Complement system – applied aspect

Dr Anitha MA BHMS,MD(Hom)
Dr.Padiyar Homoeopathic Medical College.Kerala

This term ‘complement’ is coined by Ehrlich, because this factor complemented the action of antibody. ‘Complement’ refers to a system of factors which occur in normal serum, activated by antigen antibody interaction and subsequently mediate a number of biologically significant consequences. These complement factors are heat labile (Ab. are heat stable) Majority of these complement proteins are inactive. They require proteolytic cleavage to become active. Enzymatically active form is represented by putting a ‘bar’ above its notation eg C-lr. Cleavage products of complement are represented by, a (large fragment) and b(small fragment) eg: C3a, C3b (exception to this rule is C2 where large fragment is b & small one is a).

Complement constitutes about 5% of normal serum protein and is not increased as a result of immunization. Heat labile complement proteins are destroyed in 30mts at 56oC.Serum deprived of its complement activity by heating at 560 for 30 mt is said to be inactivated.
C is activated by binding with the Fc portion of Ab. C is combined with antigen. It binds with IgM, IgG1, IgG2, IgG3 antibodies (not with IgG4, IgA, IgE,IgD)

Components of complement
Complement is a complex of fractions of proteins, C1 – C9
C1 has 3 subunits : Clq, Clr,Cls serves as a recoginition unit.C2, C3,C4and C5 serves as an enzymatic activation unit which will lead to the sequential assembling of C5, C6, C7,C8,C9. C9 serves as attack unit for alteration of the cell membrane.

Complement Activation
Begins with the reaction of complement attached to the Fc portion of Ab binds to antigen. Thereafter the proteins of complement or component of complement act as a cascade, where each enzyme act as a catalyst for the next, cleaving them into dissimilar fragments. Larger fragments join the cascade and smaller fragments contribute to defence mechanisms by favoring inflammatory process. Net result of complement activation is alteration of target cell membrane that permits excess fluid to enter the cell and leads to lysis of cell. There are 2 main pathways for complement activation. Classical and alternative pathways. Classical pathway is related to adaptive immune system through binding of immune complexes to c1q. The alternative pathway is activated by C 36 to the surface of micro organism and is concerned with innate immunity. (Inflammatory cells like phagocytes, polymorph and macrophages have complement fragment receptors on their surface, so they move towards the target cell and cause lysis).
Complement can distinguish self from non self antigens by rapid binding of C3 b to non self (micro organisms or immune complexes). But the individuals own cell surface are protected from limiting C3b deposition

Classical pathway
Explained by taking the eg : of lysis of erythrocyte (E) sensitized by its antibody (A).This is an antibody directed mechanisms for complement activation. C1 is the first enzyme complex in the cascade. C1 is Ca 2+ dependent complex consisting of a single C1q molecule, two Clr and two Cls molecules.

I step :

Binding of C1 (1q) to the antigen antibody complex. clq is the recognition unit. Clq has 6 combining sites. At least 2 binding sites should be attached to Ab for effective activation. This binding takes place in the presence of Ca 2+ leads to sequential activation of Clr and Cls

11 step :

Activated Cls (Cls) is an esterase which cleave C4 to C4a (anaphylatoxin) and C4b which binds to cell membranes along with C1

111 step
C14b in the presence of Mg+ cleaves C2 into C2a, which linked into C4b and C2b is released into fluid.C4b2a has enzymatic activity and is referred to as classical pathway C3 convertase.

1V step
C3 convertase splits C3 into C3a (anaphylotoxin) and C3b which remains bound to C4b2a to form C14b2a3b and is called C5 convertase

V step:

The membrane attack phase begins with this stage. C5 convertase cleaving C5a to anaphylatoxin, released into the medium and C5b which remain with the cascade (C14b2a3b5b) C6 and C7 then join with it.C5b react with other complement components to produce a macromolecular complex(membrane attack complex) that has the ability to alter cell membrane permeability. C8 is the most active component producing maximal cell lysis.

Mechanism of cytolysis is the production of holes on the cell membrane. Host cell bear membrane protein which protect against lysis by MAC.Other than antigen antibody complexes, classical pathway can be activated by DNA, CRP, trypsin like enzyme or some retroviruses.

Alternative Pathway
Activation of C3 is achieved without the prior participation of C142.
The internal thioester bond in native C3 is susceptible to spontaneous hydrolysis by water, generating an activated form of C3. This spontaneous C3 activation is plasma is called tick over reaction and the product is named as C3i. Then C3i binds with factor B to form C3iB. This binding B Factor is cleaved by Factor D and release Ba. The remaining complex is C3iBb which act as C3 convertase. This enzyme is labile. The factor P (properdin) stabilize this enzyme which convert C3 to C3a and C3b. Further steps occur as in the classical pathway.

Lectin Pathway
Homologous with the classical pathway., but it is activated in an antibody independent fashion ie Clq is able to bind directly to certain micro organism including mycoplasma and some retroviruses in an independent process.
So in short all these pathways lead to the formation of C3 convertase.

Regulations of C activation :
By 1. inhibitors which stops the progressive complement activation.
Eg : Serum protein
Cl-esterase inhibitor

By 2. Inactivators : enzymes that destroy complement proteins.
Eg : Factor I
C4 binding protein etc.

Biological effects of complement
Major beneficial activities:
1. Promotion of killing of micro organism
2. Efficient clearing of immune complexes
3. Induction and enhancement of antibody responses

Harmful effects:
1. If activated systematically on a large scale, anaphylactic reaction occurs eg : in gram –ve septicemia
2. If activated by tissue necrosis eg : MI
3. If activated by an autoimmune response to host tissue.

Complement promote killing of micro organism by :
1. By generation of anaphylatoxins, induce chemotaxis
2. By opsonising the micro organisms to enhance phagocytosis.
3. By insertion of MAC into cell membrane of organism

Deficiency of Complement system
Deficiency commonly causes bacterial infections. Complement is of less importance in host defense against viral infection.Deficiency of MAC are associated with increased susceptibility to Neisseria meningitides

Clinical syndromes associated with complement deficiency

  • C1 inhibitor – Hereditory angio neurotic oedema
  • C2, C4 – SLE and other collagen vascular diseases
  • C3 and C3b inactivator – Severe recurrent pyogenic infections
  • C5 to C8 – Bacteremia with gram –ve diplococci ,toxoplasmosis
  • Cq – No particular disease

Complement activity of serum is measured by estimating the highest dilution of serum lysing sheep erythrocytes sensitized by antierythrocytic antibody. Estimation of individual complement components also uses hemolytic activity in a system containing excess of all complement components except the one to be measured

Biosynthesis of Complement

  • C1 – from intestinal epithelium
  • C2, C4 – Macrophages
  • C5, C8 – Spleen
  • C3, C6, C9 – Liver
  • C7 – not known

Rise in C level (particularly C4, C3, C5, C6) is seen in acute phase of inflammation

References :
Immunology by Roitt-Brostoff- Male
Basic pathology by Kumar , Cotran, Robbins
Text book of Microbiology by Jayaram Panikkar
Boyd’s text book of pathology

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