Overview
The complement (C) system is part of the innate immune system β a cascade of plasma proteins that "complements" the killing action of antibodies. Complement can opsonize bacteria for enhanced phagocytosis, activate inflammatory cells, regulate antibody responses, and clear immune complexes and apoptotic cells. Left unchecked, it can also cause tissue damage and trigger anaphylaxis.
Discovered in the late 19th century by Buchner, Pfeiffer, Bordet, and named "complement" by Paul Ehrlich, this system activates through three major pathways β classical, lectin, and alternative β which all converge on a common terminal pathway leading to pathogen lysis.
Learning Objectives
After this lesson you will be able toβ¦- Describe the complement system and its historical discovery
- List the general properties and major components of complement
- Explain the classical, lectin, and alternative pathways of activation
- Describe how complement activation is regulated
- Explain the biologically active products of complement activation
- Discuss the effects of complement deficiencies on disease susceptibility
Clinical Story
Why This MattersA young man presents with recurrent episodes of painless facial swelling that resolve on their own without treatment. His physician suspects hereditary angioedema and orders a C1 esterase inhibitor (C1-INH) assay β a test that only makes sense once you understand how the classical complement pathway is regulated.
Core Concepts
Hans Buchner first found that blood serum contained a bacteria-killing "factor." In 1894, Richard Pfeiffer showed that immunized guinea pigs rapidly lysed cholera bacteria injected into the peritoneum β an effect lost when serum was heated to 55Β°C but restored by adding fresh, non-immune serum. Jules Bordet concluded that bacteriolysis required two components: a heat-stable specific antibody and a heat-labile non-specific substance he called "alexine."
Paul Ehrlich later renamed this heat-labile substance "complement," because it complements the action of antibodies (which he called "amboceptors") in destroying bacteria.
- Present in normal serum of humans, animals, birds, amphibians, and fishes
- Non-specific β complement from one species can react with antibodies from another species
- Heat labile β serum loses complement activity when heated at 56Β°C for 30 minutes
- Ordinarily does not bind free antigen, only antibody that has already combined with antigen
- Only IgM, IgG1, IgG2, and IgG3 fix complement
- Complement levels are not influenced by immunization
Initiated by antigenβantibody complexes. C1 (made of C1q, C1r, C1s) binds to the Fc region of IgG/IgM that has already reacted with antigen. This activates C1qrs, which cleaves C4 into C4a and C4b, and C2 into C2a and C2b. The resulting C4bC2a complex is a C3 convertase, cleaving C3 into C3a and C3b. C3b then joins C4bC2a to form C4bC2aC3b, the C5 convertase β marking the end of the classical pathway.
Very similar to the classical pathway but antibody-independent. Mannose-binding lectin (MBL) binds bacterial surface mannans, associating with MASP-1 and MASP-2 (analogous to C1r and C1s). This cleaves C4 and C2 to form the same C3 and C5 convertases as the classical pathway.
Begins with spontaneous, low-level hydrolysis of C3 to C3i, which binds Factor B. Factor D cleaves Factor B to Bb, forming the C3iBb convertase that generates more C3b (amplification loop). On an activator surface (e.g., bacterial LPS), stabilized C3bBb (further stabilized by properdin) continues generating C3a and C3b, eventually forming C3bBbC3b, the alternative C5 convertase.
This pathway provides non-specific resistance without antibody involvement β a first line of defense before adaptive immunity develops.
- Kinins β C2b (prokinin) becomes active kinin via plasmin, causing edema; controlled by C1-INH
- Anaphylatoxins β C4a, C3a, C5a (increasing potency) trigger mast cell/basophil degranulation; controlled by carboxypeptidase B (C3a-INA)
- Chemotactic factors β C5a and the membrane attack complex fragment attract neutrophils to the site of infection
- Opsonins β C3b and C4b bind complement receptors on phagocytes, promoting phagocytosis
Key regulatory proteins include C1-INH (dissociates C1r/C1s from C1q), Factors H and I (degrade C3b), and DAF (blocks Factor B binding to C3b).
Laboratory Principle
Complement activity of serum is measured by determining the highest dilution of serum capable of lysing antibody-sensitized sheep erythrocytes (the CH50 assay). Because complement proteins circulate as inactive proenzymes that activate sequentially in a cascade, this functional lysis-based assay confirms that every step in the pathway β from C1 to the membrane attack complex β is working correctly. Individual components can also be quantified by radial immunodiffusion.
