The binding of the antibody and the antigen is not enough sometimes to give an effective protection against the invader agent; that is why it is necessary the complementary action of other components of the immunological system able to neutralize or promote the neutralization of the foreign agent.
Complement system is a set of proteins that form a biochemical cascade which participates in the immunological mechanisms of the body. This system, several times, act complementary (and as result) of the antigen-antibody interaction, but some times can even act independently of the action of antibodies.
The proteins that form this system are mainly proteolytic enzymes in form of zymogens, that when the mechanism is initiated, are activated and trigger defense mechanisms that include a wide range of actions, from the activation of phagocytosis to the lysis of foreign cells. Other complement proteins act as cofactors while others act as inhibitors.
Most of the proteins that form the complement system are synthesized in the liver. Complement proteins form approximately 5 % -10 % of plasma globulins. They are components of the acute phase response and their concentration in blood is increased during infections, injuries, and traumas. Most of these proteins are named with a C letter and a number that was assigned in the order that they were discovered.
The functions of the complement system include:
1. – Cell lysis
2. – Stimulation of phagocytosis through opsonization.
3. – Attraction of phagocytic cells through chemotaxis
4. – Contribution to the inflammatory and allergic reactions, by stimulating degranulation and release of intracellular enzymes, histamine, etc.
5. – Facilitation of immune complex elimination.
Activation of the complement system can happens through any of the following mechanisms:
1. – Classic complement pathway.
2. – Alternative pathway
3. – Mannose-binding Lectine pathway.
In the classic activation pathway, the antigen-antibody interaction provokes allosteric changes in the immunoglobulin that exposes, in the constant region 2 the heavy chains (HC2), a binding site for C1q, a protein of the complement system. The binding (and subsequent activation) of C1q to the constant region of the heavy chains activates two other proteins of the complement system: C1s and C1r. C1s is a serine protease which acts on C4; when C4 is activated, C4 acts on C2. The active fragments of C4 and C2 form the complex C3 convertase, which hydrolyzes C3. (C1q can be activated also by mycoplasms, bacterial endotoxins, RNA virus, and some membranes, in the absence of antibodies)
When C3 is activated the signal is highly amplified, since C3 is the most abundant protein of the complement system, and it can experiment also self-activation. The C3 b derived from C3 binds to glycoproteins in the cell surface. Since macrophages and neutrophils have C3b receptors, they recognize the cells covered with C3b and phagocyte them.
Another part of C3b binds to C5 forming a complex that is hydrolyzed by C3 convertase (aka C3/C5 convertase).This hydrolysis produce C5a, which attracts neutrophils, and C5b. C5b form a complex in the cellular membrane with C6, C7 and C8. This complex guide the polymerization of around 15 molecules of C9, to form a pore that goes through the membrane lipid bilayer of the foreign cell, allowing the passage of ions and small molecules, and provoking the cell lysis. This complement complex is called the Complement Membrane Attack Complex (MAC).
The following video shows a version of this process:
The alternative pathway occurs in the absence of the antigen:antibody complex. Usually, a certain quantity of C3 is spontaneously hydrolyzed releasing 3a and 3b. In normal conditions, 3b is inactivated, but in the presence of bacteria, or invader particles or molecules (virus, fungus, bacteria, parasites, snake’s venoms, or Ig A) 3b can bind to the bacteria membrane and interact with other plasma protein, Factor B, forming a C3bB complex. This complex, when hydrolyzed by another protein (Factor D) releases Ba and becomes a C3bBb complex, with C3/C5 convertase activity. This complex triggers ulterior changes that provoke the formation of the Membrane Attack complex and the invader cell lysis. (Some proteins, Factor H and factor I inhibit C3 convertase, while properdin stabilizes C3 convertase active conformation)
A third form of complement activation is the Mannose-binding Lectine pathway. In this pathway, the Mannose-Binding Lectine (MBL), a serum protein that is able to link to mannose and other monosaccharides in the glycolipids and glycoproteins of the surface of the invader cells, form a complex with two serine proteases zymogens (Mannose-binding lectin Associated Serine Proteases) MASP-I and MASP-II. When the MBL binds to the oligosaccharides on the bacteria, virus and fungus surface, the serine proteases result activated and hydrolyze C4 and C2 proteins, triggering the complement cascade.
It does not matter which activation mechanism is used, the three of them converge in the formation of a complex with C3 convertase activity, formation of C3b and the progression of the cascade that culminates with the foreign cell lysis.
Even when different textbooks differ in some specific details, the fact is that the complement system main functions include:
1.- Opsonization (marking foreign cells for phagocytosis; e.g. C3b)
2.- Chemotaxis (attraction of neutrophils to the invader agent; e.g. C5a)
3.- Lysis of invader cells (Ex C5, C6, C7, C8, and C9)
4.- Contributing to the inflammatory and allergic response, by stimulating cell degranulation and release of enzymes, histamine, and other substances (Effects of C3a, C4a, C5a)
5.- Promoting the elimination of immune complexes (Ex. C3b )
This video summarizes the mechanism of action of the complement system (some small details are different; do not care about that and pay attention to the big picture):
Complement system dysfunction is related to some diseases, like acquired or congenital deficit of individual complement components. In these diseases, the patient shows an increased susceptibility to Neisseria or pyogenic infections.
There is also an important association between the deficiency of complement factors and immunological diseases of the type of Systemic Lupus Erythematosus, and other collagen and vascular diseases, as well as with some cases of chronic nephritis, angioedema, etc.
Additional information can be found in the following links:
Complement Membrane Attack Complex
When the immune system has inadequate levels of Complement proteins
Gupta, R.; Agraharkar, M.:
Complement Related Disorders
Chaganti, K.R. et al:
Glovski, M. et al:
Complement determinations in human disease