The IgG4 test is a specialized diagnostic tool that measures levels of a specific antibody subclass in the blood. It helps identify and manage certain immune-related conditions by providing insights into potential inflammatory processes.
Understanding Immunoglobulin G4
Immunoglobulin G4 (IgG4) is one of four subclasses of IgG antibodies, a major component of the immune system. Unlike other IgG subclasses, IgG4 generally does not activate the complement system, a part of the immune response that drives inflammation. IgG4 antibodies are also involved in immune tolerance, particularly in allergic reactions, where their presence can help block the effects of IgE antibodies, thereby reducing allergic responses.
Purpose of the IgG4 Test
The IgG4 test is performed to investigate specific chronic inflammatory and autoimmune conditions, rather than as a general screening tool for overall health. Its primary application lies in aiding the diagnosis and monitoring of certain disorders characterized by unusual immune responses. This test helps clinicians understand if elevated levels of this specific antibody subclass are contributing to a patient’s symptoms or disease progression.
Conditions Associated with Elevated IgG4
IgG4-Related Disease (IgG4-RD)
Elevated IgG4 levels are significantly associated with IgG4-related disease (IgG4-RD), a chronic fibroinflammatory condition that can affect almost any organ. It often leads to tumor-like masses or painless enlargement. Examples include type 1 autoimmune pancreatitis, Mikulicz’s disease (involving salivary and lacrimal glands), retroperitoneal fibrosis, and inflammatory aortic aneurysms. The disease is characterized by the infiltration of IgG4-positive plasma cells and often, but not always, elevated serum IgG4 levels. This immune cell infiltration and subsequent fibrosis can lead to organ dysfunction.
Other Conditions
While elevated IgG4 supports a diagnosis of IgG4-RD, it is not exclusive to this condition. Elevated levels can also be seen in other autoimmune diseases, allergies, and certain cancers.
How the IgG4 Test is Performed
The IgG4 test is a straightforward blood test. A healthcare professional draws a small sample of blood, typically from a vein in the arm. This sample is then sent to a specialized laboratory for analysis, where the concentration of IgG4 antibodies in the serum is measured.
Interpreting IgG4 Test Results
Interpreting IgG4 test results requires careful consideration of the patient’s complete clinical picture. Elevated IgG4 levels, often considered above 135 mg/dL, are consistent with an IgG4-RD diagnosis in the appropriate clinical context. However, not all individuals with IgG4-RD will have elevated concentrations, and elevated IgG4 levels can occur in other conditions, including allergies, autoimmune diseases, or certain cancers. Therefore, an elevated IgG4 level alone is not sufficient for a definitive diagnosis. The results must be correlated with the patient’s symptoms, physical examination findings, imaging studies, and frequently, tissue biopsies, which provide evidence of IgG4-positive plasma cell infiltration.