The Complement System: How It Works and Why It Matters

What the Complement System Does

The complement system performs three core defensive functions, each contributing to pathogen clearance through a different mechanism. First, it kills microorganisms directly by assembling membrane attack complexes (MACs) that bore holes through bacterial and viral membranes. Second, it coats pathogens with complement fragments, primarily C3b, in a process called opsonization that marks them for rapid engulfment by macrophages and neutrophils. Third, it generates small protein fragments, notably C3a and C5a, that act as powerful inflammatory mediators, attracting immune cells to the site of infection and increasing blood vessel permeability so that more immune components can reach the tissue.

Beyond these direct antimicrobial effects, complement plays a critical role in clearing immune complexes, clusters of antibodies bound to antigens that can accumulate in the blood and deposit in tissues. Red blood cells carry complement-coated immune complexes to the liver and spleen, where resident macrophages strip them off and degrade them. When this clearance mechanism fails, as it does in systemic lupus erythematosus, immune complexes deposit in kidneys, joints, and blood vessels, causing chronic inflammation and organ damage.

Complement also bridges innate and adaptive immunity. Complement fragments bound to antigens enhance B cell activation by cross-linking the B cell receptor with the complement receptor CR2 (CD21). This co-stimulatory signal lowers the threshold of antigen needed to trigger an antibody response by 100-fold to 1,000-fold, meaning complement makes the adaptive immune system more sensitive to foreign invaders. Without complement, antibody responses are sluggish and incomplete.

The Three Activation Pathways

Complement activation can be triggered through three distinct pathways, each initiated by a different molecular recognition event, but all three converge on the same central step: the cleavage of C3 into C3a and C3b. This convergence point ensures that regardless of how complement is activated, the downstream effector mechanisms are identical.

The classical pathway is triggered when C1q, a large hexameric recognition molecule, binds to the Fc regions of IgG or IgM antibodies that are already bound to a pathogen surface. C1q requires multiple Fc regions in close proximity to achieve stable binding, which is why a single IgG molecule cannot activate complement but a cluster of IgG molecules on a bacterial surface can. Once bound, C1q activates the serine proteases C1r and C1s, which cleave C4 and C2 to form C4b2a, the classical pathway C3 convertase. This enzyme cleaves hundreds of C3 molecules into C3a and C3b, amplifying the signal enormously. The classical pathway is the most antibody-dependent activation route and is a key mechanism by which adaptive immunity amplifies innate defense.

The lectin pathway operates through a similar mechanism but uses different recognition molecules. Mannose-binding lectin (MBL) and ficolins circulate in the blood and bind to specific carbohydrate patterns, such as mannose and N-acetylglucosamine, that are abundant on the surfaces of bacteria, fungi, and some viruses but rare on human cells. MBL resembles C1q structurally and, upon binding its carbohydrate target, activates MBL-associated serine proteases (MASPs) that cleave C4 and C2 to form the same C3 convertase as the classical pathway. The lectin pathway is entirely antibody-independent, making it particularly important during the early phase of a first infection, before the adaptive immune system has had time to produce specific antibodies.

The alternative pathway is the most ancient and the only pathway that is constitutively active. A small amount of C3 in the blood spontaneously hydrolyzes into a C3b-like molecule (C3(H2O)) in a process called "tick-over." This molecule can bind factor B, which is then cleaved by factor D to form the alternative pathway C3 convertase (C3bBb). On the surfaces of host cells, regulatory proteins like factor H, decay-accelerating factor (DAF/CD55), and membrane cofactor protein (MCP/CD46) rapidly inactivate any C3b that lands on them, preventing complement from damaging the body's own tissues. Pathogens lack these regulatory proteins, so C3b that lands on a microbial surface is stabilized and amplified through a positive feedback loop, rapidly coating the pathogen in C3b. The alternative pathway also serves as an amplification loop for the classical and lectin pathways: any C3b generated by those pathways can recruit factor B and factor D to generate additional C3 convertase on the target surface.

The Membrane Attack Complex

The terminal phase of complement activation produces the membrane attack complex (MAC), a ring-shaped structure that inserts into the lipid bilayer of target cells and forms a transmembrane pore approximately 10 nanometers in diameter. The MAC is assembled from five complement proteins: C5b, C6, C7, C8, and multiple copies of C9. The process begins when C5 convertase (formed by addition of C3b to the C3 convertase) cleaves C5 into C5a, a potent inflammatory mediator, and C5b, which initiates MAC assembly.

C5b sequentially binds C6, C7, and C8 on the target membrane. The C5b-C8 complex then recruits and polymerizes 10 to 16 copies of C9, which unfold and insert into the membrane to complete the pore. The resulting channel allows ions, water, and small molecules to flow freely across the membrane, destroying the osmotic balance that the cell depends on for survival. Gram-negative bacteria are particularly susceptible to MAC-mediated killing because their outer membrane is directly accessible to complement. Gram-positive bacteria are more resistant because their thick peptidoglycan cell wall shields the inner membrane from MAC insertion.

