Innate Immunity
Complement Cascade
Three pathways converge on C3 → anaphylatoxins, opsonin, and the MAC pore
Plasma complement amplifies through three pathways — classical, lectin, alternative — converging on C3. C3a/C5a inflame, C3b opsonizes, and C5b-9 drills a 10-nm pore in bacterial membranes.
- Plasma C3~1.3 mg/mL (most abundant component)
- Cascade timeFull cascade in ≈2 min after trigger
- MAC poreC5b-6-7-8-(C9)₁₀₋₁₆ · ~10 nm diameter
- Best at killingGram-negative — especially Neisseria
- Host regulatorsCD55, CD46, CD59, factor H
- Anti-C5 drugEculizumab — PNH, aHUS, gMG, NMOSD
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Three pathways, one outcome
Complement is the immune system's answer to the question: how do you amplify, in seconds, a tiny initial signal into the destruction of a pathogen, before adaptive immunity has had time to wake up? The answer is an enzymatic cascade with positive feedback. Three independent triggers can light the fuse:
- Classical pathway. IgM or two adjacent IgG molecules bound to antigen recruit C1q (a tulip-shaped hexameric protein). C1r autoactivates, cleaves C1s. C1s cleaves C4 and C2 — the C4b fragment covalently binds the pathogen surface; C2a joins to form C4b2a, the classical C3 convertase.
- Lectin pathway. Mannose-binding lectin (MBL) or ficolins, both pattern-recognition molecules, bind sugar patterns on microbial surfaces (mannose, N-acetylglucosamine — sugars rare on host glycoproteins). Associated serine proteases MASP-1 and MASP-2 cleave C4 and C2 — same downstream convertase as classical.
- Alternative pathway. Always armed via spontaneous C3 hydrolysis (~1% per hour, "tickover"). C3(H2O) recruits factor B; factor D cleaves it; the resulting Bb-containing convertase deposits surface C3b. On host cells, regulators inactivate this; on microbial surfaces, regulators are absent and a positive-feedback loop deposits thousands of C3b in seconds.
All three roads lead to one place: a C3 convertase, sitting on the pathogen, cleaving C3 (~1.3 mg/mL in plasma) into C3a and C3b. Now the cascade explodes into three parallel outcomes.
What complement does once it fires
- Anaphylatoxins. C3a, C4a, and especially C5a are small peptides released by cleavage. They bind GPCRs on mast cells (histamine release), endothelium (vasodilation, permeability), and neutrophils (chemotaxis, oxidative burst priming). C5a is the most potent neutrophil chemoattractant known — pulling neutrophils across vessel walls to the infection site within minutes.
- Opsonization. C3b covalently bonds to nearby surfaces via its reactive thioester. Phagocytes — neutrophils and macrophages — express CR1 (CD35) and CR3 (CD11b/CD18) that bind C3b and its breakdown product iC3b. A C3b-coated bacterium is engulfed orders of magnitude faster than an uncoated one. Alongside IgG, C3b is the dominant opsonin in plasma.
- Membrane attack complex. Once enough C3b accumulates, a C5 convertase forms (C4b2a3b or C3bBb3b). C5 is cleaved; C5b nucleates the terminal complex with C6, C7, C8, and 10-16 copies of C9. The assembled MAC threads a transmembrane β-barrel pore ~10 nm wide. Water rushes in. Gram-negative bacteria lyse within minutes.
Worked clinical example: meningococcal sepsis in a college freshman
An 18-year-old presents to the emergency department with petechiae, hypotension, and meningismus. Blood cultures grow Neisseria meningitidis. He has had two episodes of meningococcal disease in the last three years — extremely unusual. The infectious disease physician orders a CH50 (total hemolytic complement) — it returns 0, suggesting a terminal pathway defect. Individual component testing shows undetectable C7. He carries homozygous loss-of-function mutations in C7. The pattern fits: Neisseria spp. are uniquely vulnerable to MAC lysis because of their thin cell wall and lack of complement-resistance factors. Patients with C5-C9 deficiency have a roughly 10,000-fold increased risk of invasive meningococcal disease relative to controls. He receives quadrivalent ACWY and B meningococcal vaccines and life-long pneumococcal and Hib coverage; lifelong rifampin prophylaxis or amoxicillin standby is considered. His complement system never gets stronger, but knowing the gap lets us patch around it.
Why complement matters clinically
- Bacterial sepsis. Opsonization and MAC are crucial against encapsulated and Gram-negative organisms; deficiency causes recurrent severe infections.
- SLE and immune complex disease. Classical pathway deficiency (C1q, C2, C4) causes impaired immune complex clearance and lupus-like autoimmunity.
- PNH and aHUS. Loss of host regulators (CD55/CD59 in PNH; factor H mutations in aHUS) causes uncontrolled MAC against red cells or endothelium — eculizumab transformed both diseases.
- Hereditary angioedema. C1 inhibitor deficiency lets contact activation run unchecked, producing bradykinin and recurrent angioedema; icatibant and lanadelumab are mainstays.
- Transplant rejection. Antibody-mediated rejection deposits C4d in graft vessels — a diagnostic marker visible on biopsy.
- Lab markers. Low C3/C4 indicates active consumption — SLE flare, post-infectious glomerulonephritis, sepsis. Total CH50 screens overall function.
