Medical Analysis

Complement 4 (C4) Laboratory Assessment and Clinical Significance

The fourth component of complement (C4), a 100 kDa protein, is encoded by two closely linked genes (C4A and C4B) located within the major histocompatibility complex (MHC) on chromosome 6 [5]. Although the protein products of the two loci share most of their structure and function, there are four amino acid differences between them [5]. It is mainly synthesized by hepatocytes, but macrophages and monocytes also produce it to a lesser extent [2]. Its half-life is 3-6 days [1, 2]. It plays a crucial role in immunization, opsonization, and cell lysis [2]. It is a key component in the classical and lectin pathways, where it helps to mark pathogens for destruction and facilitates their clearance [2, 5].

Activation of Complement Cascade: Understanding Immunological Pathways

The activation of the complement cascade involves distinct pathways [2]. The Classical pathway is initiated by an antigen-antibody complex [2, 5]. The Alternative pathway is activated by surfaces like endotoxins [2]. The Mannose-Binding Lectin (MBL) Pathway is activated by mannose on pathogen surfaces [2]. These pathways integrate C1, C4, C2, C3b, Factor B, MBL, and MASP-1/-2 to form C3 convertase [2]. This leads to the splitting of C3 into C3a (anaphylatoxin) and C3b (opsonic factor) [1, 2]. Further progression involves C5 convertase, leading to C5a (a powerful anaphylatoxin), C6, and the eventual formation of the Terminal Complement Complex (TCC) [2].

Functions of Complement 4 (C4)

The activation of the classical pathway occurs as C4 is cleaved by C1s into C4a and C4b after antibody-antigen complex formation [2, 5]. C3 convertase formation is critical, where C4b binds to pathogen or cell surfaces and combines with C2a to form C4b2a (C3 convertase), which is essential for complement activation [2]. Regarding opsonization, C4b acts as an opsonin, marking pathogens for phagocytosis [1, 2]. In terms of inflammation, C4a functions as a weak anaphylatoxin, promoting inflammatory responses such as increased vascular permeability and leukocyte recruitment [1, 2]. Immune complex clearance is facilitated as C4b assists in binding and removal of immune complexes by the reticuloendothelial system [2]. Furthermore, it aids in defense against pathogens by playing a crucial role in initiating complement activation via the classical and lectin pathways, thereby aiding in pathogen elimination [2]. C4 facilitates the uptake and destruction of pathogens by phagocytic cells, which occurs by the specific recognition of bound complement components by complement receptors (CRs) on phagocytes [2]. The complement system is part of your body’s immune system that cleans up damaged cells, helps your body heal after an injury or an infection, and destroys microscopic organisms like bacteria that make you sick [2]. Your complement system is the front line of defense for your immune system [2].

C4 Test Methodology and Pathophysiology

This test measures the amount of C4 proteins in blood [5]. These proteins are part of the complement system, an important part of the immune system that helps kill disease-causing bacteria and viruses [2]. The pathophysiology of C4 is such that complement is a system of plasma proteins that interacts with pathogens to mark them for destruction by phagocytes [2]. The classical pathway is initiated by activation of the C1 complex [2, 5]. A deficiency in complement C4 levels has been linked to different forms of kidney disease and chronic hepatitis [6]. It has also been found in a number of childhood diseases, including: Henoch-Schonlein purpura, a type of inflammation of the blood vessels, and childhood diabetes mellitus [3, 4]. Various interactions of viruses (HCV, HBV, Adenovirus, Coronavirus, other Flaviviruses), bacteria (E. coli), and pathological conditions (IR, RA, Tocilizumab, Histone H3, H4) can cause the consumption or inhibition of complement C4 through the classic or lectin complement pathways [5, 6]. The regulatory mechanisms involve negative regulation between anti-inflammatory signaling (C4a, C4d) and pro-inflammatory signaling (C3a, C5a) [1, 2].

Diagnostic Indications and Methodology

Clinical indications to evaluate the activity of the classical and lectin pathway include:

• Suspecting autoimmune diseases like SLE, Discoid lupus erythematosus (DLE) [5, 6].

• Hereditary angioedema [3, 6].

• Glomerulonephritis [6].

• Nephropathy [6].

• Infections [8].

• Evaluating suspected complement deficiency [3, 4].

• Immune-mediated disorders [5].

• Monitoring response to treatment in autoimmune conditions [6].

Laboratory methods of estimation include: Immunoturbidimetric [1], Immunonephelometric [6], ELISA [1, 5], Radial Immunodiffusion [7], Western blotting technique [1], Complement functional assay [3].

Sample Collection and Clinical Interpretation

Collect 3.0 ml blood in a plain tube (Red capped) or in Lithium Heparin (Green capped). Separate serum or plasma as early as possible. If you want to store or transport the sample, it must be stored or sent in a frozen state [1, 2].

Causes of Increased and Decreased C4 Levels

Causes of increased C4 levels include Rheumatoid arthritis, Pregnancy, Acute bacterial infections, Steroids and oral contraceptives, Hepatitis, Cirrhosis, and Stress [1, 5]. Causes of decreased C4 levels include Genetic deficiency, Systemic lupus erythematosus (SLE), Autoimmune hemolytic anemia, Hereditary angioedema, Autoimmune nephritis, Collagen vascular disease, Complement Deficiencies, Liver Cirrhosis, Malnutrition, Glomerulonephritis, and Hemodialysis [3, 5, 6].

