Complement System

The complement system is a sophisticated and systematic defense strategy that can be traced back 600-700 million years. It is traditionally considered a central part of innate immunity that enhances the lethal effect of antibodies and phagocytic cells during the defense against microbes, apoptotic and necrotic cells from an organism. However, research in recent decades reveals that complement system may be a bridge between innate and adaptive immune response, because of the ability to orchestrate immune reactions by communicating with multiple cells involved in both two branches of the immune response. And the complement system can be recruited and activated by antibodies generated by the adaptive immune system. Furthermore, the complement system helps to maintain the solubility of circulating immune complexes and facilitate their elimination.

Complement System

The complement system consists of more than 30 proteins and protein fragments, including serum proteins and cell membrane receptors. Many of the constituents are circulated as zymogens, which are inactive precursors that require cleavage to become active enzymes. The activation of the complement system is usually initiated by several triggers, e.g. antigen-antibody complex, lipopolysaccharide, mannosans, peptidoglycan. Once stimulated, the proteases in the system will cleave specific proteins to release cytokines and induce an amplifying cascade of further cleavages to play an immunological role. The system is acceptable to carry out various functions:

1. Phagocytosis: Complement serves as an opsonin to mark the invading pathogens and then promote phagocytosis. The opsonization is mainly mediated by C3b, C4b, and C1q.
2. Inflammation: Complement cleavage products promote the generation of inflammatory signaling agents (e.g. histamine) to enhance capillary permeability and attract macrophages and neutrophils.
3. Membrane attack: Complement ruptures the wall of pathogens by inserting the terminal membrane attack complex (MAC).

Complement System Pathways

Depending on the activation triggers, the complement system can be stimulated by three distinct pathways: the classical, lectin and alternative pathway.

1. Classical Pathway

The classical pathway is initiated when C1q (first protein of the cascade) binds to the IgM or IgG antigen/antibody complexes. In addition, some other danger signals can also activate the classical pathway with antibody-independence, such as C-reactive protein, viral proteins, polyanions, apoptotic cells, and amyloid. The classical pathway acts as the link between the effectors of the innate and adaptive immunity.

1. Lectin Pathway

The lectin pathway is initiated when either mannose-binding lectin (MBL) or ficolin bind to mannose residues on the surfaces of pathogens. Once activated, the lectin pathway proceeds through the C4 and C2 to activate other complement proteins down in the cascade. The biological activities and the regulatory proteins of the lectin pathway are similar to the classical pathway.

1. Alternative Pathway

This pathway can be activated by when the exogenous viruses, fungi, bacteria, parasites, cobra venom, immunoglobulin A, and polysaccharides invade the organism, the component C3b will bind to factor B to start the alternative pathway. It is an important part of the defense mechanism independent of the immune response.

Fig 2. Three pathways of complement cascade. (Douglas, 2015)

1. Membrane Attack Complex

The complicated and delicate network can be activated by diverse mechanisms proceeding through distinct pathways, yet all converge on a final common ending: formation of a multimolecular complex, the membrane attack complex (MAC). The MAC inserts into cell membranes to form a functional pore, resulting in ion flux and ultimately osmotic lysis.

Fig 2. Illustration of the membrane attack complex (MAC).²

Although complement system plays a key role in defense against pathogens and in host homeostasis, it is widely regarded as a double-edged sword. The subsequent cascade of enzymatic reactions is strictly regulated to guarantee that the activation only occurred in the defense against pathogens, thus avoiding host tissue damages.

References
1. Mathern, Douglas R., and Peter S. Heeger. "Molecules great and small: the complement system." Clinical journal of the American Society of Nephrology: CJASN 10.9 (2015): 1636.
2. From Wikipedia: By SLiva2016 - Own work, CC BY-SA 3.0 https://commons.wikimedia.org/wiki/File:Membrane_Attack_Complex_(Terminal_Complement_Complex_C5b-9).png

• Classical Pathway
• Lectin Pathway
• Alternative Pathway
• Membrane Attack Complex