragment is released into the microenvironment.  Activated “C1qrs” also cleaves C2 into C2a and C2b.  C2a binds to the membrane in association with C4b, and C2b is released into the microenvironment. The resulting C4bC2a complex is a C3 convertase, which cleaves C3 into C3a and C3b.

C3 activation (generation of C5 convertase):
C3b binds to the membrane in association with C4b and C2a, and C3a is released into the microenvironment.  The resulting C4bC2aC3b is a C5 convertase.  The generation of C5 convertase is the end of the classical pathway.

B) Alternate pathway:

 In bacterial, fungal and viral cell surface substances which are immunologic, this pathway is induced. There is no Ab present as it is a new infection. These cell wall polysaccharides, endotoxins possess binding to C3 (H2O) and factor B. this complex is cleaved by a protease, factor D to produce C3bBb. This C3bBb act as C3 convertase to form C3b.

C) Lectin pathway:

The lectin pathway is very similar to the classical pathway. It is initiated by the binding of mannose-binding lectin (MBL) to bacterial surfaces with mannose-containing polysaccharides (mannans). Binding of MBL to a pathogen results in the association of two serine proteases, MASP-1 and MASP-2 (MBL-associated serine proteases). MASP-1 and MASP-2 are similar to C1r and C1s, respectively and MBL is similar to C1q. Formation of the MBL/MASP-1/MASP-2 tri-molecular complex results in the activation of the MASPs and subsequent cleavage of C4 into C4a and C4b. The C4b fragment binds to the membrane and the C4a fragment is released into the microenvironment. Activated MASPs also cleave C2 into C2a and C2b. C2a binds to the membrane in association with C4b and C2b is released into the microenvironment. The resulting C4bC2a complex is a C3 convertase, which cleaves C3 into C3a and C3b. C3b binds to the membrane in association with C4b and C2a and C3a is released into the micro-environment. The resulting C4bC2aC3b is a C5 convertase. The generation of C5 convertase is the end of the lectin pathway. The biological activities and the regulatory proteins of the lectin pathway are the same as those of the classical pathway.

Flow chart showing the classical, alternative and lectin pathways of complement system

Complement Activation Pathways

4) Regulation of complement system:

  1. At antibody level: Normally complement binding site located in heavy chain on fc region is not available for c1 component of the complement system. It is only after the binding of AG conformational changes take place which allow C1 to bind. That is why the complement is not activated even when high amount of IgM and IgG are present blood.
  2.  Regulation by serum proteins: CI Inhibitor (C1-INH)  is important regulator .it is activates protease activity of C1. Classical pathway begins with this step.
  3. C3a inactivator (C3a-INA; Carboxypeptidase B):  It inactivates C3a.
  4. Factor H & I: Alternative pathway is regulated by Binding of H factor to C3b and cleavage of this complex by factor I a protease.
  5. Decay accelerating factor [DAF]: Protection of human cells from lysis by membrane attack complex is done by decay accelerating factor [DAF], glycol protein located on surface of human cells. It acts by distabalising C3 convertase and C5 convertase .
  6. C4 binding protein (C4-BP) and Factor I; C4-BP facilitates degradation of C4b by Factor I; C4-BP also prevents association of C2a with C4b thus blocking the formation of C3 convertase.
  7. Protein S (vitronectin):  After formation of MAC, some of the C5b67 complex formed can dissociate from the membrane and enter the fluid phase. It can bind to other nearby cells and lead to their lysis. This damage is prevented by Protein S (vitronectin). Protein S binds to soluble C5b67 and prevents its binding to other cells.

 Regulation of complement system

Regulation of complement system


5) Biological effects of complement components:

  1. Opsonisation:  As the C3b receptor is present on surfaces of many macrophages. Phagocytosis of cells, Ag-Ab  complexes , viruses and other forgein invades is much more better in presence of C3b.
  2. Chemotaxis: C5a and C5b67 complex attracts neutrophils. They migrate well towards C5a, it enhances the adhisiveness of neutrophils the endothelium.
  3. Anaphylatoxin: C4a, C3a and C5a (in increasing order of activity) are all anaphylotoxins. They cause degranulation of mast cells with release of mediators eg. Histamine [increased vascular permeability , smooth muscle contraction, bronchospasm] but this anaphylaxis is less severe than anaphylaxis caused by IgE.
  4. Cytolysis:  Insertion of C5b6789 complex into cells membrane causes lysis of cell. This process is not mediated by enzymatic degradation.
  5. Enhancement of antibody production: Binding of C3b with antigen on the surface of B cells markedly enhances the production of antibodies as compare to when the only Ag is attached to B cell.

6) Disorders related to complement system:

  1. Inherited deficiency of some factors like C5-C8 greatly enhances susceptibility to infection by Neisseria.
  2. Inherited deficiency of C1 esterase inhibitors results in angiodema.
  3. Inherited deficiency of DAF results in paroxysmal nocturnal haemoglobinurea.
  4. If blood transfusion is mismatched, antigen-antibody reaction occurs that leads to formation of membrane attack complex which causes haemolysis and anaphylatoxins cause shock
  5. Immune complex bind complement and thus complement levels are less in immune complex diseases for e.g. Acute glomerulonephritis and systematic Lupus erythromytosis. Binding of complent to PMN’s causes release of enzyme-tissue damage.
Know more………

Complement system is part of innate host defense system. Alternate pathway is more important when we first time get infected by microorganisms, cells, antibodies require to trigger the classical pathway is not present. Both pathways lead to the formation of C3B molecules which is the central molecule of complement cascade.

Complement proteins are synthesized by liver. It is heat labile i.e; inactivated by heating serum at 560C for 30 minutes.

Functions of C3b

  • It combines with other molecules to form C5 convertase(which form membrane attack complex)
  • Opsonises bacteria as macrophages have C3B receptors on their surface.



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  1. Immunology: Understanding The Immune System. By Klaus D. Elgert
  2. Immunology: A Short Course. By Richard Coico, Geoffrey Sunshine.
  3. Immunology.  By David K. Male.
  4. Immunology: Essential And Fundamental. By Sulabha Pathak, Urmi Palan.

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