Key Complement System Proteins

Why This Matters

The complement system is one of the most elegant examples of a proteolytic cascade in immunology: a molecular domino effect where each activated protein triggers the next, amplifying the immune response exponentially. You're being tested on how this system bridges innate and adaptive immunity, generates inflammation, enables opsonization, and directly kills pathogens through membrane attack.

Don't just memorize protein names and numbers. Know which pathway each protein belongs to, what enzymatic reaction it catalyzes or undergoes, and what effector function it contributes to. Exams love asking you to trace a pathway from initiation to lysis, compare how different pathways converge, or explain what happens when specific components are deficient. Master the logic of the cascade, and the individual proteins will make sense.

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Pathway Initiators: Recognition Molecules

These proteins detect "danger" and kick off complement activation. Each pathway uses a different recognition strategy: antibody binding, lectin-carbohydrate interactions, or spontaneous C3 hydrolysis. All three converge on the same downstream effectors.

C1 Complex (C1q, C1r, C1s)

• Initiates the classical pathway by binding to antibody-antigen complexes on pathogen surfaces. This is the main link between adaptive immunity and complement.
• C1q is the recognition subunit, binding the Fc regions of IgG or IgM. C1r and C1s are serine proteases that activate sequentially once C1q engages its target. C1q binding triggers C1r autoactivation, and active C1r then cleaves and activates C1s.
• C1s cleaves C4 and C2 to generate the classical pathway C3 convertase ($C4b2a$), launching the cascade.

Mannose-Binding Lectin (MBL)

• Pattern recognition receptor that binds mannose and other carbohydrate arrays commonly found on bacterial and fungal surfaces but rare on healthy host cells.
• Activates the lectin pathway by associating with MBL-associated serine proteases (MASP-1 and MASP-2), which function analogously to C1r/C1s. MASP-2 is the key enzyme that cleaves C4 and C2.
• No antibodies required, making MBL critical for early defense before adaptive responses develop.

Ficolins

• Recognize acetylated carbohydrates (like $N$-acetylglucosamine) on pathogen surfaces. This is a different carbohydrate pattern than what MBL targets, broadening the range of pathogens detected.
• Activate the lectin pathway through the same MASP-dependent mechanism as MBL.
• Provide redundancy in innate recognition. Multiple lectins ensure pathogens can't easily evade complement activation by modifying a single surface sugar.

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Alternative Pathway Amplifiers

The alternative pathway doesn't wait for recognition. It's always "ticking" at low levels through spontaneous C3 hydrolysis (called C3 tickover). These proteins amplify that baseline activity specifically on pathogen surfaces, creating a powerful positive feedback loop.

Factor B

• Binds to surface-bound C3b to form the proenzyme complex $C3bB$, which is then cleaved by Factor D.
• The Bb fragment remains attached to C3b, forming the alternative pathway C3 convertase ($C3bBb$).
• Drives the amplification loop: each $C3bBb$ generates more C3b, which recruits more Factor B, exponentially increasing opsonin deposition. This loop also amplifies C3b generated by the classical and lectin pathways, making the alternative pathway the major amplifier of all complement activity.

Factor D

• Serine protease that cleaves Factor B only when B is already bound to C3b. This specificity prevents uncontrolled activation in the fluid phase.
• Rate-limiting enzyme of the alternative pathway, present in plasma at very low concentrations.
• Without Factor D, the alternative pathway cannot generate its C3 convertase.

Properdin (Factor P)

• Stabilizes the $C3bBb$ convertase by binding to it and extending its half-life on pathogen surfaces (from about 90 seconds to roughly 30 minutes).
• The only known positive regulator of complement. Most regulatory proteins inhibit the cascade, but properdin enhances it.
• Binds preferentially to microbial surfaces, helping ensure amplification happens where it's needed rather than on host cells.

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The Central Hub: C3 and Its Convertases

C3 is where all three pathways converge. Its cleavage into C3a and C3b is the single most important event in complement activation. It generates opsonins, anaphylatoxins, and the platform for MAC assembly.

C3

• Central component of all three pathways. Classical, lectin, and alternative pathways all generate C3 convertases whose job is to cleave C3.
• C3b is the major opsonin. Upon cleavage, C3b exposes a reactive thioester bond that covalently attaches to hydroxyl or amine groups on pathogen surfaces. This marks them for phagocytosis via complement receptors (especially CR1 on macrophages and neutrophils).
• C3b also forms C5 convertase by joining existing C3 convertases ($C4b2a3b$ or $C3bBb3b$), advancing the cascade toward lysis.
• C3 is the most abundant complement protein in serum (~1.2 mg/mL), reflecting its central role.