Reference Interval

Reference intervals derived from standardized pediatric and adult clinical immunology studies [7, 9].

For Non-Medicos: Clinical Implications and Utility

The clinical implications of C4 are that it is used for diagnosing and monitoring autoimmune diseases, especially systemic lupus erythematosus (SLE) and rheumatoid arthritis [6]. Low C4 levels indicate active autoimmune disease or complement activation, and may signal ongoing inflammation or immune dysregulation [6]. It helps differentiate between autoimmune, infectious, and inflammatory causes of symptoms [6]. It guides treatment decisions and response monitoring for conditions like lupus, vasculitis, and hereditary angioedema [3, 6]. Elevated C4 may reflect recovery from infection or other inflammatory states [1]. Complete or partial C4 deficiency increases susceptibility to infections and the development of autoimmune disorders [3, 5]. Monitoring C4 is critical for tracking disease progression and severity, especially in kidney and neurological immune-mediated diseases [6, 10].

Clinical Utility of C4

Comparative Analysis: C3 vs. C4

References:

1. Ricklin, D., Reis, E. S., Mastellos, D. C., Gros, P., & Lambris, J. D. (2016). Complement component C3 – The “Swiss Army Knife” of innate immunity and host defense. Immunological Reviews, 274(1), 33–58. https://doi.org/10.1111/imr.12500

2. Merle, N. S., Church, S. E., Fremeaux-Bacchi, V., & Roumenina, L. T. (2015). Complement System Part I – Molecular Mechanisms of Activation and Regulation. Frontiers in Immunology, 6, 262. https://doi.org/10.3389/fimmu.2015.00262

3. McMurray, J. C., Schornack, B. J., Weskamp, A. L., Park, K. J., Pollock, J. D., Day, W. G., et al. (2024). Immunodeficiency: Complement disorders. Allergy and Asthma Proceedings, 45(5), 305–309. https://doi.org/10.2500/aap.2024.45.240050

4. Bernacchia, A., Ginaca, A., Rotondo, S., Tejada, M. P., & Di Giovanni, D. (2024). Case Report: C3 deficiency in two siblings. Frontiers in Pediatrics, 12, 1424380. https://doi.org/10.3389/fped.2024.1424380

5. Lintner, K. E., Wu, Y. L., Yang, Y., Spencer, C. H., Hauptmann, G., Hebert, L. A., et al. (2016). Early Components of the Complement Classical Activation Pathway in Human Systemic Autoimmune Diseases. Frontiers in Immunology, 7, 36. https://doi.org/10.3389/fimmu.2016.00036

6. Sandhu, V., & Quan, M. (2017). SLE and Serum Complement: Causative, Concomitant or Coincidental? The Open Rheumatology Journal, 11, 113–122. https://doi.org/10.2174/1874312901711010113

7. Davis, C. A., Vallota, E. H., & Forristal, J. (1979). Serum complement levels in infancy: age related changes. Pediatric Research, 13(9), 1043–1046. https://doi.org/10.1203/00006450-197909000-00019

8. Tsoni, S. V., & Weis, J. J. (2009). Complement C3 Plays an Essential Role in the Control of Opportunistic Fungal Infections. Infection and Immunity, 77(11), 5069–5077. https://doi.org/10.1128/iai.00233-09

9. van Beek, A. E., Kamp, A., Kruithof, S., Nieuwenhuys, E. J., Wouters, D., Jongerius, I., et al. (2018). Reference Intervals of Factor H and Factor H-Related Proteins in Healthy Children. Frontiers in Immunology, 9, 1727. https://doi.org/10.3389/fimmu.2018.01727

10. Sun, L. (2025). Associations of Serum Complement Biomarkers With Adverse Clinical Outcomes Among Patients With Ischemic Stroke. Journal of the American Heart Association, 14(3), e046136. https://doi.org/10.1161/JAHA.125.046136

FAQ’s:

• What is C4 blood test?
  It measures C4 protein levels, a component of the immune system that helps kill bacteria and viruses.

• How is C4 test performed?
  It uses methods like immunoturbidimetric, ELISA, or immunonephelometric assays on a serum blood sample.

• What triggers C4 activation?
  C4 activates via the classical and lectin pathways triggered by antigen-antibody complexes or mannose on pathogen surfaces.

• Why are C4 levels low?
  Low levels may indicate genetic deficiency, SLE, glomerulonephritis, or ongoing consumption in immune complex-mediated diseases.

• When are C4 levels high?
  Elevated levels occur during acute inflammation, bacterial infections, pregnancy, or recovery states like hepatitis or cirrhosis.

• What does C4 deficiency cause?
  It increases susceptibility to various infections and the development of autoimmune disorders like systemic lupus erythematosus.

• How is the sample collected?
  Collect 3.0 ml blood in a plain or lithium heparin tube and store/transport the serum frozen.

• C3 vs C4 test differences?
  C3 is central to all pathways, while C4 primarily functions in the classical and lectin pathways.

• What is C4’s main role?
  C4 helps mark pathogens for destruction, facilitates opsonization, and aids in immune complex clearance.

• Can C4 monitor autoimmune diseases?
  Yes, it is crucial for tracking disease progression and treatment response in conditions like SLE and vasculitis.

Related Diagnostic Tests

• Complement 3 (C3)