Complement System Study Notes

Complement system: overview
- A set of heat-labile plasma proteins that act in a cascade to enhance microbial killing.
- Produced in the liver.
- Components include C1, C2, C3, …, C9 and many more.
- Acts as a complement to the immune system to defend against microbes.
- Clinical relevance: testing for presence of complement proteins in a patient typically involves blood samples.
- During inflammation, complement can access tissues and act on microbes in both the blood and tissues.
Major effector outcomes of complement
- Opsonization: coating of microbes to enhance phagocytosis.
- Direct lysis via the Membrane Attack Complex (MAC).
- Proinflammatory effects: release of anaphylatoxins (C3a, C5a) that recruit immune cells.
- Overall outcomes include opsonization, inflammation, and cytolysis.
Three activation pathways (overview)
- All three pathways converge on a common goal: deposition of C3b and generation of downstream effector molecules.
- Activation pathways: Classical (antibody-mediated), Alternative (spontaneous hydrolysis on microbial surfaces), Lectin (MBL/ficolins binding to pathogen carbohydrates).
- Each pathway yields a C3 convertase, which cleaves C3 into C3a and C3b, leading to formation of C5 convertases and MAC in downstream steps.
Central significance of C3 and C5 cleavage
- $ext{C3} ightarrow ext{C3a} + ext{C3b}$
- C3b is the central opsonin deposited on the pathogen surface; C3a is inflammatory.
- $ext{C5} ightarrow ext{C5a} + ext{C5b}$
- C5a is a potent inflammatory mediator and neutrophil chemotactic factor; C5b initiates MAC assembly.
- All pathways converge at C3b deposition on the pathogen surface.
Three activation pathways in detail
- ## Classical Pathway
- Triggered by antibody binding to microbes; antibodies can fix complement.
- First complement protein to bind: C1q (C1qr2s2).
- Activation complex: C1q activates C1r and C1s; C1s cleaves C4 and C2.
- Formation of the C3 convertase: $ext{C4b} + ext{C2a} ightarrow ext{C4b2a} \,( ext{C3 convertase})$
- C3 cleavage and deposition: $ext{C3} ightarrow ext{C3a} + ext{C3b}; ext{C3b covalently binds to microbe surface}$
- Formation of the C5 convertase: $ext{C4b2a} + ext{C3b} ightarrow ext{C4b2a3b} \,( ext{C5 convertase})$
- C5 cleavage: $ext{C5} ightarrow ext{C5a} + ext{C5b}$
- MAC assembly begins with C5b as the initiator.
- ## Alternative Pathway
- Triggered by spontaneous hydrolysis of C3 on all surfaces (host and microbes); typically unstable and rapidly degraded unless surfaces stabilize it.
- Microbial presence stabilizes via covalent binding of C3b to surface; C3b binds factor B, which is cleaved by factor D to form
 $ext{C3b} ext{ + } ext{Bb} = ext{C3bBb} \,( ext{C3 convertase})$
- The C3 convertase cleaves additional C3 to generate more C3b and C3a.
- Accumulated C3b on surface forms the C5 convertase: $ext{C3bBb} + ext{C3b} ightarrow ext{C3bBb3b} \,( ext{C5 convertase})$
- C5 cleavage follows as above.
- ## Lectin Pathway
- Initiated by mannose-binding lectin (MBL) and ficolins recognizing carbohydrates on pathogen surfaces.
- MASP-2 (MBL-associated serine protease) cleaves C4 and C2, forming the C4b2a C3 convertase, then proceeds like classical pathway.
- Diagrammatic summary (early steps):
 - MBL/ficolin binds microbe surface → MASP-2 activation → C4 cleavage → C4b binding → C2 cleavage → C4b2a C3 convertase → downstream C5 convertase and MAC.

Convergence and amplification (early steps)

All three pathways lead to deposition of C3b on the microbe and to the formation of a C5 convertase.
Early steps produce multiple C3convertase enzymes, which dramatically amplify the response as even a few activated molecules generate many effectors.
Macromolecular cascade ensures localized activation at site of infection and limits systemic damage via regulatory proteins.

Formation and role of the Membrane Attack Complex (MAC)

Proteins involved in MAC formation: C5b, C6, C7, C8, and multiple C9 units.
Sequence:
C5b initiates assembly by binding C6 and C7.
The complex then binds C8 and initiates polymerization of C9 to form a pore in the membrane.
Result: pore formation in membranes of pathogens leading to lysis.
Summary: $ext{C5b} ightarrow ext{MAC assembly with } ext{C6, C7, C8, nC9} ightarrow ext{cell lysis}$

Early amplification and site specificity

The cascade amplification can produce a large number of effector molecules from a small initial trigger.
Covalent attachment of activated complement proteins to the target surface helps localize effector functions to the infection site and minimize collateral damage.

Key functions of the complement system (3 major functions)

1) Opsonization and phagocytosis via C3b deposition and CRs on phagocytes.
2) Direct lysis of microbes via MAC (C5b-9 complex).
3) Stimulation of inflammation via anaphylatoxins C3a and C5a, promoting recruitment and activation of leukocytes.

