Complement C4, Serum

Complement component 4 (C4) is a key protein in the immune system that helps the body recognize and eliminate harmful microbes. It plays an important role in both the classical and lectin complement pathways, which are part of your innate immunity—the body’s first line of defense against infection. C4 is also one of the most variable proteins in the complement system.

C4 levels in the blood often rise when there is inflammation. Even mild or “low-grade” inflammation can increase C4 production. Conditions that may cause elevated C4 levels include:

• Obesity, since C4 levels tend to increase with body mass index (BMI)

• Skin conditions such as psoriasis, dermatitis, or hives

• Inflammatory bowel diseases like ulcerative colitis and Crohn’s disease

• Autoimmune joint conditions such as psoriatic arthritis or rheumatoid arthritis

• Rheumatic fever

• Inflammation linked to kidney disease or kidney failure

• Certain cancers, including Hodgkin’s lymphoma, leukemia, or sarcoma

• General infection or tissue injury

C4 levels may also rise during pregnancy or with the use of certain medications, such as cimetidine.

Like other complement proteins, C4 activity can vary in different parts of the body. For example, people with rheumatoid arthritis may show high complement levels in their blood but low levels in their joint fluid.

Because autoimmune and inflammatory conditions differ from person to person, it’s important to discuss your test results with your healthcare provider. They can help identify the underlying cause and recommend appropriate treatment or lifestyle adjustments.

Supporting a balanced immune response and reducing chronic inflammation may help keep C4 levels within a healthy range. Helpful strategies include:

• Maintaining a healthy weight

• Following a balanced, anti-inflammatory diet

• Exercising regularly

• Getting adequate sleep

• Managing stress

Lower-than-normal Complement C4 levels can occur in several conditions where the immune system is either overactive, fighting infection, or using up complement proteins. Possible causes include:

• Certain bacterial infections, especially those caused by Neisseria species

• Viral infections such as hepatitis

• Malnutrition or poor nutritional status

• Rejection after a kidney transplant

• Systemic lupus erythematosus (SLE), an autoimmune disease that can affect the skin, joints, kidneys, and other organs

• Lupus nephritis, a kidney disorder often seen in people with lupus

• Cirrhosis, a sign of advanced liver damage

• Glomerulonephritis, a type of inflammation that affects the kidneys

• Hereditary angioedema, a rare immune condition that can cause severe swelling in different areas of the body

Complement activity can also vary across different parts of the body. For example, someone with rheumatoid arthritis may have normal or high complement levels in their blood but low levels in their joint fluid.

Complement component 4 (C4) is a blood protein that plays an important role in your immune system. It’s part of the complement pathway—a defense system made up of several proteins that work together to help your body fight infections and clear away damaged or dead cells. A Complement C4 test measures how much of this protein is in your blood to help evaluate immune function and identify possible immune system disorders.