The Complement System
The experiment that found it
In 1894 Jules Bordet did something simple and got a strange result. He took serum from an animal immunised against Vibrio cholerae and added it to the bacteria. They dissolved. Then he repeated it with the same serum after heating it to 56 °C for half an hour. This time the bacteria clumped together — the antibody was clearly still working — but they did not dissolve.
The decisive step came next. He added fresh serum from a non-immunised animal, which contained no anti-cholera antibody at all, to the heated mixture. The bacteria lysed again. So killing needed two things: a heat-stable, antigen-specific component — the antibody — and a heat-labile, non-specific component present in everybody's serum. Paul Ehrlich named that second component complement, because it completes the action of antibody.
Every property that matters is already visible. Complement is in normal serum whether or not you have ever met the pathogen — so it is innate. It is destroyed by mild heat, so it is protein and fragile. And in this experiment it needed antibody to direct it — the classical pathway. What Bordet could not see was that complement has two other routes that need no antibody at all, and those turn out to do most of the work.
A collective name for a group of plasma proteins that become proteolytic enzymes after activation, and that opsonise and destroy pathogens. Complement proteins exist in blood serum, tissue fluid and on cell membranes. They are one of the major effector mechanisms of both humoral and innate immunity.
| Property | Detail |
|---|---|
| Made by | Mainly hepatocytes and macrophages |
| Normal state | Inactive zymogens — activated by proteolytic cleavage, in a cascade |
| Most abundant / least | C3 has the highest serum level; factor D the lowest |
| Heat lability | Inactivated at 56 °C for 30 min; also decays at 0–10 °C over 3–4 days. Store below −20 °C |
| Also inactivated by | UV radiation, ethyl alcohol, strong acid or base |
| Class | Acute-phase proteins — stress cytokines regulate their synthesis |
- Who discovered complement, and when? → Jules Bordet, 1894 (Nobel Prize 1919)
- Who named it, and why? → Paul Ehrlich — it 'completes the action of antibody'
- At what temperature is complement inactivated? → 56 °C for 30 minutes
- Which component has the highest serum level? → C3. The lowest is factor D
- In what form do complement proteins normally circulate? → As inactive zymogens
Components and how they are named ★★
Complement has about thirty proteins, and they fall into three functional groups. Learn the grouping before the individual names — it makes the pathways much easier to hold.
| Group | Members |
|---|---|
| ① Activation components | Classical: C1 (C1q, C1r, C1s), C2, C3, C4 Lectin (MBL): MBL, MASP, C2, C3, C4 Alternative: C3, factor B, factor D, factor P (properdin) Common terminal sequence: C5, C6, C7, C8, C9 |
| ② Regulatory components | C1 inhibitor (C1INH), C4-binding protein (C4bp), factor I, factor H, MCP (CD46), DAF (CD55), C8-binding protein |
| ③ Complement receptors | CR1–CR5, C3aR, C5aR |
The naming rules — and the one exception
- Intrinsic components of the classical pathway are numbered C1–C9. The numbers follow the order of discovery, not the order of the cascade — which is why the sequence runs C1, C4, C2, C3.
- Alternative-pathway components are factors — factor B, factor D, factor P (properdin).
- Cleavage fragments take a lower-case letter: C3a and C3b, C4a and C4b. As a rule 'b' is the larger fragment, and it is the one that stays bound to the surface.
- Enzymatically active complexes are written as the fragments that form them — C4b2a, C3bBb. Some textbooks draw a bar over them.
- Inactivated components take a lower-case i in front: iC3b.
For C3, C4 and C5 the larger fragment is the b one. C2 is the historical exception: TMU's deck and Janeway's both use C2a for the larger, active fragment — the one that joins C4b to form the C3 convertase. Janeway's states it explicitly: “In this book, C2a is used to denote the larger, active fragment of C2.”
This matters because some circulated review sheets write the classical C3 convertase as C4b2b. That is the older nomenclature and it is not what your department teaches — the TMU deck writes C4b2a seventeen times and C4b2b not once. Write C4b2a.
