Complement C4
Imunidade e inflamaçãoÚltima revisão: 7 de abril de 2026. Abordagem de fontes: contexto padrão de interpretação laboratorial, material médico de referência e orientações clínicas ou de saúde pública quando relevantes.
O que é 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 que isso 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.
Faixas de referência normais
| Grupo | Faixa | Unidade |
|---|---|---|
| Adults | 10–40 | mg/dL |
| Children | 10–40 | mg/dL |
| Newborns | 6–30 | mg/dL |
As faixas de referência podem variar entre laboratórios. Sempre compare seus resultados com as faixas fornecidas pelo seu local de exame.
O que significam níveis altos de C4
Causas comuns
- Acute phase response (C4 is a mild acute phase reactant)
- Chronic inflammation
- Malignancy (some cases)
- Corticosteroid therapy
Possíveis sintomas
- Elevated C4 does not cause specific symptoms
- Symptoms relate to the underlying inflammatory condition
O que fazer: 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.
O que significam níveis baixos de C4
Causas comuns
- 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)
Possíveis sintomas
- 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
O que fazer: 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.
Quando o exame de C4 é recomendado?
- 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
Perguntas frequentes
Biomarcadores relacionados
Referências e abordagem de revisão
As páginas do glossário de biomarcadores são explicações educativas e devem ser interpretadas junto com as faixas de referência e observações fornecidas pelo seu laboratório e pelo seu médico. Para conhecer nossos padrões editoriais e processo de revisão, veja nossa Política editorial e a nossa revisão de conteúdo.
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Enviar resultados de exames →Aviso médico: Estas informações são apenas educativas e não substituem orientação, diagnóstico ou tratamento médico profissional. As faixas de referência podem variar entre laboratórios. Sempre converse com seu profissional de saúde sobre a interpretação dos seus resultados específicos.