Complement C3

Immune & Inflammation

What is Complement C3?

Complement C3 is the most abundant and central protein of the complement system, a network of over 30 plasma proteins that form a critical arm of innate immunity. C3 serves as the convergence point for all three complement activation pathways—classical, lectin, and alternative—and its cleavage into C3a and C3b is the pivotal step in complement cascade activation. C3b opsonizes pathogens and immune complexes for phagocytosis, C3a is an anaphylatoxin that recruits inflammatory cells, and downstream activation generates the membrane attack complex (C5b-9) that directly lyses pathogens.

C3 is primarily synthesized in the liver, with additional local production by macrophages, fibroblasts, and epithelial cells at sites of inflammation. Serum C3 levels reflect the balance between hepatic production and consumption (utilization) by complement activation. In autoimmune diseases like SLE, immune complexes chronically activate the classical pathway, consuming C3 and C4 and leading to low serum levels—a hallmark of active lupus nephritis. Conversely, C3 behaves as an acute phase reactant and can be elevated in infections, inflammation, and metabolic syndrome, where increased hepatic production outpaces consumption.

Why It Matters

C3 levels are essential for monitoring disease activity in systemic lupus erythematosus, particularly lupus nephritis, where falling C3 (and C4) levels often herald a disease flare before clinical symptoms worsen. Serial C3 measurements guide treatment intensity—persistently low C3 with rising anti-dsDNA antibodies is a red flag prompting preemptive therapy escalation. Low C3 is also critical for diagnosing complement-mediated kidney diseases including C3 glomerulopathy, atypical hemolytic uremic syndrome (aHUS), and post-infectious glomerulonephritis. Genetic C3 deficiency, though rare, predisposes to severe recurrent bacterial infections, particularly by encapsulated organisms.

Normal Reference Ranges

Group	Range	Unit
Adults	90–180	mg/dL
Children (1–18 years)	80–170	mg/dL
Infants (6 months–1 year)	60–150	mg/dL

Reference ranges may vary by laboratory. Always compare results to the ranges provided by your testing facility.

What High C3 Levels Mean

Common Causes

Acute phase response (infection, trauma, surgery)
Chronic inflammation
Metabolic syndrome and obesity
Diabetes mellitus
Biliary obstruction
Certain malignancies

Possible Symptoms

Elevated C3 itself does not cause specific symptoms
Symptoms relate to the underlying inflammatory or metabolic condition

What to do: Mildly elevated C3 as part of an acute phase response is common and typically resolves as the underlying condition improves. Persistently elevated C3 may be associated with metabolic syndrome, insulin resistance, or chronic inflammation. Address the underlying condition—metabolic syndrome management, infection treatment, or inflammatory disease control. Elevated C3 alone rarely requires specific investigation unless accompanied by other concerning findings.

What Low C3 Levels Mean

Common Causes

Active systemic lupus erythematosus (particularly lupus nephritis)
Post-infectious (post-streptococcal) glomerulonephritis
C3 glomerulopathy (C3 glomerulonephritis, dense deposit disease)
Atypical hemolytic uremic syndrome (aHUS)
Hereditary complement deficiency
Severe liver disease (decreased production)
Cryoglobulinemia
Serum sickness
Membranoproliferative glomerulonephritis
Severe sepsis or disseminated intravascular coagulation (consumption)

Possible Symptoms

Symptoms depend on the cause—often kidney-related: blood or protein in urine, edema, hypertension
In SLE: joint pain, rash, fatigue, fever
Recurrent bacterial infections (with hereditary deficiency)
Dark or foamy urine

What to do: Low C3 requires urgent evaluation in the context of renal disease or lupus. Check C4 simultaneously (low C3 with normal C4 suggests alternative pathway activation; both low suggests classical pathway activation as in SLE). Obtain anti-dsDNA antibodies, urinalysis, renal function tests, and potentially a kidney biopsy. In active lupus nephritis, immunosuppressive therapy escalation may be needed. For complement-mediated kidney diseases, complement-targeted therapies (eculizumab, ravulizumab) may be indicated. In hereditary deficiency, vaccination against encapsulated organisms is essential.

When Is C3 Testing Recommended?

When monitoring disease activity in known SLE (serial measurements with C4 and anti-dsDNA)
In the workup of glomerulonephritis (particularly membranoproliferative or post-infectious)
When hereditary complement deficiency is suspected (recurrent infections by encapsulated bacteria)
In the evaluation of atypical hemolytic uremic syndrome (aHUS)
When cryoglobulinemia or immune complex disease is suspected

Frequently Asked Questions

Measuring both C3 and C4 helps determine which complement activation pathway is involved. If both C3 and C4 are low, the classical pathway is likely activated—this is the typical pattern in active SLE, where immune complexes trigger the classical cascade. If C3 is low but C4 is normal, the alternative pathway is preferentially activated—seen in C3 glomerulopathy and some infections. If C4 is low but C3 is normal, a C4 null allele (genetic) or early classical pathway activation may be responsible. The pattern guides diagnosis and management.

In patients with active lupus, C3 and C4 are typically checked every 1–3 months, often alongside anti-dsDNA titers and urinalysis. During disease remission, every 3–6 months may suffice. The key is to detect trends: a falling C3 often precedes a clinical flare by weeks, providing a window for preemptive treatment adjustment. Each patient's baseline complement levels should be established, as some lupus patients have constitutively low complement.

Diet does not directly raise complement levels, but factors that increase inflammation or hepatic protein synthesis can modestly affect C3 since it is an acute phase protein. Obesity and metabolic syndrome are associated with chronically elevated C3 levels. There are no supplements proven to normalize low C3 in the context of autoimmune disease—treatment requires addressing the underlying immune-mediated consumption. Weight management may help normalize mildly elevated C3 associated with metabolic dysfunction.

Related Biomarkers

C4Complement C4 ANAAntinuclear Antibodies Anti-dsDNAAnti-dsDNA Antibodies CRPC-Reactive Protein ESRErythrocyte Sedimentation Rate

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Medical Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Reference ranges may vary between laboratories. Always consult your healthcare provider for interpretation of your specific test results.