Complement C4

Function

Complement C4 is a protein that is part of the body's complement system, a complex group of proteins that work with the immune system to clear pathogens and promote inflammation. C4 is specifically involved in the 'classical pathway' of complement activation, which is often triggered by antibodies binding to antigens.

Why it is Ordered

The C4 test is primarily used to diagnose and monitor autoimmune diseases. Because complement proteins are 'consumed' (used up) during an inflammatory immune response, low levels of C4 often indicate that the immune system is overactive or attacking the body's own tissues.

Associated Conditions

• Systemic Lupus Erythematosus (SLE): C4 levels are often low during a lupus flare-up and are used to monitor disease activity and treatment effectiveness.
• Hereditary Angioedema: A genetic condition characterized by low C4 levels even when the patient is not having an attack.
• Rheumatoid Arthritis: May occasionally show altered complement levels.
• Chronic Hepatitis: Certain viral infections can deplete complement proteins through the formation of immune complexes.

Clinical Interpretation

By measuring C4 alongside C3, doctors can determine which pathway of the immune system is active, helping to differentiate between various types of kidney disease or autoimmune syndromes.