- What are the three components of C1? → C1q, C1r, C1s
- How many C1r and C1s per C1 complex? → Two each, with one C1q
- Which fragment is usually larger, a or b? → b — except for C2, where C2a is the larger active fragment
- What does a lower-case i mean, as in iC3b? → Inactivated
- Name the three alternative-pathway factors. → Factor B, factor D, factor P (properdin)
The classical pathway ★★★
The classical pathway is the antibody-dependent one — the route Bordet saw. It begins not with free antibody but with an immune complex: antibody that has already bound its antigen. That distinction is the mechanical heart of the whole pathway.
You are carrying grams of circulating IgG right now, and none of it is triggering complement. The reason is the hinge, from Unit 3. Before antigen binds, the antibody sits in a T shape and the C1q-binding site in CH2 is not accessible. Bind antigen, and the arms swing into a Y shape, exposing that site. So complement activation is automatically restricted to antibody that has actually found a target — the safety catch is built into the geometry.
There is a second safeguard. C1 must bind at least two antibody molecules simultaneously to be activated, because a single contact is too unstable. C1q's six arms have to grip two Fc regions that are close enough together. This has a direct consequence you can predict: IgM is the best complement activator, because as a pentamer it presents five Fc regions in one molecule — a single bound IgM can do it. IgG needs two separate molecules to land close together on the surface, which takes far more antibody.
- Immune complex forms — Ag bound by IgM, or by IgG1/IgG2/IgG3.
- C1q binds the Fc — CH2 of IgG, CH3 of IgM. At least two antibody molecules must be engaged.
- C1q activates C1r, which in turn activates C1s.
- Activated C1s cleaves C4 → C4a + C4b. C4b binds covalently to the pathogen surface.
- C1s cleaves C2 → C2b + C2a. C2a joins the surface-bound C4b.
- C4b2a = the C3 convertase.
- C3 convertase cleaves C3 → C3a + C3b.
- C3b binds the convertase → C4b2a3b = the C5 convertase.
Requires Ca²⁺ and Mg²⁺. Note the order of the numbers: C1 → C4 → C2 → C3. The numbering reflects discovery, not sequence.
The classical pathway runs 1, 4, 2, 3. One way to hold it: the convertase you are building is C4b2a — the 4 comes before the 2 in the name of the product, and that is exactly the order the components are cleaved in. Build the name and the sequence comes with it.
- What initiates the classical pathway? → An immune complex — Ag bound to IgG or IgM
- Can free antibody activate complement? → No — antigen binding must expose the C1q site
- How many antibody molecules must C1 engage? → At least two
- Why is IgM the best activator? → It is a pentamer, so one molecule presents several Fc regions at once
- Give the C3 convertase of the classical pathway. → C4b2a
- Give the C5 convertase. → C4b2a3b
- Which ions are required? → Ca²⁺ and Mg²⁺
The alternative pathway ★★★
The alternative pathway needs no antibody, and it is always quietly running. C3 in plasma undergoes spontaneous hydrolysis at a low rate — a slow, continuous trickle of C3b generated for no particular reason. On your own cells and in fluid phase that C3b is immediately inactivated and nothing happens. But if it lands on a microbial surface, which lacks the regulatory proteins your cells carry, it survives — and the cascade takes off.
Think of the spontaneous C3 tick-over as the immune system dropping a lit match onto every surface in the body, over and over. On your own cells the match goes out instantly, because your membranes carry extinguishers — DAF, MCP, factor H. On a bacterium there are no extinguishers, so the match catches. The alternative pathway does not recognise the pathogen; it recognises the absence of self-protection. That is a genuinely different strategy from an antibody looking for a specific epitope, and it is why §9 on regulation is not an afterthought — the regulators are the discrimination.
- Spontaneous cleavage of C3 → C3a + C3b, at a low rate.
- In fluid phase, or on host cells, C3b is hydrolysed and inactivated.
- On a microbial surface, C3b survives and binds factor B → C3bB.
- Factor D cleaves factor B → Ba is released, Bb stays bound.
- C3bBb = the C3 convertase, stabilised by properdin (factor P).
- C3bBb cleaves more C3 → more C3b → more C3bBb. This is the amplification loop.
- A further C3b joins → C3bBb3b = the C5 convertase.
Requires Mg²⁺ only. Initiators: microbial surface components such as LPS, peptidoglycan, zymosan and dextran.
