The complement system is a complex cascade involving proteolytic cleavage of serum glycoproteins often activated by cell receptors. This cascade ultimately results in induction of the inflammatory response, phagocyte chemotaxis and opsonization, and cell lysis.
Complement factors C3a, C5a and C4 can induce vasodilatation, increased capillary permeability, and expression of leukocyte adhesion molecules. Complements C3a and C4b are opsonins that bridge phagocytes to microorganisms. Complements C3a and C4a promote phagocyte chemotaxis. Complement C3b may be an opsonin for antigen-antibody complexes which helps prevent damage from the formation of large, insoluble immune aggregates. Complement C5a, like C3a is an anaphylatoxin, and is a chemotactic attractant for induction of neutrophilic release of antimicrobial proteases and oxygen radicals. A complex of complements C5b,C6, C7, and C8 mediates the polymerization of up to eighteen C9 molecules into a tube-like membrane attack complex that is inserted into the plasma membrane of an unwanted organism such as of gram-negative bacteria and viral infected cells. This channel through the lipid bilayer results in lysis of the cell. Ischaemic infarction may also cause initiation of the complement cascade. Excessive deposits of membrane attack complexes in tissues may occur following ischaemic injury. Other deleterious effects of complement activation include, degranulation of neutrophils, basophils and mast cells, unwanted release of the neutrophil products elastase and oxygen radicals, and extracorporeal blood circulation. Complement inhibitors are being studied as potential therapeutics for immune diseases and Alzheimer’s.
Three pathways have been elucidated through which the complement cascade can be initiated; Classical, Alternate and Lectin Pathways. All three pathways merge through at common intersection, complement C3.
The classical pathway mediates specific antibody responses. The classical pathway is initiated by the binding of antibodies to cell surface antigens. Subsequent binding of the antibody to complement C1q subunits of C1 result in catalytically active C1s subunits. The two activated C1s subunits are then able to catalyze the assembly of the C3 convertase (complement C4b2a) from complements C2 and C4.
The alternate pathway does not require the action of antibodies to inititate the cascade, but is initiated by foreign cell surface components. In the alternate pathway complement C3 undergoes spontaneous cleavage resulting in complement B binding to C3b. Diffusion of the Ba subunit results in an active alternate pathway C3 convertase (C3bBb). C3bBb is stabilized by binding to properdin prior to merging into the common pathway and conversion of C3.
The lectin pathway is similar to the classical pathway. C1q is not involved in the lectin pathway. Instead an opsonin, mannan binding protein (MBP), is involved in the initiation process.