What is Complement System?

Definition of the Complement System

The immunity complement system is part of the natural immune response and promotes antibodies and phagocytes in purging organisms of a pathogen or sickly cells. It is the first line of defense against infection.

The complement system consists of enzymatic reactions that build upon one another to regulate pathogens, inflamed cells, destroyed foreign bodies and interconnect natural and adaptive immunity. Complement pathway is triggered by 3 different routes: the classical, alternative and lectin routes. Complement activation must be carefully regulated so that unregulated inflammation and cell damage do not ensue. Studying immune disorders and researching immunotherapies depends on a robust appreciation of the complement system.

Fig. 1 The complement reaction.¹

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What Makes Up Your Complement System?

Complement is made of more than 30 soluble and membrane-associated proteins. These complement proteins immune system are mainly produced by liver hepatocytes and diffuse in the blood stream as inert forms. Upon activation, they work in a cascade-like mechanism to eliminate threats. The complement system is divided into three primary pathways:

• Classical pathway
• Lectin pathway
• Alternative pathway

Despite their distinct initiation triggers, all pathways converge at the generation of C3 convertase, a pivotal enzyme in the cascade.

Proteins of the Complement System

Table. 1 Proteins and their functions of different pathways in the complement system.

Pathway	Overview	Component	Function
Classical Pathway	This pathway is antibody-dependent, triggered by the binding of C1 complex (C1q, C1r, and C1s) to antigen-antibody complexes.	C1q	Recognizes and binds to immune complexes
		C1r & C1s	Serine proteases that activate downstream complement proteins
		C4 & C2	Form C4bC2a, the classical pathway C3 convertase
Lectin Pathway	This pathway is activated by pattern recognition molecules like mannose-binding lectin (MBL) or ficolins binding to microbial surfaces.	MBL	Binds to mannose residues on pathogens
		MASPs	Activate C4 and C2, forming the C3 convertase
		Ficolins	Recognize acetylated sugars
Alternative Pathway	Unlike the other pathways, the alternative pathway is antibody-independent and activated spontaneously through the hydrolysis of C3 or pathogen surfaces.	C3	Undergoes spontaneous hydrolysis to form C3(H2O)
		Factor B	Combines with C3b to form C3bBb (C3 convertase)
		Factor D	Cleaves Factor B to stabilize the C3 convertase
		Properdin	Stabilizes the C3 convertase complex
Terminal Pathway	The terminal complement pathway involves the assembly of the membrane attack complex (MAC), which is responsible for pathogen lysis.	C5b	Initiates MAC assembly
		C6, C7, C8, C9	Sequentially form the MAC
		MAC	Creates pores in the pathogen membrane, leading to lysis

Regulatory Proteins of the Complement System

Given its potent effects, the complement system is tightly regulated to prevent host tissue damage.

Table. 2 Regulatory proteins and their functions in the complement system.

Regulatory Protein	Function
Factor H	Inhibits the alternative pathway
C1 Inhibitor (C1-INH)	Blocks C1 activation in the classical pathway
CD55 (DAF)	Decays C3 convertase
CD59	Prevents MAC formation on host cells

What Does the Complement System Do?

The complement system in immune system can be activated by three different pathways: the classical pathway, the lectin pathway, and the alternative pathway. Although the initiation of these pathways depends on different molecules, they converge to produce the same set of effector molecules. The activation of the complement results in a sequence cascade of enzymatic reactions, producing a wide range of activities, from cell lysis to immune clearance. In summary, the complement system has four major functions.

Fig. 2 Complement system in physiological conditions.^{2, 3}

Complement System Function

What is Complement Activation?

Complement activation definition is the pathway of biochemical reaction that is triggered by complement proteins to kill pathogens, purge immune complexes, and stimulate inflammation. This stimulation sets off a cascade of proteolytic breakage and molecular assembly reactions that end in the destruction of invader microbes and immune responses.

The hallmark of complement system activation is its amplification ability, where a small initial stimulus triggers an exponentially larger response.

How Do Complement System Proteins Activate?

The activation typically involves three critical steps:

• Initiation: A specific trigger, such as a pathogen surface or immune complex, activates the complement pathway.
• Amplification: Proteolytic cleavage of complement proteins generates active fragments, which serve as enzymes for subsequent reactions.
• Terminal activation: Assembly of the MAC, a lytic structure that disrupts the integrity of target cell membranes.

Table. 3 Key proteins involved in different steps of complement activation.

Step	Key Proteins Involved	Outcome
Initiation	C1q, MBL, Factor D	Recognition of pathogens or immune complexes
Amplification	C3, C5 convertases	Generation of active complement fragments
Terminal Activation	C5b-9	Formation of the MAC

Each pathway has distinct initiation mechanisms but converges at the central cleavage of C3.

