Residency · Residency · Allergy Immunology

Complement System - Pathways and Deficiencies

Overview

The complement system comprises more than 50 plasma and membrane-bound proteins that collectively constitute approximately 15 percent of the serum globulin fraction. This ancient proteolytic cascade system serves as a critical effector arm of both innate and adaptive immunity, performing five principal functions: opsonization of pathogens for enhanced phagocytosis, chemotactic recruitment of inflammatory cells, direct lysis of target cells through membrane attack complex formation, clearance of immune complexes and apoptotic cells, and bridging of innate and adaptive immunity through enhancement of B cell responses. The majority of complement proteins are synthesized by hepatocytes in the liver, with the notable exception of C1q, which is produced primarily by tissue-resident macrophages and dendritic cells. The complement nomenclature system designates numbered components (C1 through C9), factors (B, D, H, I, P/properdin), and various regulatory proteins.

Classical Pathway

Activation

The classical pathway is initiated by the binding of C1q to antigen-antibody complexes, representing the most direct link between adaptive humoral immunity and the complement effector system. C1q recognizes the Fc region of antibodies that have engaged antigen, with the efficiency of complement activation varying significantly by immunoglobulin class and subclass. A single pentameric IgM molecule provides sufficient C1q binding sites for activation, whereas IgG requires at least two molecules in close proximity (a "doublet") on the target surface to create a platform for C1q engagement. Among IgG subclasses, complement-fixing efficiency follows the hierarchy IgG3 > IgG1 >> IgG2, while IgG4 does not activate the classical pathway at all.

The C1 complex consists of C1q associated with two molecules each of the serine proteases C1r and C1s, assembled in a calcium-dependent manner. Upon C1q binding to immune complexes, conformational changes activate C1r, which in turn cleaves and activates C1s. Activated C1s cleaves C4 into C4a and C4b, and subsequently cleaves C2 into C2a and C2b. C4b and C2a associate on the target surface to form the classical pathway C3 convertase (C4b2a), which cleaves C3 into C3a and C3b. The addition of C3b to the C4b2a complex generates the classical pathway C5 convertase (C4b2a3b), which initiates the terminal pathway.

Clinical Correlations of Classical Pathway Deficiency

Deficiencies of classical pathway components are strongly associated with the development of systemic lupus erythematosus-like autoimmune disease, reflecting the critical role of this pathway in clearing apoptotic cells and immune complexes. | Complement Deficiency | Prevalence | Primary Clinical Association | Notes |

C1q deficiency carries the strongest genetic association with SLE of any single gene, with greater than 90 percent of homozygous C1q-deficient individuals developing lupus-like disease. The pathogenesis is attributed to impaired clearance of apoptotic cells, which become a source of self-antigens that drive autoimmune responses, and impaired solubilization and clearance of immune complexes that deposit in tissues and promote inflammation.

C1r and C1s deficiencies similarly predispose to SLE-like disease and have additionally been associated with Ehlers-Danlos-like connective tissue features. C4 deficiency is strongly associated with SLE, with C4A null alleles (C4A*Q0) carrying a stronger lupus association than C4B null alleles, reflecting the differential functional properties of these isotypes: C4A preferentially binds amino groups and is more important for immune complex clearance, while C4B preferentially binds hydroxyl groups on pathogen surfaces. Low C4 gene copy number (fewer than four copies total) independently increases SLE risk in population studies.

C2 deficiency is the most common hereditary complement deficiency in Caucasian populations, with an estimated prevalence of approximately 1:20,000. Approximately 30 percent of C2-deficient individuals develop SLE-like disease, while the remainder may present with recurrent pyogenic infections, reflecting the role of C2 in generating the C3 convertase necessary for opsonization.

Lectin Pathway

Activation

The lectin pathway is activated when mannose-binding lectin (MBL) or ficolin proteins bind to specific carbohydrate patterns on pathogen surfaces. MBL is a collectin that recognizes mannose, N-acetylglucosamine, and fucose residues arranged in the specific spatial patterns characteristic of microbial surfaces but absent on mammalian glycoproteins. MBL-associated serine proteases (MASP-1, MASP-2, and MASP-3) are the lectin pathway equivalents of C1r and C1s. MASP-2 is the principal effector protease, cleaving C4 and C2 in a manner functionally analogous to C1s, and generating the same C3 convertase (C4b2a) as the classical pathway.

