Summary
Complement C3 is a key protein of the complement system — part of the immune defence that helps clear pathogens and immune complexes. C3 is measured to investigate and monitor autoimmune diseases, particularly systemic lupus erythematosus (SLE), and certain kidney diseases. Low C3 often indicates the complement system is being consumed by active immune-mediated disease.
The complement system is a cascade of proteins that, once activated, amplifies the immune response, marks pathogens for destruction, and clears immune complexes. C3 sits at the convergence point of all complement activation pathways, making it a central and informative marker.
In active autoimmune diseases like lupus, immune complexes activate complement, consuming C3 and C4 faster than they are produced — so levels fall. A low C3 (often with low C4) supports active disease and is used to monitor flares and treatment response. As C3 is also an acute phase protein, levels can rise with inflammation, which sometimes masks consumption.
C3 is also important in certain kidney diseases (such as membranoproliferative glomerulonephritis and C3 glomerulopathy) and in assessing inherited or acquired complement deficiencies, which predispose to recurrent infections.
What It Is
Complement C3 is the most abundant complement protein, synthesised mainly by the liver. It is the central component where the classical, alternative, and lectin activation pathways converge. C3 is cleaved into C3a (an anaphylatoxin) and C3b (an opsonin that tags surfaces for phagocytosis and drives the membrane attack complex).
C3 is consumed during complement activation, so falling levels indicate ongoing activation — as in immune complex diseases. It is also a positive acute phase reactant, so concurrent inflammation can raise it, occasionally offsetting consumption. The C3 and C4 pattern helps distinguish classical pathway activation (both low, as in lupus) from alternative pathway activation (low C3, normal C4).
Reference ranges: approximately 0.75–1.65 g/L, varying by laboratory. Low C3 suggests consumption or, less commonly, deficiency; high C3 reflects an acute phase response.
Functions
Immune defence
Central to the complement cascade — opsonises pathogens, recruits immune cells, and helps form the membrane attack complex.
Lupus disease activity
Low C3 (with low C4) supports active SLE and is used to monitor disease flares and treatment response.
Kidney disease assessment
Important in diagnosing and monitoring complement-mediated kidney diseases such as C3 glomerulopathy.
Complement consumption marker
Falling C3 indicates the complement system is being consumed by active immune-complex disease.
Reference Ranges
Complement C3
Measured in g/L| Status | Range (g/L) | Range (mg/dL) | What it means |
|---|---|---|---|
| Low | < 0.75 | < 75 | Low C3 — complement consumption (active autoimmune disease) or deficiency. Investigate. |
| Normal | 0.75–1.65 | 75–165 | Normal complement C3 — no evidence of significant consumption. |
| High | > 1.65 | > 165 | Elevated — acute phase response from inflammation or infection. |
Reference ranges vary between laboratories. C3 is both consumed in immune-mediated disease and raised as an acute phase reactant. Interpret with C4, clinical context, and serial measurements.
Symptoms of Imbalance
C3 levels themselves cause no symptoms; abnormalities reflect underlying autoimmune, kidney, or immune-deficiency conditions.
- Symptoms of active lupus (joint pain, rash, fatigue, kidney involvement)
- Recurrent infections (with complement deficiency)
- Features of glomerulonephritis (blood or protein in urine, swelling)
- Fatigue and malaise
- Usually reflects an acute phase response — symptoms of the underlying inflammation or infection
- No specific symptoms from a high C3 itself
Causes of Imbalance
- Active systemic lupus erythematosus (SLE)
- Membranoproliferative and post-infectious glomerulonephritis
- C3 glomerulopathy
- Inherited or acquired complement deficiency
- Severe sepsis (consumption)
- Cryoglobulinaemia
- Acute phase response (infection, inflammation, injury)
- Some malignancies
- Inflammatory conditions in their non-active or recovering phase
FAQs
C3 and C4 are both complement proteins, and their pattern helps identify which activation pathway is involved. In classical pathway activation — as in active lupus — both C3 and C4 typically fall. In alternative pathway activation, C3 falls but C4 stays normal. Measuring them together therefore gives more diagnostic information than either alone, and both are used to monitor autoimmune disease activity.
A low C3 usually means the complement system is being consumed by active immune-mediated disease — most commonly systemic lupus erythematosus or certain kidney diseases. Less often, it reflects an inherited or acquired complement deficiency, which can predispose to recurrent infections. A low C3 is always interpreted with C4, autoantibodies, and the clinical picture to identify the cause.
Yes. In many people with lupus, C3 and C4 fall during disease flares as complement is consumed, and recover as the disease comes under control. Serial measurements are therefore useful for tracking disease activity and response to treatment, alongside clinical assessment and other markers such as anti-dsDNA antibodies. Trends over time are more informative than a single result.
C3 is a dual-natured protein: it is consumed during complement activation but also produced more during general inflammation as an acute phase reactant. In some situations these effects offset each other, so C3 can appear normal despite ongoing complement activation. This is why doctors look at the C3 and C4 pattern together and follow trends, rather than relying on a single C3 value.
References
- Walport MJ. Complement. First of two parts. N Engl J Med. 2001;344(14):1058–1066. View source
- Sturfelt G, Truedsson L. Complement in the immunopathogenesis of rheumatic disease. Nat Rev Rheumatol. 2012;8(8):458–468. View source
- Ricklin D, et al. Complement in disease: a defence system turning offensive. Nat Rev Nephrol. 2016;12(7):383–401. View source