Complement System

Why complement matters

• Speed and abundance. Complement is on within seconds — proteins are pre-positioned in plasma at gram-per-liter concentrations (C3 ~1 mg/mL is among the most abundant proteins in blood, comparable to fibrinogen). No transcription, no protein synthesis, no cell trafficking required. Pathogens face it the moment they enter the bloodstream.
• Three independent triggers. Antibody-bound? Classical pathway. Microbial sugars exposed? Lectin pathway. Surface lacking host regulators? Alternative pathway. The redundant triggers mean evading one doesn't help — pathogens have evolved separate countermeasures for each.
• The amplification loop is enormous. A single initial C3b deposition seeds the alternative pathway feedback loop: C3b binds factor B, factor D cleaves it, the resulting C3bBb is itself a C3 convertase that generates more C3b. One activation event becomes thousands of C3b molecules on a target in seconds, blanketing it for phagocytosis.
• Critical for fighting Neisseria. C5-C9 deficient patients have ~10,000-fold higher rates of Neisseria meningitidis disease — the MAC is the dominant defense against this otherwise complement-evading Gram-negative. Eculizumab (anti-C5) carries a black-box warning for meningococcal sepsis.
• Disease driver when dysregulated. Atypical hemolytic uremic syndrome (factor H/I/MCP/C3 mutations), paroxysmal nocturnal hemoglobinuria (CD55/CD59 loss via GPI-anchor defect), age-related macular degeneration (factor H Y402H polymorphism), and C3 glomerulopathy all stem from complement going off the rails. The therapeutic market is rapidly growing — eculizumab, ravulizumab, pegcetacoplan, danicopan are among approved drugs.
• Bridge to adaptive immunity. C3d, the final processed C3 fragment, binds CR2/CD21 on B cells lowering the BCR activation threshold ~10-100x. Complement-tagged antigens are vastly more immunogenic, which is why some adjuvants are designed to drive complement deposition.
• Evolved before adaptive immunity. Sea urchins and other invertebrates have functional alternative pathway homologs (~600 million years old), making complement one of the oldest molecular defense systems in animals — predating both lymphocytes and antibodies.

Common misconceptions

• Complement only works with antibodies. The alternative and lectin pathways are antibody-independent. The alternative pathway runs constantly in plasma whether or not adaptive immunity has engaged — it is a true innate system.
• MAC is the most important effector. For Gram-positive bacteria the MAC is largely irrelevant — thick peptidoglycan blocks C9 insertion. Opsonization (C3b/iC3b tagging for phagocytes) and anaphylatoxin recruitment (C3a, C5a) are usually more impactful for non-Neisseria infections.
• "Complement" is one molecule. It's a cascade of about 30 distinct proteins (C1q, C1r, C1s, C2, C3, C4, C5, C6, C7, C8, C9, factor B, factor D, properdin, MBL, ficolins, MASP-1/2, factor H, factor I, C1 inhibitor, C4BP, DAF, CR1, CR2, CR3, CR4, CD59, etc.). Each has a distinct role; "complement" refers to the system collectively.
• The alternative pathway is for "alternative" cases. It's actually the dominant pathway by activation volume in vivo — the constant tickover means most C3 cleavage in healthy plasma is alternative-pathway driven and surveils for unprotected surfaces continuously.
• C3 deficiency is incompatible with life. It's not — C3-null patients exist but suffer recurrent encapsulated bacterial infections (Streptococcus pneumoniae, Haemophilus influenzae) due to lost opsonization. They survive with vaccinations and antibiotics, demonstrating that opsonization, not lysis, is the most life-critical complement function for most pathogens.
• Complement can't hurt host cells. It can and does — autoimmune hemolytic anemia, ABO-incompatible transfusion reactions, and hyperacute transplant rejection are complement-mediated. The protective regulators on host cells are not absolute, and any antibody bound to a host-cell antigen (autoimmune or transfusion mismatch) can recruit complement against self.

How the cascade unfolds

The classical pathway begins when C1q — a hexameric collagen-like molecule with six globular heads — engages two or more adjacent Fc regions of IgG (or one IgM pentamer) on a coated target. This conformational change activates C1r and C1s zymogens to cleave C4 and C2, generating the classical C3 convertase C4b2a. The lectin pathway is structurally homologous: MBL or ficolins replace C1q, MASP-1/MASP-2 replace C1r/C1s, and the same C4b2a convertase is produced. The alternative pathway works differently — C3 in plasma spontaneously hydrolyzes its internal thioester at ~1% per hour, generating C3(H2O) that mimics C3b conformationally. C3(H2O) binds factor B; factor D cleaves bound factor B into Ba and Bb; the resulting C3(H2O)Bb is a fluid-phase C3 convertase. It cleaves C3 into C3b and C3a. C3b's exposed thioester (TED domain) reacts within milliseconds with hydroxyl or amino groups on nearby surfaces — covalently labeling them. On surfaces with host regulators (factor H, DAF), the deposited C3b is rapidly inactivated; on microbes without these protections, C3b binds factor B/Bb forming C3bBb (the surface-bound alternative pathway C3 convertase), and the amplification loop begins.

