• It is a part of immune system which consists of series of proteins that interacts with one another in an organized manner to eliminate pathogens.
  • Complement pathway has a role to play in removal of damaged cells and pathogens by helping antibodies and phagocytic cells in this process.
  • Proteins taking part in complement system are known as complement proteins and works together as biological cascade (sequence of reactions, in which each being the catalyst for the other).
  • Mostly, complements are soluble proteins and glyco-proteins produced by special cells called hepatocytes.
  • These complements circulate in the body in form of Zymogens (inactive forms) and more than 20 such types have been recognized in the serum.
  • When an antibody binds to an antigen, it triggers complement pathway. It can also be triggered by few components of innate immunity. This system can also work in acquired immunity.
  • During inflammation, these complements gets activated and reaches infected area of the intestinal tissue through dilated blood vessels, which are activated by proteolytic cleavage and exposes active site of the complements.


  • Complements were identified as heat-sensitive components in the blood in the year 1895 by Jules Bordet.
  • Complement levels will be low during recurrent microbial infections, auto-immune diseases. (Lupus disease).

Overview and components of Compliment pathways:

  • C letter is used to denote complement proteins along with numbers like C1, C2, C3 and so forth. Some can also be denoted by B and D and some are represented by names like homologous restriction factor.
  • Initial step varies in different complement pathways, however, all pathways forms enzyme complexes.
  • C3 is converted into C3a and C3b by C3 convertase. C5 gets cleaved by C5 convertase into C5a and C5b. C3 convertase is bound by C3b to form C5 convertase.
  • Initiation of late components of complement system to form Membrane attack complex (MAC) and to kill pathogen is done by C5 convertase generated by various complement pathways.
  • These are some of the common steps in different complement system pathways.

Types of Complement Pathways:

  1. Classical Pathway:
  2. Initial step in the classical pathway is the formation of Antigen-Antibody complex. When antibody binds to antigen, a conformational change is induced in the FC (fragment crystallization) portion of the antibody. This FC region exposes binding site for C1 protein.
  3. C1 is composed of c1q and two molecules of c1r and c1s each. C1q remains bounded to the FC portion. C4 and C2 are cleaved with the help of proteases, c1s and c1r.
  4. C1 bounded to the immune complex, calls for another protein C4 which gets cleaved into two; C4a and C4b. C4b is activated and attaches to target surface near to C1q whereas C4a goes away(eliminated).
  5. C4b than cleaves C2 into C2a and C2b. C4b2a complex is formed upon C2a binding to C4b, whereas C2b goes away (eliminated).
  6. C3 complex is then activated by C4b2a. This complex is also called as C3 convertase complex, as C3 is converted to an active form by separating C3a and C3b.
  7. To cleave large number of C3 molecules, one molecule of C4b2a is enough. C3b can bind to both, to the microbial surface or to the convertase itself.
  8. C4bC2aC3b complex is formed upon C3b binding to C3 convertase. C5 gets activated by C5 convertase into C5a and C5b. C5b gets stabilized by binding to C6 while C5a is eliminated. C5bc6 complex is formed, which than binds to C7.
  9. Complex called C5BC6C7 formed, binds to phospho-lipid bilayer of the cell membrane and further binds to C8.
  10. At the end all this C5b678, results in activation of C9 which forms a macromolecular structure called Membrane attack complex. (MAC).   
  11. Due to the formation of membrane attack complex, hole is produced in the bacterium resulting in leaking of cellular contents and unwanted substances can get in. As a result of this cell looses its osmotic stability resulting in lysis by influx of water and loss of electrolytes.
  12. The system (MAC complex formation) is found to be more effective in gram negative than in gram positive bacteria as MAC complex can be easily formed in thin peptidoglycan layers of gram negative bacterial cell walls rather than thick layered gram positive cell walls.
  13. There are few exceptions where, some of the c3b molecules do not associate with C4b2a. Instead these molecules are known to coat microbial cell surfaces and immune complexes and works their as Opsonins (an antibody or cell that binds to foreign antigen and exposes them to phagocytosis process). This process of opsonin formation is called as Opsonisation.

2. Alternate Pathway:

  • In this pathway there is no formation of antigen-antibody complex.
  • In this pathway, the complement system is initiated by Cell surface Constituents, which are foreign to the host. Example- Lipopolysaccharide.
  • During inflammation, bacteria enter host body and reaches to the site where C3 is directly bounded to antigens surface, and becomes active.
  • C3 contains thioester bonds which undergo hydrolysis to give C3a and C3b.C3b now binds to surface of the foreign particle and then binds to Factor B.
  • This factor B exposes site which serves as enzymatic substrate for serum protein D. As a result of this, factor D cleaves B into Ba and Bb. This results in formation of C3 convertase (C3bBb).
  • C5 convertase gets formed by C3 convertase and C5 then forms MAC complex, similar to the Classic pathway.

3. Mannose Binding Lectin Pathway (MBL):

  • Without Antibody and Endotoxin, complement pathway can be activated with the help of MBL pathway. When circulating lectin binds to Mannose residues on carbohydrates surface of Micro-organisms, this MBL pathway gets activated.
  • Salmonella, Listeria and Neisseria strains can induce MBL pathway.
  • Concentration of MBL increases during inflammation as it is an acute phase protein.
  • Lectin recognizes and binds carbohydrates of target cell to activate this MBL pathway. This pathway has similarities with that of classical pathway in terms of C4 and C2 to produce complement proteins.
  • MBL resembles C1q in structure and works in similar fashion to that of C1q.
  • Masp1 (mbl-associated serene proteases) and Masp 2 are the two components that binds to MBL after lectin binds to carbohydrates and activates MBL pathway.
  • A tetrameric complex is formed by Masp1 and 2 similar to complex formed by c1s and c1r and than cleaves C2 and C4 to form C3 convertase.
  • Than in the next steps, C5 convertase is formed, which results in MAC complex and the rest process is same as that of classical pathway.

Functions of Complements:

  • Opsonisation and Phagocytosis: proteins involved in this process are C3b. It is bounded to the surface of pathogen and activates phagocytic cells by binding to specific receptors present on the surface of phagocytic cells.
  • Cell lysis: C5b6789 forms a membrane complex which ruptures microbial cell surface and kills them.
  • Chemo taxis: Neutrophils and macrophages are attracted to an area where antigens are present by complement fragments. These cell surfaces have specific receptors for C5a, C3a and thus, run towards site of inflammation, called Chemo taxis.
  • Antibody Production: C3b receptors are present in b cells. When C3b binds to b cells more antibodies are produced. Thus, C3b is an antibody amplifier and can convert this into defense mechanism against invading Micro-organisms.
  • Immune Clearance: immune complexes are removed from circulation and are deposited in liver and spleen. Thus, complement protein acts as anti-inflammatory function. Solubilisation of these complexes is facilitated by Complement proteins and also helps in their phagocytosis.

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