Before We Start
Three starting points, one shared destination
The complement system can be triggered three different ways — by antibody, by a sugar pattern, or spontaneously — but no matter which path starts it, all three converge on the exact same molecule, which then drives three distinct downstream effects.
💡 Memory Trick
Complement pathways: Classical (antibody) · Lectin (mannose) · Alternative (spontaneous) → all converge at C3. From there: opsonization, inflammation, and membrane attack — three outcomes from one convergence point.
The Key Points
Three activation routes, then convergence into three outcomes
CP
Classical — triggered by antibody
The classical pathway is activated when antibody (IgG or IgM) binds an antigen, forming an immune complex that C1q recognizes. It's the one complement pathway that's technically part of the adaptive immune response, since it specifically requires antibody to get started.
🦠 An IgM-antigen complex forming during an early immune response activates C1q, kicking off the classical pathway — this pathway can't fire without antibody already present.
LP
Lectin — triggered by mannose, no antibody required
The lectin pathway is activated when mannose-binding lectin (MBL) recognizes mannose sugar residues on a microbial surface — no antibody required at all. This makes it a purely innate recognition mechanism, distinct from the antibody-dependent classical pathway.
🦠 MBL binding directly to mannose residues on a bacterial cell wall activates the lectin pathway immediately, without needing any prior antibody response to have developed.
AP
Alternative — spontaneous, always-on background activation
The alternative pathway activates spontaneously and continuously at a low background level, providing baseline surveillance that gets specifically amplified on microbial surfaces lacking the regulatory proteins that normally protect host cells from complement activation.
🦠 The alternative pathway's constant low-level background activity means it's already primed to amplify rapidly the moment it encounters a microbial surface lacking the host's own protective regulatory proteins.
C3
Convergence — all roads lead to C3, then three distinct outcomes
All three pathways converge on C3 convertase, which cleaves C3 into C3b and C3a. C3b coats the pathogen for opsonization, making it easier for phagocytes to engulf. C3a (along with C5a) acts as an anaphylatoxin, triggering mast cell degranulation and chemotaxis. Finally, C5b through C9 assemble into the Membrane Attack Complex (MAC), which punches a lethal pore directly into the pathogen's membrane.
🦠 In a Gram-negative bacterial infection, the MAC (C5b-9) is what actually lyses the bacterial outer membrane — which is exactly why deficiencies in these late complement components leave a patient specifically vulnerable to Neisseria species.
🏥 Applied Scenario
A patient with recurrent, severe bacterial infections is found to have very low C3 levels.
Step 1
Explain the broad vulnerability: Why does low C3 cause such broad susceptibility? Because C3 is the convergence point for all three pathways — without it, opsonization fails across the board, regardless of which pathway would have originally triggered the response.
Step 2
Contrast with a narrower deficiency: A different patient with recurrent Neisseria infections specifically (and normal C3) is found to have a C5-C9 deficiency instead — since the Membrane Attack Complex is what specifically kills Neisseria, a deficiency there causes a much narrower, more specific vulnerability rather than broad susceptibility.
Step 3
Recognize the exam-relevant contrast: This contrast — broad susceptibility with a C3 deficiency versus narrow, Neisseria-specific susceptibility with a C5-C9 deficiency — is exactly how exams differentiate complement deficiency questions from one another.
Step 4
Conclusion: Where in the cascade a deficiency occurs — before or after the C3 convergence point — determines whether the resulting vulnerability is broad or narrow, a distinction worth checking specifically on any complement deficiency question.
📌 Exam Application
Exams test which pathway is antibody-dependent (classical) versus antibody-independent (lectin, alternative), what C3 convertase produces (C3b for opsonization, C3a/C5a as anaphylatoxins), and complement deficiency syndromes: C1q deficiency resembles SLE, C3 deficiency causes severe recurrent bacterial infections, and C5-C9 deficiency specifically predisposes to Neisseria infections.
⚠️ The Trap — Assuming All Complement Activation Requires Antibody
The most common trap is assuming all complement activation requires antibody — only the classical pathway does. The lectin and alternative pathways are purely innate and fire without any antibody involved at all. A second common trap: confusing C3 deficiency (broad susceptibility to many bacteria) with C5-C9 deficiency (narrow, Neisseria-specific susceptibility) — these two are frequently tested directly against each other.
✓ Quick Self-Test
Answer before checking:
1. Which complement pathway requires antibody to activate?
2. What triggers the lectin pathway?
3. What do all three complement pathways converge on?
4. What does C3b do, and what does the C5b-C9 complex do?
5. What is the clinical difference between a C3 deficiency and a C5-C9 deficiency?
Answers:
1. The classical pathway — it's triggered by antibody (IgG or IgM) bound to antigen.
2. Mannose-binding lectin recognizing mannose sugar residues on a microbial surface, without antibody.
3. C3 convertase, which cleaves C3 into C3b and C3a.
4. C3b opsonizes pathogens for phagocytosis; C5b-C9 forms the Membrane Attack Complex (MAC), which lyses the pathogen's membrane.
5. C3 deficiency causes broad, severe recurrent bacterial infections; C5-C9 deficiency causes a narrow, specific susceptibility to Neisseria infections.