MBL Deficiency

MBL deficiency is remarkably common, with approximately 5 to 10 percent of the population having low functional MBL levels and approximately 25 percent carrying variant alleles that reduce MBL production or function. In immunocompetent adults, MBL deficiency is usually clinically silent, as redundant complement activation through classical and alternative pathways provides adequate protection. However, MBL deficiency may increase susceptibility to infections in neonates, whose adaptive immune system is still immature, and in immunocompromised patients, such as those receiving chemotherapy, where other immune defenses are concurrently impaired. In otherwise healthy individuals, MBL deficiency alone rarely causes significant clinical disease.

<image>A comprehensive diagram showing all three complement activation pathways (classical, lectin, alternative) converging on C3 convertase and proceeding to the membrane attack complex. Classical pathway: C1q binding IgM/IgG immune complexes, activating C1r/C1s, cleaving C4 and C2 to form C4b2a. Lectin pathway: MBL binding mannose residues on bacterial surface, activating MASP-1/2, forming C4b2a. Alternative pathway: spontaneous C3 hydrolysis (tick-over), Factor B binding, Factor D cleavage forming C3bBb, stabilized by properdin. All three converge on C3 cleavage, then C5 convertase formation, then the terminal pathway (C5b-C6-C7-C8-C9n forming MAC). Key regulatory proteins shown at their sites of action: C1-INH, C4BP, Factor H, Factor I, DAF (CD55), MCP (CD46), CD59. Use distinct colors for each pathway.</image>

Alternative Pathway

Activation

The alternative pathway is unique among complement activation pathways in that it is constitutively active at a low level through a process called C3 tick-over. Spontaneous hydrolysis of the internal thioester bond in C3 generates C3(H2O), a conformationally altered form of C3 that can bind Factor B. Factor D, a serine protease that circulates in its active form, then cleaves Factor B into Ba and Bb, generating the fluid-phase C3 convertase C3(H2O)Bb. This convertase cleaves native C3 to generate C3b, which deposits on nearby surfaces. Surface-bound C3b recruits Factor B, which is again cleaved by Factor D to form the surface-bound alternative pathway C3 convertase C3bBb. This convertase has a short half-life of approximately 90 seconds unless stabilized by properdin (Factor P), which extends its activity and amplifies complement activation.

The alternative pathway functions as a powerful amplification loop for complement activation initiated by any of the three pathways. C3b generated through classical or lectin pathway activation can seed the alternative pathway amplification loop, dramatically increasing the total amount of C3b deposited on the target surface. This amplification mechanism accounts for the majority of total C3 activation in most complement-mediated responses.

Regulation

Because the alternative pathway is constitutively active, tight regulation is essential to prevent complement-mediated damage to host tissues.

Factor H is a key regulator that binds C3b on host cell surfaces, where it recognizes sialic acid and glycosaminoglycans as markers of self, and serves as a cofactor for Factor I-mediated cleavage of C3b to the inactive fragment iC3b. Decay-accelerating factor (DAF/CD55) is a GPI-anchored membrane protein that accelerates the dissociation of C3 and C5 convertases on host cell surfaces. Membrane cofactor protein (MCP/CD46) is a transmembrane protein that serves as a cofactor for Factor I-mediated cleavage of both C3b and C4b on the cell surface.

Dysregulation of the alternative pathway has major clinical consequences. Mutations in Factor H, or the development of autoantibodies against Factor H, are the most common cause of atypical hemolytic uremic syndrome (aHUS), a thrombotic microangiopathy driven by uncontrolled complement activation on endothelial surfaces. Properdin (Factor P) deficiency, the only X-linked complement deficiency, confers susceptibility to fulminant meningococcal disease due to loss of the stabilizing factor for the alternative pathway C3 convertase.

Terminal Pathway and Membrane Attack Complex (MAC)

MAC Formation

The terminal pathway is initiated when the C5 convertase cleaves C5 into C5a, a potent anaphylatoxin and chemotactic factor, and C5b, which initiates MAC assembly. C5b sequentially binds C6, C7, and C8, and the C5b-8 complex then recruits multiple C9 molecules, which polymerize to form the completed MAC pore (C5b-9). This pore creates a transmembrane channel with an inner diameter of approximately 10 nm, sufficient to disrupt the osmotic integrity of the target cell and cause lysis. At sublytic densities, MAC insertion can activate endothelial cells and platelets without causing lysis, contributing to inflammatory signaling and coagulation activation.

