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Complement C4

Published: July 27, 2026Updated: August 25, 2026

Complement C4 is a protein that plays a crucial role in the immune system's response to inflammation and infections. It is a part of the complement system, which enhances the ability of antibodies and phagocytic cells to clear pathogens and damaged cells.

Quick summary

Complement C4 is a key protein in the immune system's inflammatory response.

Medically reviewed by WellAlly Editorial TeamLast reviewed August 25, 2026Editorial policy

Key facts at a glance

Units
mg/dL
Clinical utility
diagnostic
Result kind
quantitative
FDA-cleared
Roche C4, Abbott C4, Siemens C4
interpretation: {"text":"May indicate autoimmune disorders such as SLE, or certain types of kidney disease.","label":"Low levels"}; {"text":"May suggest inflammation, infection, or other immune-related conditions.","label":"High levels"}

Complement C4 is a protein that is integral to the complement system, a part of the immune system that enhances the ability of antibodies and phagocytic cells to clear pathogens and damaged cells from the body. It is measured in milligrams per deciliter (mg/dL).

The normal reference range for Complement C4 is typically between 15–45 mg/dL. Levels outside this range can indicate various health conditions, including autoimmune disorders and certain infections. Low levels of C4 are often associated with conditions like systemic lupus erythematosus (SLE) and other autoimmune diseases, while high levels may suggest inflammation or infection.

Complement C4 testing is primarily used as a diagnostic tool to help identify and monitor autoimmune disorders, particularly those affecting the connective tissues. It is often ordered alongside other tests to provide a comprehensive picture of immune system function.

Frequently asked questions

What does a low Complement C4 level mean?
A low Complement C4 level can indicate autoimmune disorders such as systemic lupus erythematosus (SLE) or other conditions where the immune system attacks the body's own tissues.
Can high levels of Complement C4 be harmful?
High levels of Complement C4 may suggest inflammation or infection but are not necessarily harmful on their own. They indicate that the immune system is actively responding to a threat.
  1. 1
    Complement C4: MedlinePlus Medical TestSource
  2. 2
    Complement C4 - Lab Tests OnlineSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 25, 2026. · Read our editorial policy