One number from the deck is worth carrying because it reframes the whole system: regardless of which pathway starts the reaction, up to 90% of the C3b eventually deposited is generated through the alternative pathway's amplification loop. The classical pathway may light the fuse, but the alternative loop does most of the burning. Do not think of the three pathways as three separate systems — think of one amplifier with three different triggers.
- What initiates the alternative pathway? → Spontaneous C3 hydrolysis, sustained on microbial surfaces (LPS, peptidoglycan, zymosan)
- Is antibody required? → No
- Give the C3 convertase. → C3bBb
- Give the C5 convertase. → C3bBb3b
- What stabilises the C3 convertase? → Properdin (factor P)
- Which ion is required? → Mg²⁺ only
- How does the pathway distinguish self from non-self? → Host cells carry regulatory proteins that inactivate C3b; microbes do not
The lectin (MBL) pathway ★★
The third route is the most recently described and the easiest to learn, because it is the classical pathway with a different trigger. Mannose-binding lectin (MBL) is a soluble protein that is structurally similar to C1q — and it binds mannose and other sugar residues that appear on microbial surfaces but not on human glycoproteins.
Once MBL binds a microbial sugar, MBL-associated serine proteases (MASP) attach to it and behave exactly as C1r and C1s do. MASP-2 cleaves C4 and C2, producing the same C4b2a C3 convertase and the same C4b2a3b C5 convertase as the classical pathway. MASP-1 can additionally cleave C3 directly, feeding the alternative pathway's amplification loop.
MBL is C1q's innate twin. Both are recognition molecules with the same job — grip a surface, then activate a pair of serine proteases. The difference is only in what they grip: C1q grips antibody, MBL grips sugar. Downstream of that first contact, the two pathways are identical, which is why they share convertases. So there are really only two distinct chemistries — the C4/C2 route and the factor B/D route — reached by three different recognition events.
- What does MBL bind? → Mannose (and galactose) residues on microbial surfaces
- Which molecule is MBL structurally similar to? → C1q
- What replaces C1r and C1s? → MASP — MBL-associated serine proteases
- Which MASP cleaves C4 and C2? → MASP-2
- What does MASP-1 do? → Cleaves C3 directly, reinforcing the alternative pathway loop
- Which convertases does the lectin pathway use? → The classical ones — C4b2a and C4b2a3b
- Which ion is required? → Ca²⁺
The three pathways compared ★★★
This is the table the exam draws on, and it is worth being able to reproduce from memory. Read down the C3 convertase row first: two of the three pathways use the same one, and that is where most marks are lost.
| Classical | Alternative | Lectin (MBL) | |
|---|---|---|---|
| Initiator | Ag–Ab immune complex (IgM, IgG1–IgG3) | Microbial surfaces — G⁻ bacteria, LPS, peptidoglycan, zymosan | Mannose and other carbohydrates on microbial surfaces |
| First component | C1q | C3 | MBL |
| Components | C1, C4, C2, C3 | C3, factor B, factor D, properdin | MBL, MASP, C4, C2, C3 |
| Antibody needed? | Yes | No | No |
| Ions | Ca²⁺ and Mg²⁺ | Mg²⁺ | Ca²⁺ |
| C3 convertase | C4b2a | C3bBb | C4b2a |
| C5 convertase | C4b2a3b | C3bBb3b | C4b2a3b |
| Immunity | Adaptive | Innate | Innate |
| Timing in infection | Later stage | Initial stage; has a positive feedback loop | Early stage; promotes the other two |
There are only two distinct chemistries to learn, not six. C4b2a → add 3b → C4b2a3b (classical and lectin). C3bBb → add 3b → C3bBb3b (alternative). In both cases the C5 convertase is just the C3 convertase with a C3b stuck on. Learn one rule instead of four names.
- Which component is shared by all three pathways? → C3 — this is MCQ Q5
- Which pathways share the same convertases? → Classical and lectin — both use C4b2a and C4b2a3b
- Which pathway needs antibody? → Classical only
- Ions for each? → Classical Ca²⁺ + Mg²⁺; alternative Mg²⁺; lectin Ca²⁺
- How is any C5 convertase built from its C3 convertase? → By adding a C3b
- Which pathway acts earliest in infection? → Alternative and lectin (innate); classical acts later
The membrane attack complex ★★
All three pathways converge on the same ending. Whatever produced it, the C5 convertase cleaves C5 into C5a and C5b — and from C5b onwards there is only one route, the common terminal sequence. No more enzymes are involved: the remaining proteins simply assemble, in order, into a structure that punches a hole.
