Introduction to Complement
- Complement is the name given to a system of some non-specific proteins present in normal human and animal serum.
- These proteins have got the ability to lyse or damage cells (bacteria, virus, tumour cells) and to stimulate some antigen and antibody reactions.
- This system consists of approximately 20 serum proteins which include components of complement (C1-C9), properdin system and regulatory proteins.
- Some of these proteins are enzymes, some are control molecules while others are structural proteins without any enzymatic action.
- Complement in general is heat-labile and inactivated at 56oC for 30 minutes.
- Components of the complement are activated sequentially through a cascade manner either by classical or alternative pathway and both the ways have same results.
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- This pathway is initiated by binding of complement to aggregates of antibody and antigen on the cell surface or an immune complex.
- The model explains the lysis of erythrocyte by antibody in the presence of complement.
- In lytic action, all the nine components of the complement (C1-C9) comprising of 11 proteins are involved.
- These reacts together one after the other in a cascade or sequential manner once the reaction has been triggered.
- Only IgG and IgM possess complement binding sites and capable of sensitizing red cells to complement lysis.
- Other immunoglobulins (IgA, IgD and E) do not have a binding site for C1q.
- IgM is a better complement binding antibody than IgG.
- The complement components are grouped under three functional units: C1, the recognition unit; C4, C2, C3, the activation unit; and C5-C9, the membrane attack unit.
A) C1- the recognition unit
- C1 is a complex of three subunits; C1q, C1r and C1s held together by calcium ions.
- One molecule each of C1q and C1s with two molecules of C1r, compose the C1 complex or recognition unit.
- The complement binding site is not exposed in the native state of Ig molecule and it gets exposed only after configurational changes in the molecule following antigen-antibody interaction.
- C1q facilitates binding of the recognition unit (C1 complex) to the exposed site on the Fc region of the cell surface antigen-antibody complexes.
- Activation of classical complement cascade requires linkage of C1q with two molecules of IgG or one molecules of IgM through their Fc regions.
- Binding of C1q, activates C1r (referred to now as C1r*).the enzymes C1r* then converts C1s to an active serine esterase (C1s*, C1s esterase).
B) Activation Stage
- C1s esterase activates C4 which gets split into C4a (anaphylatoxin) and reactive C4b, and C2 to C2a and C2b.
- C4a is liberated into fluid phase, and C4b attaches to erythrocyte surfaces, bacterial cell membranes and other antigens.
- C2a (larger fragments) binds with C4b forming cell bound C4b2a complex, known as the classical pathway C3 convertase.
- C2b (minor fragment) is released into fluid phase.
- C3 is a heart of complement cascade.
- C4b2a (C3 convertase) binds to the cell membrane and acts on C3 and splits into a small 3a and a large C3b subunit on the surface of the cell.
- C3a is an anaphylatoxin.
- The C3 convertase amplifies the response by splitting many C3 molecules.
- A small amount of C3b generated will bind to the activating surface and act as focus for further complement activation.
C) Membrane Attack Complex
- The terminal stage of the complement pathway involves creation of the membrane attack complex, which is known as lytic unit.
- The C5 convetases are generated from C4b2a of the classical and C3bBb of the alternative pathway by addition of another C3b molecules.
- These membrane bound tri-molecular complexes selectively bind C5 and cleave it into a small C5a released in body fluids (anaphylatoxic and chemotactic) and a large membrane bound C5b.
- The formation of rest of the membrane attack complex is non-enzymatic.
- C6 binds to C5b and the joint complex C5b6 is released from the C5 convertase.
- C7 binds to the C5b6 complex forming a heat stable tri-molecular complex C5b67.
- The C5b67 is highly lipophilic and binds to membrane bilayer in the vicinity of initial activation site, where it lies as a high affinity receptor for C8.
- C5b678 binds and polymerizes C9 forming the membrane attack complex (MAC).
- As many as 12-16 C9 molecules can be polymerized by one C5b678 complex.
- A (C5b678)1(C9)n complex forms and drills a hole in the cell membrane, which leads to the hypotonic lysis of cells.
Classical complement pathway