Complement C4

Immune & Inflammation

What is Complement C4?

Complement C4 is a glycoprotein and essential component of the classical and lectin complement activation pathways. It is encoded by two genes, C4A and C4B, located within the major histocompatibility complex (MHC) on chromosome 6p21. The number of C4 gene copies varies widely in the population—individuals may carry 2 to 8 copies total—which accounts for the broad normal range of C4 protein levels. When the classical pathway is activated (typically by antigen-antibody immune complexes binding C1q), C1s cleaves C4 into C4a (a small anaphylatoxin) and C4b, which binds covalently to the target surface and participates in forming the C3 convertase (C4b2a) that amplifies the complement cascade.

C4 is an important clinical biomarker primarily in two contexts: monitoring autoimmune disease activity (especially SLE) and diagnosing hereditary angioedema (HAE). In active SLE, immune complexes chronically consume classical pathway components, leading to decreased C4 and C3. Importantly, C4 is often the first complement component to decline in a lupus flare, sometimes before C3 drops. In hereditary angioedema types I and II, C4 is characteristically low even between attacks due to chronic unregulated C1 activation from C1-inhibitor deficiency, making C4 an excellent screening test for this condition.

Why It Matters

Low C4 is among the earliest laboratory indicators of a lupus flare, often declining before clinical worsening or C3 changes. Serial C4 monitoring, combined with C3 and anti-dsDNA levels, forms the cornerstone of lupus activity surveillance. C4 is also the first-line screening test for hereditary angioedema—a potentially life-threatening condition causing recurrent episodes of swelling in the face, extremities, gastrointestinal tract, and airway. A normal C4 level between attacks effectively excludes types I and II HAE. Additionally, low C4 due to genetic C4 null alleles (C4A or C4B deficiency) is itself a risk factor for developing SLE, linking C4 to both disease susceptibility and disease monitoring.

Normal Reference Ranges

Group	Range	Unit
Adults	10–40	mg/dL
Children	10–40	mg/dL
Newborns	6–30	mg/dL

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

What High C4 Levels Mean

Common Causes

Acute phase response (C4 is a mild acute phase reactant)
Chronic inflammation
Malignancy (some cases)
Corticosteroid therapy

Possible Symptoms

Elevated C4 does not cause specific symptoms
Symptoms relate to the underlying inflammatory condition

What to do: Elevated C4 is typically a nonspecific finding reflecting acute phase response and rarely requires specific investigation. It may be seen during infections, after surgery, or during corticosteroid therapy. Address the underlying condition as appropriate. Mild elevations do not have established clinical significance as an independent finding.

What Low C4 Levels Mean

Common Causes

Active systemic lupus erythematosus (classical pathway consumption)
Hereditary angioedema types I and II (chronic C1-inhibitor deficiency)
Genetic C4 null alleles (partial C4A or C4B deficiency—common, affecting ~25% of population)
Cryoglobulinemia
Immune complex-mediated glomerulonephritis
Serum sickness
Severe liver disease (decreased synthesis)
Bacterial endocarditis (immune complex formation)

Possible Symptoms

In lupus: joint pain, fatigue, rash, fever, kidney dysfunction
In hereditary angioedema: recurrent episodes of facial, limb, abdominal, or laryngeal swelling without urticaria
Recurrent infections in severe complement deficiency

What to do: The clinical context is essential. In known or suspected SLE, low C4 (especially if declining) should prompt assessment for disease flare with anti-dsDNA, urinalysis, and clinical evaluation; immunosuppressive therapy may need escalation. If hereditary angioedema is suspected, check C1-inhibitor level and function, and C1q level. Persistently low C4 without active autoimmune disease may reflect genetic C4 null alleles—these can be confirmed by C4 gene copy number analysis and are associated with increased SLE susceptibility. Genetic C4 deficiency warrants baseline rheumatologic evaluation.

When Is C4 Testing Recommended?

When monitoring SLE disease activity (in combination with C3 and anti-dsDNA)
When hereditary angioedema is suspected (recurrent angioedema without urticaria)
As part of the workup for suspected immune complex disease or glomerulonephritis
When evaluating for complement deficiency in recurrent infections
When cryoglobulinemia or endocarditis-related immune complex disease is considered

Frequently Asked Questions

Hereditary angioedema (HAE) is a genetic condition caused by deficiency (type I) or dysfunction (type II) of C1-inhibitor, a protein that regulates the complement and contact systems. Without functional C1-inhibitor, uncontrolled C1 activation chronically consumes C4, keeping it low even between swelling episodes. A low C4 is 95–98% sensitive for types I and II HAE, making it an excellent screening test. If C4 is low, C1-inhibitor level and functional assays are ordered to confirm the diagnosis.

Persistently low C4 without symptoms most commonly results from genetic C4 null alleles (partial C4A or C4B gene deletion), which are surprisingly common—present in about 25% of the population. While typically benign, C4A deficiency is a known genetic risk factor for developing SLE. It is reasonable to discuss baseline autoimmune screening (ANA) with your doctor and be aware of symptoms that might suggest early autoimmune disease. No treatment is needed for asymptomatic genetic C4 deficiency.

In SLE, immune complexes activate the classical complement pathway in a sequential manner: C1 is activated first, which cleaves C4, then C2, and the resulting C4b2a complex (C3 convertase) cleaves C3. C4 is consumed early in this cascade and has lower serum concentrations than C3, so its depletion is detectable earlier. Additionally, C3 has multiple activation pathways and a higher production rate, providing more buffering capacity. This is why C4 is often the first complement component to decline during a lupus flare.

Related Biomarkers

C3Complement C3 ANAAntinuclear Antibodies Anti-dsDNAAnti-dsDNA Antibodies CRPC-Reactive Protein

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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.