Complement receptors and cellular interactions

Complement receptors (CRs) expressed on monocytes, macrophages, and B cells participate in:
Opsonization and phagocytosis (via recognition of C3b deposited on microbes).
Example: CR1 on phagocytes binds C3b-coated microbes to promote phagocytosis.

Roles in disease and immune regulation

Deficiency effects (from lecture/exam references):
C3 deficiency: central component loss; impaired opsonization; poor clearance of immune complexes; increased susceptibility to bacterial infections.
C2 or C4 deficiency: impaired classical/lectin pathways; decreased clearance of immune complexes; higher risk of encapsulated bacterial infections; immune complex disease risk (e.g., SLE).
C5–C9 deficiency: MAC formation impaired; increased susceptibility to Gram-negative infections; lack of direct bacterial lysis.
Systemic lupus erythematosus (SLE) and immune complexes: classical pathway is normally involved in clearing immune complexes; deficiency in early complement components can worsen IC clearance and promote disease.

Anaphylatoxins and inflammatory responses

C3a and C5a are anaphylatoxins that bind their receptors on various cells and promote inflammation:
Smooth muscle contraction, degranulation of mast cells and basophils, release of histamine and other vasoactive amines.
Increase vascular permeability.
C5a is a potent neutrophil chemoattractant.
Overall effect: recruitment and activation of leukocytes to infection sites.

Regulation and safety mechanisms

Complement regulatory proteins prevent unwanted activation and tissue damage; deficiency or dysregulation can lead to inflammatory disease.
Complement can be activated spontaneously on surfaces, but regulatory proteins keep activation localized to intended sites.

Connections to broader immune defense (innate and adaptive)

Innate defense: complements work with phagocytes, antimicrobial peptides, and pattern-recognition receptors to control infections.
Adaptive defense: antibodies can fix complement (classical pathway) to enhance microbe killing and clearance.
Timeline context: Innate responses act in minutes to hours, followed by adaptive responses over days to weeks (illustrated in lecture visuals).

Quick review: matching learning objectives to components

Learning Objective 1: Understand the complement system and the source of complement molecules.
Learning Objective 2: Differentiate classical vs alternative pathways of activation.
Learning Objective 3: Identify components and activation cascade leading to MAC formation.
Learning Objective 4: List the 3 major functions of the complement system and match cascade components to their activities.

Quick diagnostic/quiz prompts (based on content)

Which antibodies can activate or fix complement? A: IgG and IgM.
Name the three main outcomes of complement activation.
What is the C3 convertase of the classical/lectin pathway? A: $ext{C4b2a}$
What is the C3 convertase of the alternative pathway? A: $ext{C3bBb}$
What constitutes the C5 convertase in the classical pathway? A: $ext{C4b2a3b}$
What are the final components of the MAC? A: $ext{C5b, C6, C7, C8, multiple C9}$

References to course context

Abbas et al., Basic Immunology: Functions and Disorders of the Immune System, Chapter 8 (Learning Objectives overview).
Visual timelines show innate to adaptive transition and the rapid early response of complement in hours to days after infection.

Practical implications and take-home messages

Complement provides a rapid, multi-pronged assault on microbes: tagging for phagocytosis, direct lysis, and warning/inflammation.
Defects in early components (C1–C4–C3) severely impact opsonization and immune complex clearance, increasing risk for encapsulated bacteria and autoimmune complications like SLE.
Regulatory proteins are crucial to prevent unwanted activation and tissue damage.

Quick glossary

Anaphylatoxins: C3a, C5a – promote inflammation and recruit leukocytes.
Opsonins: C3b – enhances phagocytosis via CR receptors.
MAC: Membrane Attack Complex – pore-forming complex causing lysis.
C3 convertase: enzyme that cleaves C3; forms via different pathway components (C4b2a in classical/lectin; C3bBb in alternative).
C5 convertase: enzyme that cleaves C5; forms as above (C4b2a3b or C3bBb3b).

Note on visuals from transcript

The transcript emphasizes: (i) a general overview of the three pathways, (ii) convergence at C3, (iii) MAC assembly, and (iv) the functional outcomes (opsonization, lysis, inflammation).
It also highlights clinical correlations (SLE, encapsulated bacteria) and the roles of antibodies in fixing complement (IgG and IgM).

Key takeaways for exam preparation
- You should be able to:
- Explain the source and role of complement proteins.
- Differentiate classical, alternative, and lectin pathways and identify their initiating events and convertases.
- Describe the cascade leading to MAC formation and enumerate MAC components.
- List the three major functions of complement and match components (e.g., C3b with opsonization; C5a with inflammation; MAC with lysis).
- Recognize clinical implications of deficiencies (C3, C2, C4, C5–C9) and their associated diseases (e.g., susceptibility to encapsulated bacteria, SLE).
- Recall which antibodies can fix complement (IgG, IgM).