What is Complement Component 4?

What is Complement component 4 (C4):

Complement component 4 (C4) is a blood test that measures the activity of a certain protein. This protein is part of the complement system.

What is the complement system?

The complement system is a part of the immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promotes inflammation, and attacks the pathogen’s cell membrane.

Complement may also be activated during reactions against incompatible blood transfusions, and during the damaging immune responses that accompany autoimmune disease. Deficiencies of individual complement components or inhibitors of the system can lead to a variety of diseases.

There are nine major complement proteins. They are labeled C1 through C9.

C3 and C4 are the most commonly measured complement components. When the complement system is turned on during inflammation, levels of complement proteins may go down. Complement activity may be measured to determine how severe a disease is or if treatment is working.

What is a complement test being used for?

A complement test may be used to monitor people with an autoimmune disorder. For example, people with active systemic lupus erythematosus may have lower-than-normal levels of the complement proteins C3 and C4.

Complement activity varies throughout the body. In people with rheumatoid arthritis, complement activity may be normal or higher-than-normal in the blood, but much lower-than-normal in the joint fluid.

More details on C4:

C4 is the most second-most abundant of the 11 complement proteins (after C3). It is also among the best studied. Levels of C4 are known to increase during inflammation and tissue destruction, and decrease during autoimmune disorders. The Complement C4 Blood Test helps determine the levels of C4 in blood. It is used to determine if deficiencies in the complement system are contributing to an immune disorder.

Reference ranges:

Normal ranges for C4 is 15 to 45 milligrams per deciliter (mg/dL) (0.15 to 0.45 g/L).

High levels:

Higher-than-normal complement C4 values may be a sign of cancer or ulcerative colitis.

Low levels:

Lower-than-normal complement C4 values could indicate:

bacterial infections (especially Neisseria)
hepatitis
malnutrition
rejection of a kidney transplant
systemic lupus erythematosus, which is an autoimmune disease affecting the skin, joints, kidneys, and other organs
lupus nephritis, which is a kidney disorder that often occurs as a result of systemic lupus erythematosus
cirrhosis, which indicates significant liver damage
glomerulonephritis, which is a type of kidney disease
hereditary angioedema, which is a rare but serious autoimmune disease that causes swelling in various parts of the body

Complement component activity varies throughout the body. People with rheumatoid arthritis, for example, may have high complement levels in their blood but low complement levels in their joint fluid.

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