All three pathways converge at C3. Once enough C3b accumulates, the C3 convertase recruits another C3b to become a C5 convertase (C3bBbC3b for alternative, C4b2aC3b for classical/lectin). C5 cleavage produces C5a (anaphylatoxin, recruits neutrophils chemotactically at femtomolar concentrations) and C5b. C5b binds C6 forming C5b6, then C7 — the conformational change exposes hydrophobic surfaces and the complex inserts into the target lipid bilayer. C8 joins, then 10-18 copies of C9 polymerize around the nucleus into a beta-barrel pore approximately 10 nm in diameter. The pore is large enough for water and ions to flood, dissipating membrane potential and osmolality, and lysing the cell. The MAC is the only complement effector that directly kills; C3b opsonization and C3a/C5a recruitment do most of the actual antibacterial work.

Throughout this cascade, regulators control where and how much activation happens. Factor H (~600 µg/mL plasma) binds host sialic acids and glycosaminoglycans, accelerates decay of the alternative C3 convertase, and serves as cofactor for factor I-mediated C3b cleavage. Decay-accelerating factor (DAF/CD55) and membrane cofactor protein (MCP/CD46) on host cells block convertase assembly. CD59 specifically blocks MAC formation by binding C8 and preventing C9 polymerization. C1 inhibitor (C1-INH, ~240 µg/mL) controls C1 and the lectin pathway proteases; its deficiency causes hereditary angioedema. The system is therefore a constant biochemical equilibrium between activation and inhibition — tipped toward activation only by signals (antibody binding, foreign sugars, missing-self) that distinguish pathogens.

Classical vs alternative vs lectin pathway

Feature	Classical	Lectin	Alternative
Trigger	Antibody-bound C1q (IgM, IgG1/3)	MBL/ficolins on microbial sugars	Spontaneous C3 tickover
Recognition molecule	C1q	MBL, M-/L-/H-ficolin, collectin-K1	None — surface-deposited C3b amplifies
Initiating proteases	C1r, C1s	MASP-1, MASP-2	Factor D (constitutive, low conc.)
C3 convertase	C4b2a	C4b2a (same as classical)	C3bBb (stabilized by properdin)
C5 convertase	C4b2aC3b	C4b2aC3b	C3bBbC3b
Adaptive dependency	Yes (needs antibody) — but natural IgM works	No — innate pattern recognition	No — runs constitutively
Speed of activation	Hours to days (needs antibody)	Minutes (innate sugar binding)	Seconds (always pre-active)
Major regulators	C1-INH, C4BP, DAF, MCP	C1-INH, C4BP, DAF, MCP	Factor H, factor I, DAF, MCP, properdin (positive)

Famous experiments and case studies

• Pfeiffer 1894 and Bordet 1895. Richard Pfeiffer observed that fresh guinea pig serum could kill Vibrio cholerae but heat-treated serum could not — the heat-labile factor became known as "alexin" then "complement." Jules Bordet refined the concept and won the 1919 Nobel for his work on complement and antibody. The system was named complement because it "complements" the action of antibody.
• Müller-Eberhard 1968 alternative pathway characterization. Hans Müller-Eberhard and colleagues at Scripps separated the antibody-independent activation pathway (initially called the "properdin pathway" after Pillemer's discovery in the 1950s) from classical activation, identifying C3, factor B, and factor D as the central players and establishing the amplification loop.
• Eculizumab 2007 FDA approval. Anti-C5 monoclonal antibody approved for paroxysmal nocturnal hemoglobinuria — the first complement-targeted drug, and proof that selective inhibition of the lytic phase could be tolerated long-term. PNH patients went from monthly transfusions to near-normal hemoglobin. Now expanded to atypical HUS, myasthenia gravis, and others.
• Hereditary angioedema and C1 inhibitor deficiency. Patients with C1-INH deficiency (about 1 in 50,000) suffer episodic uncontrolled bradykinin generation due to unrestrained complement and contact-system activation, producing recurrent edema attacks lasting days. Plasma-derived and recombinant C1-INH replacement, plus icatibant (bradykinin receptor antagonist), are FDA-approved therapies.
• Factor H Y402H and AMD. A single coding polymorphism in factor H (Tyr402His) increases risk of age-related macular degeneration ~5-fold in homozygotes — the strongest non-Mendelian genetic risk in AMD. The mechanism is impaired factor H binding to retinal extracellular matrix, allowing chronic alternative pathway activation in the macula. Multiple anti-complement drugs (pegcetacoplan, avacincaptad pegol) are now FDA-approved for geographic atrophy.