C4

• Cleaved by C1s or MASPs into C4a (weak anaphylatoxin) and C4b (opsonin that binds covalently to surfaces via a thioester, similar to C3b).
• C4b combines with C2a to form the classical/lectin pathway C3 convertase ($C4b2a$).
• Deficiency is linked to autoimmunity. Without C4, immune complexes aren't cleared properly, increasing risk of systemic lupus erythematosus (SLE). This is one of the strongest single-gene associations with lupus.

C2

• Cleaved by C1s or MASPs into C2a and C2b. C2a is the enzymatic (protease) subunit that cleaves C3.
• Forms the C3 convertase $C4b2a$ with C4b. This complex is essential for classical and lectin pathway progression.
• The amount of C3 convertase generated determines how much C3b is deposited, making this an amplification checkpoint.

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Terminal Pathway: The Membrane Attack Complex

Once C5 is cleaved, the cascade shifts from enzymatic cleavage to sequential protein assembly. C5b through C9 form a physical pore in the target membrane with no further enzymatic steps required.

C5

• Cleaved by C5 convertase ($C4b2a3b$ or $C3bBb3b$) into C5a (potent anaphylatoxin) and C5b (initiates MAC assembly).
• C5a is the strongest chemotactic factor in the complement system, recruiting neutrophils and macrophages to infection sites. It also activates these cells, priming them for enhanced killing.
• C5b is unstable and must quickly bind C6 or it loses activity. This prevents MAC formation at sites distant from the target surface.

C6

• Binds to C5b immediately after cleavage, forming the stable $C5b6$ complex.
• Keeps C5b active and creates a platform for C7 recruitment.
• No enzymatic activity. C6 is purely structural, stabilizing the growing MAC complex.

C7

• Binds to $C5b6$ and undergoes a conformational change that exposes a hydrophobic domain.
• Inserts into the target membrane. This is the first component that actually penetrates the lipid bilayer.
• Once C7 inserts, the complex is anchored and cannot dissociate. This commits the MAC to that specific membrane.

C8

• Binds to membrane-inserted $C5b67$ and penetrates deeper into the bilayer.
• Initiates pore formation by creating a small channel that begins to disrupt membrane integrity.
• Recruits and catalyzes C9 polymerization. Without C8, C9 cannot assemble into the final lytic pore.

C9

• Polymerizes to form the complete MAC pore. Up to 18 C9 molecules assemble into a ring structure resembling perforin (the pore-forming protein used by cytotoxic T cells and NK cells).
• Creates a transmembrane channel approximately 10 nm in diameter that allows free flow of ions and water.
• Causes osmotic lysis. The target cell cannot maintain ion gradients, swells with water, and bursts. This is particularly effective against gram-negative bacteria, whose outer membrane is vulnerable to MAC insertion.

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Inflammatory Mediators: Anaphylatoxins

These small cleavage fragments don't participate in opsonization or lysis. Instead, they diffuse away from the activation site and orchestrate the inflammatory response by signaling to immune cells and blood vessels.

C3a and C5a (Anaphylatoxins)

• Released during C3 and C5 cleavage. Their production is a direct indicator of active complement activation at an infection site.
• C3a increases vascular permeability and causes smooth muscle contraction. C5a does all of this plus powerful chemotaxis for neutrophils, monocytes, and eosinophils.
• Both bind to G protein-coupled receptors (C3aR and C5aR1) on mast cells, neutrophils, and macrophages, triggering degranulation, respiratory burst, and directed migration toward the site of activation.
• Anaphylatoxins are rapidly inactivated by carboxypeptidase N, which removes the C-terminal arginine. This limits their range of action and prevents systemic inflammatory damage.

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Quick Reference Table

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Self-Check Questions

1. Which two proteins serve analogous recognition functions in different pathways, and what distinguishes the molecules they recognize?

2. Trace the alternative pathway amplification loop: starting with spontaneous C3 hydrolysis, which proteins are required to generate more C3b, and what stabilizes the convertase?

3. Compare and contrast C3a and C5a. What inflammatory functions do they share, and what makes C5a the more potent mediator?

4. A patient has a genetic deficiency in C3. Explain why this affects all three complement pathways and which effector functions would be lost.

5. If a question asks you to explain how complement directly kills gram-negative bacteria, which proteins would you discuss and in what order are they recruited to the target membrane?