Three pathways at a glance
| Classical | Lectin | Alternative | |
|---|---|---|---|
| Trigger | IgM/IgG-antigen complex | Mannose on microbe | Spontaneous C3 tickover |
| First sensor | C1q | MBL / ficolins | C3(H2O) + factor B |
| C3 convertase | C4b2a | C4b2a | C3bBb |
| Antibody required? | Yes | No | No |
| Amplifies itself? | Modest | Modest | Yes (positive feedback) |
| Deficiency pattern | SLE-like + pyogenic infections | Mild; pediatric infections | Severe pyogenic + Neisseria |
Common misconceptions
- Complement is one protein. About 30 components plus regulators, cascading like a clotting system.
- Only antibodies trigger it. Lectin and alternative pathways are antibody-independent — the system is innate.
- MAC kills all bacteria. Mostly Gram-negative; Gram-positive cell walls block C9 access.
- High C3 means strong immunity. C3/C4 levels are consumption markers; low values often signal active disease.
- Eculizumab is safe alone. Mandatory meningococcal vaccination — fatal Neisseria infections reported in unvaccinated patients.
- The cascade is slow. Once primed, full activation runs in about 2 minutes — innate speed.
Frequently asked questions
How do all three pathways converge on C3?
Classical and lectin pathways both produce the C4b2a complex, a C3 convertase, on the pathogen surface. The alternative pathway produces C3bBb, a different C3 convertase. Both convertases cleave plasma C3 (1.3 mg/mL, the most abundant complement protein) into C3a (anaphylatoxin, released) and C3b (opsonin, covalently bound to any nearby surface via its reactive thioester). Once C3b is deposited it can join factor B to form more C3bBb, creating an amplification loop unique to the alternative pathway — one trigger event can deposit thousands of C3b molecules within seconds. This positive feedback is what makes complement explosive.
How does the membrane attack complex actually kill?
C5 convertase (C4b2a3b or C3bBb3b) cleaves C5 into C5a and C5b. C5b binds C6 in solution, then C7 — at which point the complex inserts hydrophobic side into the lipid bilayer. C8 binds, anchoring the assembly, and recruits the first C9. C9 polymerizes — 10 to 16 copies threading through the membrane to form a transmembrane β-barrel pore about 10 nanometers in diameter. Water and ions flood in along their gradients; osmotic lysis follows within minutes for Gram-negative bacteria. Gram-positive bacteria are largely resistant because their thick peptidoglycan blocks C9 access to the membrane; enveloped viruses can also be lysed.
Why do C5-9 deficient patients get Neisseria infections?
Neisseria meningitidis and Neisseria gonorrhoeae have thin Gram-negative cell walls and do not form spores or biofilms in the bloodstream — they are uniquely vulnerable to MAC lysis. For most encapsulated pathogens, opsonization (C3b coating + phagocytosis) handles the job, so deficiency of C3 or the upstream cascade causes diverse pyogenic infections. For Neisseria, MAC is the dominant defense, so terminal deficiency (C5, C6, C7, C8, or C9) yields a 10,000-fold increased risk of meningococcal disease. This is why all patients on eculizumab (anti-C5) must be vaccinated against meningococcus.
How do host cells avoid being attacked?
Multiple regulators on every host cell. CD55 (DAF, decay-accelerating factor) dissociates C3 convertases. CD46 (MCP, membrane cofactor protein) is a cofactor for factor I, which inactivates surface-bound C3b. CD59 prevents C9 polymerization, blocking MAC assembly even when upstream cascade fires. Factor H in plasma binds host sialic acids and accelerates C3b decay. Microbes do not present these self-markers, so the cascade runs to completion on them. Loss of these regulators causes disease — PNH (paroxysmal nocturnal hemoglobinuria) is GPI-anchor deficiency that strips CD55 and CD59 from red cells, causing chronic intravascular hemolysis.
What are C3a and C5a doing in real time?
Anaphylatoxins — small peptides cleaved off during cascade. They bind G-protein-coupled receptors (C3aR, C5aR1/CD88, C5aR2). C3a triggers mast cell histamine release and vasodilation. C5a is the strongest neutrophil chemotactic factor known — pulls neutrophils across endothelium toward the site, primes their respiratory burst, and increases vascular permeability. C5a peak concentration at the infection site can reach micromolar levels within minutes. In sepsis, excess systemic anaphylatoxins contribute to vasodilatory shock and DIC; in trauma they drive ARDS. C5a inhibitors (vilobelimab) are in trial for severe COVID-19 and ANCA-associated vasculitis.
What is the alternative pathway tickover?
C3 has a buried thioester bond in its core. Spontaneously, about 1% of circulating C3 per hour hydrolyzes to C3(H2O), which can act as a C3b mimic and recruit factor B. Factor D cleaves factor B; C3(H2O)Bb is a fluid-phase C3 convertase that generates the first surface C3b. On host cells, regulators immediately inactivate any deposited C3b. On microbial surfaces lacking these regulators, the C3b remains and amplifies — recruiting more factor B, generating more convertase, depositing more C3b. The tickover plus surface amplification means the alternative pathway is always armed, ready to discriminate self from non-self by regulator presence alone. No antibody required.
How is complement targeted therapeutically?
Eculizumab (Soliris) — humanized anti-C5 antibody, blocks cleavage to C5a and C5b. Approved for PNH, atypical HUS, generalized myasthenia gravis, NMOSD. Ravulizumab is a longer-acting C5 inhibitor with 8-week dosing. Pegcetacoplan is a C3 inhibitor, suppressing both intra- and extravascular hemolysis in PNH. Iptacopan targets factor B, blocking the alternative pathway. Sutimlimab blocks classical pathway C1s for cold agglutinin disease. Avacopan blocks C5aR1 for ANCA-associated vasculitis. All terminal-pathway inhibitors require meningococcal vaccination. Annual costs of eculizumab historically reached USD 500,000 — newer entrants are reshaping pricing.