- The complement system is defense mechanism that act nonspecifically against invading organisms.
- There are different proteins that function as enzyme precursors.
- The 11 main proteins are named C1, C2, C3, C4, C5, C6, C7, C8, C9, B and D (present in plasma and in tissue fluid).
Mechanism of activation of complement system:-
Classical Pathway
- This is activated by antigen-antibody reaction. As the antibody binds with antigen, a specific reactive site on constant portion of antibody becomes uncovered and binds directly with protein C1.
- This produces a cascade of sequential reactions causing activation of successive proteins leading to multiple products causing following effects:
- Opsonization: Activation of neutrophils and macrophages to engulf bacteria.
- Lysis: Destruction of bacteria by rupturing cell membrane.
- Chemotaxis: Attraction of leucocytes to site of antigen-antibody reaction.
- Agglutination: Clumping of foreign bodies like red blood cells.
- Neutralization: Covering toxic sites of antigenic products.
- Activation of mast cells and basophils: This causes liberation of histamine. Histamine causes dilatation of blood vessels and increases capillary permeability; this helps plasma proteins enter tissues and antigenic products are inactivated.
Alternate Pathway
- Compliment system can be activated without antigen-antibody reaction. A protein in circulation called factor I binds with polysaccharide present in cell membrane of invading organism.
- This binding activates C3 and C5, which attack antigenic products of the invading organism.
- It is one of the first lines of defense against invading organisms. This pathway occurs in response to large polysaccharide molecule in the cell membrane of invading organism.
- Complement proteins promote phagocytosis by neutrophils, monocytes, and macrophages.
- They activate B cells and thus activate humoral immunity.