CD59 (Protectin)

CD59 is a GPI-anchored regulatory protein present on host cell membranes that prevents the final step of MAC assembly by blocking C9 polymerization. In paroxysmal nocturnal hemoglobinuria (PNH), a somatic mutation in the PIGA gene (which is required for GPI anchor synthesis) results in the absence of GPI-anchored proteins, including both CD59 and DAF (CD55), from the surface of affected hematopoietic cells. The loss of these complement regulatory proteins renders red blood cells exquisitely susceptible to complement-mediated intravascular hemolysis. PNH also carries a significant risk of thrombosis, which is the leading cause of morbidity and mortality. Treatment with eculizumab or ravulizumab (anti-C5 antibodies) prevents terminal pathway activation and dramatically reduces hemolysis. Pegcetacoplan, an anti-C3 agent, provides upstream complement blockade for patients with residual hemolysis on anti-C5 therapy.

Terminal Pathway Deficiencies

Deficiency of any terminal pathway component (C5, C6, C7, C8, or C9) results in a profoundly increased risk of Neisseria infections, both meningococcal and gonococcal. The risk of meningococcal disease is increased 5,000 to 10,000-fold compared to the general population, reflecting the unique dependence of anti-Neisseria defense on complement-mediated bactericidal activity. C9 deficiency is the most common terminal complement deficiency globally, particularly in Japan, and is often asymptomatic because the C5b-8 complex can form rudimentary pores without C9 polymerization, though these pores are less efficient. All patients with terminal complement deficiency should receive both meningococcal conjugate vaccine (MenACWY) and serogroup B meningococcal vaccine (MenB).

Complement Anaphylatoxins

C3a, C4a, C5a

The small cleavage fragments generated during complement activation, C3a, C4a, and C5a, function as anaphylatoxins with potent inflammatory activities. C5a is the most potent, functioning both as the strongest chemotactic factor for neutrophils (acting through the C5aR1/CD88 receptor) and as a powerful activator of neutrophils, monocytes, and mast cells. C3a binds the C3a receptor expressed on eosinophils and mast cells and contributes to mast cell activation and modulation of adaptive immune responses. C4a is the weakest anaphylatoxin and may not have a dedicated receptor. All three anaphylatoxins are rapidly inactivated by serum carboxypeptidase N, which removes the C-terminal arginine residue to generate the des-Arg forms, which have substantially reduced biological activity.

Complement Regulatory Proteins and Disease

C1 Esterase Inhibitor (C1-INH, SERPING1)

C1 esterase inhibitor is a serine protease inhibitor (serpin) with broad specificity. Within the complement system, it inhibits the activated serine proteases C1r, C1s, MASP-1, and MASP-2, thereby regulating both classical and lectin pathway activation. Critically, C1-INH also inhibits key proteases of the contact activation (kinin) system, including kallikrein, Factor XIIa, and Factor XIa. This dual regulatory function explains why C1-INH deficiency causes hereditary angioedema (HAE) through uncontrolled bradykinin generation rather than through complement-mediated pathology. HAE is discussed in detail in lecture 16. Type I HAE (85 percent of cases) is characterized by low C1-INH levels and function, while Type II HAE (15 percent) features normal or elevated C1-INH protein levels with reduced functional activity due to mutations in the reactive center loop.

Complement in Kidney Disease

Dysregulated complement activation is increasingly recognized as a driver of kidney disease. C3 glomerulopathy (C3G) encompasses dense deposit disease (DDD) and C3 glomerulonephritis (C3GN), both characterized by uncontrolled alternative pathway activation on the glomerular basement membrane with C3 deposition in the absence of significant immunoglobulin. C3 nephritic factor, an autoantibody that stabilizes the alternative pathway C3 convertase and prevents its natural decay, is a common finding in C3G.

Atypical hemolytic uremic syndrome (aHUS) is a thrombotic microangiopathy driven by complement dysregulation on endothelial surfaces. Causative mutations have been identified in Factor H, Factor I, MCP, C3, Factor B, and thrombomodulin (THBD), and autoantibodies against Factor H can cause an acquired form. Treatment with eculizumab, which blocks C5 cleavage and prevents both MAC formation and C5a generation, has transformed the prognosis of aHUS.

<image>A clinical algorithm for evaluating suspected complement deficiency. Starting point: "Recurrent infections or autoimmune disease suggestive of complement deficiency." First branch: measure CH50 (total hemolytic complement) and AH50 (alternative pathway hemolytic activity). If CH50 low/absent and AH50 normal: classical/lectin pathway defect - measure C1q, C4, C2, MBL, MASP-2. If AH50 low/absent and CH50 normal: alternative pathway defect - measure Factor B, Factor D, Properdin. If both CH50 and AH50 low: terminal pathway defect (C3, C5-C9) or common component (C3). If both normal: consider functional defects or intermittent consumption. Include specific disease associations at each deficient component (SLE for C1q/C4/C2, Neisseria for C5-C9, aHUS for Factor H/I).</image>

Complement in Allergic Disease

Complement activation contributes to allergic inflammation through several mechanisms. The anaphylatoxins C3a and C5a can directly activate mast cells and amplify degranulation during anaphylaxis, creating a positive feedback loop that enhances the severity of the allergic response. Elevated C3a and C5a levels have been documented during allergen-provoked bronchoconstriction, and complement activation has been implicated as a contributor to asthma severity independent of IgE-mediated mechanisms. These observations have raised interest in complement-targeted therapies as potential adjuncts in severe allergic disease, although this remains an area of active investigation.

