Immunoglobulin G (IgG) is the most prevalent type of antibody circulating in the blood, representing approximately 75% of all antibodies in the serum. Its primary function is to provide long-lasting defense against previously encountered viruses, bacteria, and toxins. IgG is responsible for the adaptive immune system’s “memory,” allowing for a rapid response upon re-exposure to a pathogen. Determining the level of this protein is often the first step in assessing a person’s overall immune competence.
Understanding Normal IgG Ranges
IgG levels are measured with a quantitative blood test and compared to a laboratory’s established reference range. For most adults, the normal range for total serum IgG is between 600 to 1,700 milligrams per deciliter (mg/dL), though this can vary slightly between testing facilities. A single value outside this range does not automatically signify disease but serves as a flag for further investigation into immune function.
The IgG level is only one piece of the puzzle; clinicians often examine the levels of other immunoglobulins (IgA and IgM) and specific antibody responses to vaccines. Levels below the normal range are termed hypogammaglobulinemia, while levels above are known as hypergammaglobulinemia. The clinical context, such as a patient’s history of infections, is always considered alongside the numerical value to determine if intervention is required.
When Low IgG Requires Immunoglobulin Replacement
A significantly low IgG level, especially when paired with recurrent, severe infections, often necessitates immunoglobulin replacement therapy. This deficiency can be due to primary immunodeficiencies, such as Common Variable Immunodeficiency (CVID), or secondary causes like certain blood cancers or immunosuppressive medications. In these situations, the body is unable to mount a protective immune response, leaving the person highly susceptible to serious bacterial infections, particularly in the sinuses and lungs.
The decision to initiate replacement therapy, which involves administering concentrated IgG (Intravenous Immunoglobulin, or IVIg, and Subcutaneous Immunoglobulin, or SCIg), is based on the absolute IgG number and the severity of symptoms. For patients with chronic lymphocytic leukemia (CLL) or other secondary immunodeficiencies and a history of serious infections, some guidelines recommend starting therapy if the IgG level falls below 500 mg/dL. Other regulatory bodies suggest considering replacement when the IgG level is below 400 mg/dL, provided the person experiences severe or repeated infections.
The goal of treatment is to maintain a protective level over time, specifically focusing on the “trough” level, which is the lowest point the IgG reaches just before the next infusion is due. Clinicians aim to keep this trough level above 500 mg/dL, as this threshold has been shown to reduce the frequency and severity of acute bacterial infections. For patients with chronic sinopulmonary disease or those who still experience infections at this target, a higher trough level may be necessary to achieve clinical benefit. Replacement therapy provides the missing antibodies, substituting for the body’s failed immune system and restoring long-term immune memory.
Treating Conditions Associated with High IgG
When a blood test reveals high IgG levels (hypergammaglobulinemia), it signals an underlying disorder rather than a problem requiring direct treatment to lower the antibody count. This elevated level is the immune system’s response to chronic stimulation. Common reasons for this sustained immune activity include chronic infections like viral hepatitis, HIV, or tuberculosis, as well as autoimmune diseases such as systemic lupus erythematosus or rheumatoid arthritis.
The presence of high IgG can also be a marker for plasma cell disorders, including Monoclonal Gammopathy of Undetermined Significance (MGUS) or Multiple Myeloma. In these cases, a specific type of plasma cell produces a large amount of a single, non-functional antibody, which inflates the total IgG count. The treatment strategy for hypergammaglobulinemia is to identify and manage the root cause of the immune activation.
For a chronic infection, treatment involves the use of antivirals or antibiotics, allowing the immune system to resolve the threat and the IgG level to return toward the normal range. For autoimmune conditions, the focus is on immunosuppressive or anti-inflammatory medications to dampen the inappropriate immune attack. This approach contrasts with the replacement therapy used for low IgG, where the treatment is the immunoglobulin itself. Only in rare instances, such as when high levels cause the blood to become dangerously thick (hyperviscosity), might an intervention like plasmapheresis be used to immediately lower the protein concentration.