Complement System – Comprehensive Study Notes

Page 1

Faculty & Text References
• Lecturer: Bidyut Mohanty, PhD – VCOM–Carolinas (Class 5, 07 / 24 / 2025)
• Core text: Abbas & Lichtman, Basic Immunology
– Chapter 4 (pp 86-88) – Complement
– Chapter 13 (pp 294-310) – Immune‐mediated injury

Page 2 – Course Objectives

1. Define: complement, zymogen, protease.

2. Explain complement protein nomenclature ($C1$–$C9$, fragments $a$/$b$).

3. Compare the three activation pathways (classical, alternative, lectin).

4. Trace the common (late) pathway that produces the membrane attack complex (MAC).

5. Describe how complement kills microbes, esp. Gram-negative bacteria.

6. Identify complement opsonins ($C3b, C4b$) and their function.

7. Identify complement-derived chemotaxins (C5a>C3a>C4a) and their effects.

8. Describe complement-mediated clearance of immune complexes.

9. Explain the role of complement in B-cell activation (CR2–$C3d$ interaction).

10. List major physiologic control mechanisms (C1-INH, DAF/CD55, CD59, MCP, CR1, etc.).

11. Summarize inherited complement deficiencies and their clinical consequences.

12. Outline the etiopathogenesis of hereditary angioedema (HAE).

13. Discuss quantitative (e.g. $CH_{50}$) vs qualitative complement testing.

Page 3 – Context & Prior Knowledge

Already mastered: physical barriers, innate & adaptive immunity, hematopoiesis, lymphoid organs, PRRs (TLR, NLR, RLR, CLRs, DNA sensors), endocytosis variants, phagocytosis.
Today’s focus: three complement pathways, MAC, opsonization, chemotaxis, B-cell activation, HAE.

Page 4 – Position of Complement in Immunity

Innate immunity arm = phagocytes + complement.
Complement = heat-labile serum & membrane proteins that cooperate with antibodies and innate cells to eliminate microbes.

Page 5 – Core Definitions

• Complement: Plasma protein cascade that
1) opsonizes microbes, 2) recruits inflammatory cells, 3) directly lyses microbes.
• Nomenclature: $C1$–$C9$ → fragments $a$ (small, soluble, inflammatory) + $b$ (larger, binds cell). Example: $C3\xrightarrow{protease} C3a + C3b$.
• Zymogen / Pro-enzyme: inactive precursor activated by cleavage.
• Protease: enzyme that cleaves proteins; complement proteases sequentially activate downstream zymogens.
• Opsonin: molecule (e.g. $C3b, C4b, IgG, IgA$) that coats a target to enhance phagocytosis.

Page 6 – Three Phases of Complement Action

A. Activation (trigger): microbial surfaces, bound antibodies, or lectins.
B. Covalent Attachment: active fragments (esp. $C3b$) bind targets via exposed thioester.
C. Regulation: host cells express inhibitory proteins that terminate activation; microbes lack these.

Page 7 – Big Picture Pathway Map

Early (pathway-specific) steps create $C3$ convertases → generate $C3b$.
Late common steps: convert $C5$ → $C5b$ → MAC ($C5b$ + $C6$ + $C7$ + $C8$ + poly-$C9$).
Effector fragments:
• $C3a, C5a$ = inflammation; $C3b$ = opsonization; MAC = lysis.

Page 8 – Overview of the Three Pathways

• Alternative: purely innate, antibody-independent; triggered by spontaneous $C3$ hydrolysis $[C3 \rightarrow C3(H_2O)]$ or $C3b$ binding to microbial surface.
• Classical: complement of adaptive immunity; requires IgG/IgM bound to antigen.
• Lectin: innate; MBL or ficolin binds mannose/N-acetylglucosamine on microbes; MASP proteases mimic $C1r/C1s$.

Page 9 – Initiation & Transition to Late Steps

Trigger → pathway-specific $C3$ convertase
→ deposits $C3b$
→ $C3b$ joins convertase to form $C5$ convertase
→ late steps are identical in all pathways (MAC formation).

Page 10 – Alternative Pathway Details

1. “Tick-over” reaction: $C3\rightarrow C3a + C3b$ (low level).

2. If $C3b$ lands on microbe, it binds Factor B.
   $C3b + B \xrightarrow{D} C3bB \xrightarrow{D} C3bBb$ (active $C3$ convertase).
   Properdin ($P$) stabilizes $C3bBb$.