Therapeutic Complement Inhibition

Approved Agents

The last decade has witnessed a remarkable expansion of complement-targeted therapies, reflecting the growing appreciation of complement dysregulation in diverse diseases. Eculizumab (Soliris) is a humanized monoclonal antibody that binds C5 and prevents its cleavage into C5a and C5b, thereby blocking both anaphylatoxin generation and MAC formation. It is approved for PNH, aHUS, generalized myasthenia gravis (gMG), and neuromyelitis optica spectrum disorder (NMOSD). Because C5 blockade eliminates MAC-dependent bactericidal activity, patients must receive meningococcal vaccination at least 2 weeks before initiating therapy. Dosing is 900 mg IV every 2 weeks after a loading phase. Ravulizumab (Ultomiris) is a long-acting anti-C5 antibody engineered for extended half-life, permitting dosing every 8 weeks.

Pegcetacoplan (Empaveli) is a pegylated compstatin analog that inhibits C3, providing more proximal complement blockade than anti-C5 agents. It is administered subcutaneously and is approved for PNH, particularly for patients with residual hemolysis on anti-C5 therapy due to ongoing C3-mediated extravascular hemolysis. Iptacopan (Fabhalta) is an oral Factor B inhibitor approved for PNH, representing the first oral complement inhibitor for this disease. Sutimlimab (Enjaymo) is an anti-C1s monoclonal antibody approved for cold agglutinin disease, where it blocks the classical pathway activation that drives complement-mediated hemolytic anemia. Avacopan (Tavneos) is an oral C5a receptor (C5aR1) antagonist approved as adjunctive therapy for ANCA-associated vasculitis, where C5a-mediated neutrophil activation drives vascular injury.

<image>A timeline illustration showing the evolution of complement-targeted therapies. A horizontal complement cascade is shown at top (C1-C2-C4-C3-C5-C6-C9/MAC). Below, each approved drug is placed at its point of inhibition along the cascade: sutimlimab at C1s, pegcetacoplan at C3, iptacopan at Factor B, eculizumab/ravulizumab at C5, avacopan at C5aR1. Each drug box includes: generic name, brand name, route of administration, approved indications, and year of FDA approval. Color-coded by mechanism (monoclonal antibody vs small molecule vs peptide).</image>

Key Clinical Pearls

• CH50 = 0 suggests complete deficiency of a classical/terminal pathway component; AH50 screens the alternative pathway
• C1q deficiency has the highest penetrance for SLE among all complement deficiencies (>90%)
• C2 deficiency is the most common hereditary complement deficiency in Caucasians
• Terminal complement deficiency (C5-C9) increases Neisseria infection risk 5,000-10,000-fold; vaccinate with MenACWY + MenB
• C3 is at the convergence of all three pathways; C3 deficiency causes severe recurrent pyogenic infections and immune complex disease
• Eculizumab blocks C5 cleavage, preventing MAC formation and C5a generation; carries meningococcal risk requiring vaccination
• Low C4 with normal C3 in the setting of angioedema without urticaria should prompt evaluation for C1-INH deficiency (HAE)

References

1. Merle NS, Church SE, Fremeaux-Bacchi V, Roumenina LT. Complement system part I - molecular mechanisms of activation and regulation. Front Immunol. 2015;6:262.
2. Ricklin D, Hajishengallis G, Yang K, Lambris JD. Complement: a key system for immune surveillance and homeostasis. Nat Immunol. 2010;11(9):785-797.
3. Brodszki N, et al. European Society for Immunodeficiencies (ESID) and European Reference Network on Rare Primary Immunodeficiency, Autoinflammatory and Autoimmune Diseases (ERN RITA) complement guideline. J Clin Immunol. 2020;40(4):576-591.
4. Mastellos DC, Ricklin D, Lambris JD. Clinical promise of next-generation complement therapeutics. Nat Rev Drug Discov. 2019;18(9):707-729.
5. Skattum L, van Deuren M, de Groot PG, Truedsson L. Complement deficiency states and associated infections. Mol Immunol. 2011;48(14):1643-1655.

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