Complement Activation Antibody

Some antibodies, mostly IgM and IgG (IgG1 and IgG3), are needed to activate complement through the classical pathway. When attached to antigens, these antibodies release their Fc regions for contact with C1q. The strength of complement activation depends on:

• Antibody isotype: IgM is highly efficient in activating complement due to its pentameric structure.
• Antigen-antibody complex density: Higher density increases complement activation likelihood.
• Affinity of Fc-C1q interaction: Determines the strength of the classical pathway trigger.

Table. 4 Efficiency of antibody isotype in complement activation.

Antibody Isotype	Efficiency in Complement Activation
IgM	High
IgG1 and IgG3	Moderate
IgG2 and IgG4	Low

What is the End Product of Complement Activation?

The ultimate goal of complement activation is the formation of the MAC, a structure composed of complement proteins C5b, C6, C7, C8, and multiple C9 molecules.

• Function: The MAC forms pores in target cell membranes (complement activation pores in membrane), leading to cell lysis.
• Outcome: Elimination of pathogens and damaged cells.

In addition to MAC formation, complement activation generates other biologically active fragments:

• C3a and C5a: Potent anaphylatoxins that mediate inflammation by recruiting immune cells.
• C3b: An opsonin that enhances phagocytosis by marking pathogens for destruction.

Fig. 3 Complement anaphylatoxins.^{2 ,3}

What is Complement Immunology?

Complement system immunology is the bedrock of the immune system, merging both natural and adaptive immunity to defend the body against germs. Complement immunity - The defence mechanism of the system and its control mechanisms against self-damage. It enhances:

• Innate immunity: By directly targeting pathogens.
• Adaptive immunity: By influencing B-cell and T-cell responses.

Complement and the Immune System

The complement immune system is a vital player in the broader immune response. Its functions are diverse, ranging from direct microbial lysis to enhancing the capabilities of other immune components.

• Opsonization
• Cell lysis
• Immune clearance
• Inflammation

The complement immune response is rapid and highly effective. Key components include:

Table. 5 Key components and their functions in complement immune response.

Component	Function	Clinical Relevance
C3b	Opsonization	Deficiency can lead to increased infections
C5a	Chemotactic and pro-inflammatory	Implicated in autoimmune diseases
MAC (C5b-C9)	Pathogen lysis	Deficiencies linked to susceptibility to Neisseria infections

Complement Deficiency

Complement system deficiencies predispose us to infection, autoimmune disease and immune dysfunction. Almost all complement defects are caused by genetic mutations that delete or misfunction proteins. They are autosomally dominant or recessive mutations.

Table. 6 Classification of complement deficiencies according to the pathway or protein affected.

Deficiency Type	Affected Protein	Primary Effects	Common Associated Conditions
Classical Pathway	C1q, C1r, C1s, C4, C2	Impaired clearance of immune complexes, reduced opsonization	Systemic lupus erythematosus (SLE)
Lectin Pathway	MBL, MASP-1, MASP-2	Reduced pathogen recognition	Recurrent bacterial infections
Alternative Pathway	Factor D, Factor B, Properdin	Reduced amplification of C3 convertase	Susceptibility to meningococcal disease
Terminal Pathway	C5, C6, C7, C8, C9	Incomplete MAC formation, impaired lysis of pathogens	Recurrent Neisserial infections
Regulatory Protein Deficiency	Factor H, Factor I, CD59, DAF	Loss of regulation, leading to overactivation or dysregulation of complement	Atypical hemolytic uremic syndrome (aHUS) Paroxysmal nocturnal hemoglobinuria (PNH)

Complement System Solutions

Besides its known immune role, the complement cascade is increasingly seen as a generalizable target for immunology, oncology, neurobiology and therapeutic development.

• The immune complement system functions in both ways and has become one of the main topics of immunotherapy research.
• Complement proteins are being developed as biomarkers and therapy agents in oncology.
• Abnormalities of complement proteins such as C1q and C3 drive neuroinflammation and synaptic degeneration.

Complement Technology

Reliable assays are essential for studying complement dynamics. These tools can be applied for:

• Pathway activation analysis: Classical, lectin, and alternative pathways.
• Protein quantification: Measuring complement components such as C3, C5, and MAC.
• Functional studies: Assessing hemolytic activity or opsonization potential.
• Genetic analysis: Identifies mutations in complement-related genes.

Table. 7 Overview of complement assay types.

Assay Type	Measurement	Applications
ELISA-based assays	Protein levels (e.g., C3, C5)	Quantifying pathway activation
Hemolytic assays	Functional activity (CH50)	Classical and alternative pathways
Multiplex assays	Multiple proteins	High-throughput screening

For more information on high-quality complement assays, Creative Biolabs offers tailored solutions to meet diverse research needs.

Resources

References

1. From Wikipedia: https://commons.wikimedia.org/wiki/File:Complement_pathway.svg
2. Merle, Nicolas S., et al. "Complement system part I - molecular mechanisms of activation and regulation." Frontiers in immunology 6 (2015): 262.
3. Distributed under Open Access license CC BY 4.0, without modification.

• How Does Complement System Work?
• Complement Protein Fragment Nomenclature