The complex of complement components C5b, C6, C7, C8 and C9 (written C5b–C9), formed in the terminal steps of the classical, lectin and alternative pathways, which mediates cell lysis by creating a pore in the target cell membrane.
- C5 convertase cleaves C5 → C5a (released) + C5b.
- C5b binds the cell surface, and C6 binds C5b.
- C7 binds C5b6, and the complex inserts into the phospholipid bilayer.
- C8 binds and also inserts into the bilayer.
- Several C9 molecules polymerise onto the complex and form the pore.
Only C9 is present in multiple copies — it is the polymerising component that actually makes the channel. The result is an open pore: the cell can no longer hold its osmotic gradient and lyses.
- What is the MAC composed of? → C5b, C6, C7, C8, C9 — i.e. C5b–C9
- Which component polymerises to form the pore? → C9
- Which is the first to insert into the membrane? → C7, as part of C5b67
- How does the MAC kill? → It forms a membrane pore, destroying the osmotic gradient
Biological functions of complement ★★
The TMU deck's contents slide lists a Part IV on biological functions, but those slides are not in the file you have been given — the deck ends at the three-pathway comparison. Everything in §8 and §9 is therefore taken from Janeway's ch. 2 and cited to Janeway's. It is standard, examinable material and worth knowing, but if a TMU-specific emphasis exists it is not recoverable from the deck as supplied.
Lysis by the MAC is the most dramatic thing complement does and the least important. Most of the system's value lies in the fragments generated along the way — the small ones that float off and call for help, and the large ones that stay stuck to the pathogen and label it for destruction.
| Function | Mediator | What happens |
|---|---|---|
| Opsonisation | C3b (and iC3b) | C3b coats the microbe; phagocytes bind it through CR1 (CD35) and ingest it. Quantitatively the most important function of the whole system |
| Inflammation | C3a, C4a, C5a — the anaphylatoxins | Small fragments released into fluid; they trigger mast-cell degranulation, increase vascular permeability and recruit cells |
| Chemotaxis | C5a (the most potent) | Draws neutrophils and monocytes to the site, and activates them — C5a binding is what makes a phagocyte actually ingest a CR1-bound microbe |
| Cell lysis | MAC (C5b–C9) | Pore formation. Critical against Neisseria; less important against most other organisms |
| Immune complex clearance | C3b + CR1 on erythrocytes | Red cells ferry complexes to liver and spleen for removal — failure of this contributes to immune-complex disease (Unit 15) |
| Linking to adaptive immunity | C3d + CR2 (CD21) | Complement-tagged antigen binds the B-cell co-receptor and lowers the threshold for B-cell activation (Unit 8) |
Look at Janeway's account of phagocytosis carefully: C3b on a microbe binds CR1 on a phagocyte, and on its own that is not enough to trigger ingestion. The phagocyte also needs C5a acting on its C5a receptor to switch it into an eating mood. So the two arms are complementary in a very literal way — C3b says where, C5a says go. That pairing is also why C5a is simultaneously the most potent chemoattractant and the most dangerous fragment when complement runs out of control.
- Which fragment is the main opsonin, and which receptor binds it? → C3b, bound by CR1 (CD35)
- Name the three anaphylatoxins. → C3a, C4a, C5a
- Which is the most potent chemoattractant? → C5a
- Which complement receptor links to B cells? → CR2 (CD21), binding C3d
- Which organism is MAC-dependent defence critical against? → Neisseria
Regulation, and what happens without it ★★
A self-amplifying proteolytic cascade sitting in your plasma is obviously dangerous, and the alternative pathway is deliberately indiscriminate. What keeps it off your own cells is a set of complement-control proteins — and because they are the only thing standing between you and self-lysis, the diseases caused by losing them are unusually instructive.
