Complement C4
Inmunidad e inflamaciónÚltima revisión: 7 de abril de 2026. Enfoque de fuentes: contexto estándar de interpretación de laboratorio, material médico de referencia y orientación clínica o de salud pública cuando corresponde.
¿Qué es 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.
Por qué importa
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.
Rangos de referencia normales
| Grupo | Rango | Unidad |
|---|---|---|
| Adults | 10–40 | mg/dL |
| Children | 10–40 | mg/dL |
| Newborns | 6–30 | mg/dL |
Los rangos de referencia pueden variar entre laboratorios. Compara siempre tus resultados con los rangos proporcionados por tu laboratorio.
Qué significan los niveles altos de C4
Causas comunes
- Acute phase response (C4 is a mild acute phase reactant)
- Chronic inflammation
- Malignancy (some cases)
- Corticosteroid therapy
Posibles síntomas
- Elevated C4 does not cause specific symptoms
- Symptoms relate to the underlying inflammatory condition
Qué hacer: 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.
Qué significan los niveles bajos de C4
Causas comunes
- 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)
Posibles síntomas
- 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
Qué hacer: 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.
¿Cuándo se recomienda la prueba de C4?
- 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
Preguntas frecuentes
Biomarcadores relacionados
Referencias y enfoque de revisión
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Subir resultados de laboratorio →Aviso médico: Esta información es solo educativa y no sustituye el consejo, diagnóstico ni tratamiento médico profesional. Los rangos de referencia pueden variar entre laboratorios. Consulta siempre a tu profesional sanitario para interpretar tus resultados concretos.