3. $C3bBb + C3b \rightarrow C3bBbC3b$ = $C5$ convertase.

4. $C5b$ seeds the MAC.

Page 11 – Internal Thioester Chemistry

Proteolysis exposes thioester in $C3b$; nucleophilic attack by –OH or –NH₂ on microbial surface → covalent linkage.

Page 12–14 – Classical Pathway & $C1$ Complex

Structure: $C1 = C1q + 2C1r + 2C1s$.
• C1q: six-armed collagen-like stalk + globular heads that bind Fc of IgM/IgG.
• Two or more Fc regions must be in proximity:
– Soluble monomeric IgG cannot activate.
– Surface-bound IgG (clustered) or planar IgM (after Ag binding) permits engagement.
Sequence: $C1q$ binding → autoactivation of $C1r$ → activation of $C1s$ (serine protease) → cleavage $C4 \rightarrow C4a + C4b$ → $C4b$ binds surface → binds $C2$, $C1s$ cleaves $C2$ to $C2a$ → $C4b2a$ = classical/lectin $C3$ convertase.

Page 15–16 – Lectin Pathway

• Pattern recognition: MBL binds terminal mannose; ficolins bind $N$-acetylglucosamine.
• Associated serine proteases MASP-1 & MASP-2 (structural analogs of $C1r/C1s$).
• MASP-2 cleaves $C4$ and $C2$ → same $C4b2a$ convertase as classical pathway.

Page 17 – Convergence Diagram (Key Convertases)

• Alternative $C3$ convertase: $C3bBb$; $C5$ convertase: $C3bBbC3b$.
• Classical/Lectin $C3$ convertase: $C4b2a$; $C5$ convertase: $C4b2a3b$.

Page 18–19 – Late Steps & MAC Assembly

1. $C5$ convertase: $C5 \rightarrow C5a + C5b$.

2. $C5b$ binds $C6$ → $C5b6$ binds membrane → recruits $C7$ (inserts) → $C8$ (forms loose pore) → multiple $C9$ molecules polymerize forming a $\text{poly-C9}$ channel ≈ 10 nm → osmotic lysis.

3. $C5a, C3a$ concurrently act as potent inflammatory mediators.

4. Thin-walled Gram-negative bacteria (e.g. Neisseria, Shigella) highly susceptible.

Page 20 – Synergy with Antibodies

Innate alone < antibodies alone < antibodies + complement + phagocytes (optimal microbe clearance).

Page 21 – Function Portfolio

Host defense: opsonization, lysis, inflammation/chemotaxis.
Waste disposal: immune complex & apoptotic debris clearance.
Immune regulation: B-cell & T-cell co-stimulation.

Page 22–23 – Opsonization Mechanics

• $C3b, C4b$ coat microbe → bind CR1, CR3 on neutrophils/macrophages.
• Resolution of electrostatic repulsion → enhanced phagocytosis (“molecular Velcro”).

Page 24–25 – MAC-Mediated Lysis

• Effective mainly for microbes with thin peptidoglycan & minimal glycocalyx.
• Illustrated example: Shigella destroyed by MAC.

Page 26–27 – Anaphylatoxins & Chemotaxis

Fragments C5a > C3a > C4a:
• Smooth-muscle contraction, ↑vascular permeability.
• Induce P-selectin / E-selectin on endothelium → leukocyte extravasation.
• Direct neutrophil chemotaxis.
• Trigger mast-cell degranulation (histamine release).
Excessive systemic release → anaphylactoid shock.

Page 28 – Additional Roles

• Immune-complex clearance: CR1 on erythrocytes binds $C3b$(Ag-Ab) → transport to liver/spleen.
• Apoptotic body removal facilitated by $C1q$.
• B-cell activation: $C3d$ (from $C3b$) ligates CR2 (CD21) + BCR → lowers activation threshold.

Page 29–32 – Regulation: RCA Proteins

Goal = prevent host damage, localize reaction.

1. C1 Inhibitor (C1-INH):
   $\text{C1-INH} + C1r<em>2s</em>2 \rightarrow$ dissociation from C1q → terminates classical pathway.

2. Decay-Accelerating Factor (DAF/CD55):
   Dissociates $C4b2a$ & $C3bBb$ on host cells (competitive displacement of C2a or Bb).