| Regulator | Acts on | What it does | Disease if deficient |
|---|---|---|---|
| C1 inhibitor (C1INH) | C1r, C1s | A serpin — irreversibly inactivates them, limiting C4 and C2 cleavage | Hereditary angioedema |
| DAF (CD55) | C3 convertase | Decay-accelerating factor — displaces Bb and C2a from C3b and C4b | Paroxysmal nocturnal haemoglobinuria |
| MCP (CD46) | C3b, C4b | Membrane cofactor of proteolysis — cofactor for factor I | Atypical haemolytic uraemic syndrome |
| Factor I | C3b | Cleaves C3b to inactive iC3b | — |
| Factor H | C3b | Cofactor for factor I; discriminates host surfaces | — |
| CD59 (protectin) | C8 | Inhibits MAC formation | Paroxysmal nocturnal haemoglobinuria |
Hereditary angioedema. Lose C1INH and C1 activates spontaneously, consuming C4 and C2 unchecked. The result is episodic, non-itchy swelling of the face, airway and gut — and because it is not histamine-driven, antihistamines and adrenaline do not work on it. That single clinical fact tells you the mechanism is complement, not allergy.
Paroxysmal nocturnal haemoglobinuria. Both DAF and CD59 are anchored to the membrane by a GPI anchor. A somatic mutation in the anchor pathway strips both regulators from the red-cell surface at once — so the cells lose their protection against the constantly ticking alternative pathway and are lysed by the patient's own complement. It is the clearest possible demonstration of §4's point: the alternative pathway spares host cells only because host cells are protected.
- Which regulator is missing in hereditary angioedema? → C1 inhibitor (C1INH)
- Why do antihistamines fail in hereditary angioedema? → The swelling is complement-mediated, not histamine-mediated
- Which two regulators are lost in PNH, and why together? → DAF (CD55) and CD59 — both are GPI-anchored
- What does factor I do? → Cleaves C3b to inactive iC3b, with factor H or MCP as cofactor
- What does DAF do? → Displaces Bb and C2a from the C3 convertase
Revision layer
The exam map for this unit
| Section | Item | From |
|---|---|---|
| III. MCQ Q5 | C3 is shared by all three pathways | §6 |
| II. Fill in blanks | Classical: C4b2a · C4b2a3b · MAC — set 2019 | §3, §6 |
| II. Fill in blanks | Alternative: C3bBb · C3bBb3b · MAC — set 2020 | §4, §6 |
| II. Fill in blanks | C1 = C1q + C1r + C1s — set 2019 | §2 |
| — | Also supports Unit 3's C1q-binding site (CH2 IgG, CH3 IgM) and Unit 14's type II hypersensitivity | §3, §8 |
The whole unit on one screen
| Question | Answer |
|---|---|
| Who found complement, who named it? | Bordet 1894; Ehrlich named it |
| Inactivated by? | 56 °C for 30 minutes |
| Highest / lowest serum level? | C3 highest; factor D lowest |
| C1 composition? | C1q + 2×C1r + 2×C1s |
| Classical initiator? | Immune complex — IgM or IgG1–IgG3 |
| Best complement activator? | IgM — one pentamer presents several Fc regions |
| Classical convertases? | C4b2a → C4b2a3b |
| Alternative convertases? | C3bBb → C3bBb3b |
| Lectin convertases? | Same as classical — C4b2a → C4b2a3b |
| Shared by all three? | C3 |
| Stabilises C3bBb? | Properdin (factor P) |
| MAC? | C5b–C9; C9 polymerises to form the pore |
| Main opsonin / receptor? | C3b / CR1 (CD35) |
| Anaphylatoxins? | C3a, C4a, C5a — C5a most potent |
| C1INH deficiency? | Hereditary angioedema |
| DAF + CD59 loss? | Paroxysmal nocturnal haemoglobinuria |
- Name the three pathways and their triggers. → Classical (immune complex) · alternative (microbial surface) · lectin (mannose)
- Which needs antibody? → Classical only
- Which component do all three share? → C3
- Give all four convertases. → C4b2a, C4b2a3b (classical + lectin); C3bBb, C3bBb3b (alternative)
- How is a C5 convertase made from a C3 convertase? → Add a C3b
- What is C1 made of? → C1q, C1r, C1s
- Why can't free antibody activate complement? → The C1q site is hidden until antigen binding changes the shape
- What is the MAC and what does it do? → C5b–C9; forms a membrane pore causing lysis
- Which fragment opsonises and which recruits? → C3b opsonises; C5a recruits and activates