Immunoglobulin G (IgG) Subclass 4 (C4) Antibody, Serum

What This Test Measures

The test detects the presence and concentration of IgG4 antibodies in your serum. IgG4 is one of several types of antibodies (immunoglobulins) your body produces to fight infections and other threats. In some cases, IgG4 antibodies can become overactive, contributing to conditions like certain autoimmune diseases or inflammatory disorders.

Why It Might Be Ordered

Your healthcare provider may recommend this test if you have symptoms or a medical history suggestive of conditions where IgG4 antibodies play a role, such as autoimmune diseases, certain lymphomas, or organ-specific inflammation (e.g., pancreatitis, thyroiditis). It may also be used to monitor treatment progress or diagnose specific disorders like IgG4-related disease.

Common Conditions Linked to IgG4 Antibodies

While IgG4 antibodies are generally part of your immune system’s normal response, elevated levels can be associated with conditions like IgG4-related disease, a rare but distinct autoimmune disorder affecting multiple organs. This condition may cause swelling, inflammation, or damage to glands, organs, or tissues.

What to Expect Before the Test

No special preparation is typically needed. You may be asked to fast for 8–12 hours before the test if it involves drawing blood, but this depends on the lab’s protocol. Bring your medical records or any medications you’re currently taking to your appointment.