3. Membrane Cofactor Protein (MCP/CD46) & CR1: act as cofactors for Factor I–mediated cleavage of $C3b$/$C4b$.

4. CD59 (Protectin): blocks $C9$ polymerization → prevents MAC on self.
   Clinical links: mutations → atypical hemolytic uremic syndrome, dense-deposit disease, macular degeneration.

Page 33–34 – Genetic Complement Deficiencies

• C2 (most common): immune-complex diseases (e.g. SLE), recurrent infections.
• C3: severe recurrent pyogenic infections, often fatal early; immune-complex disease.
• MAC components (C5-C9): recurrent Neisseria meningitidis / gonorrhoeae infections with lower mortality than C3 deficiency.
• MBL deficiency: susceptibility to encapsulated bacteria & Saccharomyces.

Page 35–37 – Hereditary Angioedema (HAE)

Etiology: autosomal dominant C1-INH deficiency → unrestrained classical pathway, kallikrein-bradykinin system activation → episodic non-pruritic angioedema (face, extremities, GI, airway).
Clinical pearls:
• Not IgE mediated, no urticaria.
• Precipitated by stress/trauma; onset childhood, worsens at puberty.
• Airway obstruction is life-threatening.
Treatment: C1-INH concentrate, bradykinin receptor antagonists (icatibant), kallikrein inhibitors.

Page 38 – Complement Testing

Quantitative: ELISA of individual components.
Functional (qualitative):
• $CH<em>{50}$ – classical pathway integrity (serum dilution lysing 50% Ab-sensitized sheep RBC). • $AH</em>{50}$ – alternative pathway.
Low level causes: consumption (active disease) vs production deficit.
Indications: angioedema, recurrent infection, autoimmune flare.

Page 41–42 – High-Yield Review Bullets

• Opsonization: $C3b$ (±$C4b$).
• Chemotaxis & anaphylatoxin: C5a>C3a>C4a.
• MAC: $C5b-C9$ – lyses thin-walled microbes (Neisseria).
• B-cell costimulation: $C3d$ → CR2 (CD21).
• Regulation: C1-INH, DAF/CD55, CD59, MCP, Factor I.

Page 43–44 – Bare-Bones Pathway Memory Aid

Classical/Lectin: $C1$ or MBL → $C4b2a$ → $C4b2a3b$ → MAC.
Alternative: $C3bBb$ → $C3bBb3b$ → MAC.

Page 45 – Three Cardinal Complement Effects

A. $C3b$ opsonization
B. MAC cytolysis
C. Anaphylatoxin-driven inflammation

Page 46 – Pharmacologic/Pathologic Inhibition

• C1-INH: targets $C1r<em>2s</em>2$ (classical only).
• DAF/CD55 & MCP/CR1: act on $C3$ convertases (all pathways).
• CD59: blocks MAC (all pathways).

Page 47 – Summary Questions (Self-Check)

1. Name 3 pathways & their triggers.

2. Convergence point? (Answer: $C3$ cleavage)

3. End result? (MAC)

4. Main functions? Lysis, opsonization, inflammation, waste disposal, immune regulation.

5. Anaphylatoxins? $C5a,C3a,C4a$.

6. Opsonins? $C3b,C4b$.

7. Regulatory protein family? RCA/CCPs.

8. Most common deficiency? C1-INH → HAE.

Page 48–49 – Practice Question Key

SLE is strongly associated with early classical pathway deficiencies (C1q/r/s, C4, C2).
Likely deficiency: $\boxed{C1q}$.

Page 50–53 – Top 10 Takeaways

1. Complement = 20+ serum proteins acting in cascade.

2. Three initiation pathways but common terminal sequence.

3. $C3$ is central hub; $C3b$ is workhorse opsonin.

4. MAC ($C5b$–$C9$) lyses Gram-negative bacteria.

5. $C5a,C3a$ orchestrate inflammation & chemotaxis.

6. Complement bridges innate & adaptive immunity (CR2-$C3d$).

7. Host cells protected by RCA (C1-INH, DAF, CD59).

8. Early component deficiencies → immune-complex disease (SLE).

9. Terminal component deficiency → Neisseria infection.

10. C1-INH deficiency causes hereditary angioedema (bradykinin-mediated swelling).

Study smart: master triggers, convertases, major fragments, and regulatory checkpoints; correlate each deficiency with its clinical picture.