The MAC is not the only, or even the most important, effector mechanism of complement for most infections. Opsonization with C3b is generally more critical for bacterial clearance, because phagocytes can engulf and destroy opsonized bacteria with high efficiency. However, MAC-mediated lysis is essential for defense against Neisseria species, the bacteria that cause meningitis and gonorrhea. People with inherited deficiencies in MAC components (C5 through C9) are 5,000 to 10,000 times more susceptible to Neisseria infections than the general population, while their susceptibility to most other infections remains normal.

Complement Regulation

Because complement activation is rapid, powerful, and self-amplifying, the system requires tight regulation to prevent damage to host tissues. The body produces a suite of regulatory proteins that operate at multiple steps in the cascade, ensuring that complement activation is limited to foreign surfaces and quickly shut down on healthy cells.

C1 inhibitor (C1-INH) inactivates the serine proteases C1r, C1s, and MASPs, preventing excessive activation of the classical and lectin pathways. Inherited deficiency of C1 inhibitor causes hereditary angioedema, a condition characterized by episodes of severe, painful swelling of the skin, intestinal wall, and upper airway. The swelling is caused by uncontrolled generation of bradykinin and complement fragments, and airway episodes can be life-threatening without treatment.

Factor H is the principal regulator of the alternative pathway. It binds to C3b on host cell surfaces, where it accelerates the decay of the alternative pathway C3 convertase and serves as a cofactor for factor I, which cleaves C3b into inactive iC3b. Factor H recognizes sialic acid residues on host cell surfaces, allowing it to distinguish self from non-self. Many pathogenic bacteria have evolved surface molecules that recruit factor H to their own surfaces, effectively disguising themselves as host cells and evading complement attack. Mutations in factor H are associated with atypical hemolytic uremic syndrome (aHUS) and age-related macular degeneration, demonstrating the consequences of impaired complement regulation.

CD59 (protectin) is a membrane protein that blocks the final step of MAC assembly by preventing C9 polymerization. Together with DAF and MCP, CD59 protects host cells from bystander damage during complement activation. In paroxysmal nocturnal hemoglobinuria (PNH), a somatic mutation in the PIGA gene prevents the synthesis of glycosylphosphatidylinositol (GPI) anchors, which are required to attach both DAF and CD59 to the cell surface. Red blood cells lacking DAF and CD59 are highly vulnerable to complement-mediated lysis, causing chronic hemolytic anemia, hemoglobinuria, and increased risk of thrombosis. The anti-C5 monoclonal antibody eculizumab (Soliris) was developed specifically to treat PNH by blocking MAC formation.

Complement Deficiencies and Disease

Inherited deficiencies have been identified for virtually every complement component, and the clinical consequences depend on which pathway or effector function is affected. Deficiencies in early classical pathway components (C1q, C1r, C1s, C4, C2) are strongly associated with systemic lupus erythematosus. More than 90 percent of individuals with complete C1q deficiency develop lupus, making it the strongest single-gene risk factor for the disease. The mechanism is thought to involve impaired clearance of apoptotic cells and immune complexes, which leads to the accumulation of self-antigens that trigger autoimmune responses.

C3 deficiency is the most severe complement deficiency because C3 sits at the convergence point of all three activation pathways. Individuals with C3 deficiency suffer from recurrent, severe bacterial infections, particularly with encapsulated organisms like Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis. They also have impaired antibody responses due to the loss of complement-mediated B cell co-stimulation.

Terminal pathway deficiencies (C5 through C9) specifically predispose to invasive Neisseria infections. The frequency of these deficiencies varies by population, with C9 deficiency being relatively common in Japanese populations (approximately 1 in 1,000) but rare in European populations. Factor H and factor I deficiencies cause uncontrolled alternative pathway activation, leading to consumption of C3 and secondary C3 deficiency, with susceptibility to both infections and kidney disease.

Complement in Modern Medicine

The complement system has become an important drug target. Eculizumab, the first complement-targeting therapy approved by the FDA (in 2007), blocks C5 cleavage and is used to treat PNH, aHUS, and generalized myasthenia gravis. Its successor ravulizumab (Ultomiris) has a longer half-life, allowing dosing every eight weeks instead of every two. These drugs cost approximately $500,000 per year per patient, making them among the most expensive therapies in medicine, but they are transformative for patients with these previously untreatable conditions.

Newer complement-targeting drugs aim at upstream components. Pegcetacoplan (Empaveli), a C3 inhibitor, was approved in 2021 for PNH and provides broader complement blockade than anti-C5 antibodies. Factor B inhibitors, factor D inhibitors, and inhibitors of the lectin pathway protease MASP-2 are in clinical trials for conditions ranging from geographic atrophy (the dry form of age-related macular degeneration) to IgA nephropathy to COVID-19-associated complement activation. The growing pipeline of complement therapeutics reflects the recognition that excessive or misdirected complement activation contributes to a wide range of diseases beyond the rare genetic deficiencies.

Research continues to reveal new roles for complement in contexts far removed from infection. Complement proteins participate in synaptic pruning during brain development, with C1q and C3 tagging excess synapses for elimination by microglia. Dysregulated complement-mediated pruning has been implicated in schizophrenia and Alzheimer's disease. Complement also influences tissue regeneration, organ transplant rejection, and the tumor microenvironment, where complement activation can either promote or suppress anti-tumor immunity depending on the context.