What is the Complement System?

Complement System is a complicated system that is involved in the Immune System of an organism. This system is a cascade system that begins with biochemical processes that subsequently cause several reactions.

Some aspects of the immune system are adaptable however, the complement system is not one of these areas that are changeable. Within the blood are several small proteins that play an important role in the complement system. Several proteins while circulating in the blood are inactive.

An enzyme called protease affects several proteins which in turn releases cytokines that enhances the cascade affect of this system. The end result of the cascade is the activation of the “Membrane Attack Complex”  which is the cell killing complex to protect against foreign invaders.

There are over 20 molecules that are involved in this system and most are synthesized in the liver.

3 Main Types:

1.)  Classical Pathway
2.)  Alternative Pathway
3.)  Lectin Pathway


Classical Pathway

Protein Players

–  C1 complex
–  C2a, C2b
–  C4b, C4a
–  C3 convertase
–  others

Most proteins are found in the blood and are triggered by antibodies attached to antigens. The molecule binds to C1 complex and begins this portion for the system

When antigen-bound antibody stimulates a C1 component to attach to the antibody, a reaction is initiated.

Alternative Pathway

Believed to be older than the classic complement system. The goal is the “opsonization” of a pathogen which results in the pathogen’s “death”.

No antibodies are required in this system. Often this may result in a faster and somewhat effective system. Attaches to the membrane surface of a foreign pathogen

Acts like a “chain saw” and a “membrane attack complex” (MAC) which punches a hole and causes lysis (death) to the cell

Protein Players:

–  C3a
–  C3b
–  C5a
–  C5b
–  C6
–  C7
–  C8
–  C9

Lectin Pathway

–  This pathway is very similar to the above mentioned classic pathway
–  The difference is the “mannose binding lectin” (MBL) and ficolins instead of a C1q protein – which is apart of the C1 complex

Regulation of Complement

–  The regulation for this system is very intense a protected.
–  If not, the damage to the host organism would be substantial.
–  The proteins involved must visualize the difference from host (self) cells and foreign (Non-self)

Proteins involved:

1.)  Complement Receptor 1
2.)  Membrane co-factor protein
3.)  Factor H
4.)  C4b-binding protein
5.)  Decay-Accelerating factor

These proteins can be found on every human cell and prevent for complement to occur to these cells. This regulation protects “self” cells and improves the overall regulation of this system.

Cell debris, virus, bacteria, antibody-antigen complexes and more items are not protected by these proteins and are subject to the complement system.

Complement has been argued to have a component or factor in certain disease.

–  Lupus
–  Flomerulonephritis
–  Multiple Sclerosis
–  Asthma
–  others