Flow Diagram
Quality Control
CH50 (total hemolytic complement) assays require fresh serum handled and stored strictly at 2β8Β°C, since complement is heat-labile and rapidly loses activity. Positive and negative control sera (known complement-sufficient and complement-heat-inactivated) should be run alongside each batch to confirm assay validity.
Laboratories offering complement component assays (C3, C4, CH50) typically participate in external proficiency testing schemes to verify inter-laboratory reproducibility, given how sensitive complement proteins are to sample handling and storage temperature.
Reference Values
Key Factsβ οΈ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| C1-INH deficiency | Overproduction of C2b (prokinin) causes hereditary angioedema | Treat with Danazol or Ξ΅-aminocaproic acid |
| C1, C2, C4 deficiency | Immune complexes not opsonized, predisposes to SLE | Monitor for autoimmune features |
| C3 deficiency | Severe susceptibility to bacterial infection; most serious complement deficiency | Aggressive infection prophylaxis and monitoring |
Common Errors & How to Avoid Them
Cause: Complement is heat-labile and degrades quickly outside cold storage.
Prevention: Always use fresh serum, kept cold, and never heated above 56Β°C before testing.
Cause: Both pathways converge on the same C3/C5 convertase machinery.
Prevention: Remember β classical needs antigen-antibody complexes; alternative is triggered directly by microbial surfaces, no antibody required.
Cause: Focusing only on the activation cascade.
Prevention: Always mention C1-INH, Factors H and I, and DAF when describing pathway control β deficiencies here cause real disease.
Laboratory Tips from the Bench
If a CH50 result is unexpectedly zero, check the specimen handling first β improperly stored or repeatedly frozen-thawed serum destroys complement activity and can produce a false deficiency result.
Low C3 with normal C4 points toward alternative pathway activation; low C3 AND low C4 together suggest classical pathway (and often immune complex) activation.
"C-3 is the C-enter" β all three pathways (classical, lectin, alternative) converge at C3 cleavage, the central hub of the entire complement cascade.
Important Notes
Among C3a, C4a, and C5a, C5a is the strongest activator of mast cells and basophils and is also a powerful chemotactic factor for neutrophils.
The alternative pathway is thought to be evolutionarily more primitive, with the classical and lectin pathways likely having developed from it over time.
Interactive Quiz
Test Your KnowledgeFlashcards
Tap to flipClick or tap any card to reveal the answer.
Clinical Case Study
Apply Your KnowledgeVikram has had three episodes of meningococcal meningitis in the past two years β an unusually high recurrence rate for a healthy young adult. His physician orders a complement work-up.
Normal C3 and C4 but deficient terminal components (C5βC9) mean the membrane attack complex cannot form. Without the MAC, the body cannot lyse the outer membrane of Gram-negative bacteria like Neisseria meningitidis, explaining his recurrent meningococcal infections.
- βTerminal complement deficiencies (C5-C9) specifically predispose to Gram-negative (Neisseria) infections
- βNormal C3/C4 does not rule out complement deficiency further down the cascade
- βMeningococcal vaccination is especially important in these patients
Frequently Asked Questions
Both pathways are structurally very similar and produce the same C3/C5 convertases, but the classical pathway is triggered by antigen-antibody complexes binding C1, while the lectin pathway is triggered by mannose-binding lectin (MBL) recognizing bacterial surface sugars β no antibody required.
Because all three pathways converge on C3 cleavage, a C3 deficiency knocks out opsonization AND the ability to generate the membrane attack pathway across every route of activation, leaving the patient broadly susceptible to bacterial infection.
Total complement activity is measured by finding the highest serum dilution that still lyses antibody-sensitized sheep erythrocytes (CH50 assay). Individual components can also be measured directly by radial immunodiffusion in agar.
Quick Revision
10-Minute ReviewKey Takeaways
- Complement is a set of serum proteins that plays a major role in the innate immune response
- Some complement components lyse foreign cells; others mediate inflammation and attract phagocytes
- Complement acts in a cascade, with activation of one proenzyme triggering the next
- There are three pathways of complement activation: classical, lectin, and alternative
- C3a and C5a fragments activate mast cells and recruit neutrophils to the injury site
- Genetic deficiencies of complement components predispose to specific patterns of infection or autoimmune disease
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling (NIOS). Microbiology Module β Lesson 58: Complement.
- Roitt IM, Delves PJ. Roitt's Essential Immunology. Wiley-Blackwell.
- Kuby J. Immunology. WH Freeman & Company.