Immunoglobulin G
IgG· also: Immunoglobulin G
Clinical Overview
IgG is the most abundant immunoglobulin in blood, comprising approximately 75% of total immunoglobulins. It provides long-term humoral immunity, crosses the placenta to protect newborns, and opsonizes pathogens for phagocytosis. IgG is produced by plasma cells as the predominant antibody in secondary (memory) immune responses.
Why This Test Matters
Low IgG (hypogammaglobulinemia) causes increased susceptibility to bacterial infections, especially recurrent sinusitis, pneumonia, and otitis media from encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae). The most common cause in adults is Common Variable Immunodeficiency (CVID). High IgG can indicate chronic infection, autoimmune liver disease, or multiple myeloma (usually monoclonal IgG — detected as a paraprotein spike on serum protein electrophoresis). Immunoglobulin levels should always be measured alongside serum protein electrophoresis when myeloma is suspected.
Reference RangesWHO/IFCC standards
| Age Group | Reference Range | Unit | Notes |
|---|---|---|---|
| Adults (18–64) | 700 – 1600 | mg/dL | — |
| Children / Pediatric | 400 – 1200 | mg/dL | Age-dependent; rises through childhood |
Also reported in: g/L.
What Causes Abnormal Results?
High IgG Causes
- Chronic infections (tuberculosis, HIV, hepatitis C, osteomyelitis)
- Autoimmune liver disease (autoimmune hepatitis, primary biliary cholangitis)
- IgG multiple myeloma (monoclonal — paraprotein spike on SPEP)
- Connective tissue diseases (SLE, RA)
- Polyclonal hypergammaglobulinemia (chronic inflammation, liver disease)
Low IgG Causes
- Common Variable Immunodeficiency (CVID) — most common primary immunodeficiency in adults
- X-linked agammaglobulinemia (XLA) — severe congenital B cell deficiency
- Secondary immunodeficiency: CLL, lymphoma, post-rituximab therapy, high-dose steroids
- Protein-losing enteropathy or nephrotic syndrome (loss of IgG)
- Physiological nadir in infants (3–6 months): maternal IgG depleted, own production not yet established
Signs & Symptoms to Watch For
How to Prepare for This Test
No fasting required. Quantitative immunoglobulins (IgG, IgA, IgM) are measured by nephelometry or turbidimetry. For suspected myeloma, add serum protein electrophoresis (SPEP) and immunofixation.
Factors That Can Affect Results
- Recent IVIG infusion falsely elevates IgG (measure 3–6 months after last infusion for baseline)
- Rituximab therapy markedly suppresses IgG for 6–12 months
- Age-dependent normal ranges (IgG rises from birth to adulthood)
- Hemolysis may interfere with some nephelometry assays
Related Topics
Frequently Asked Questions
What is Common Variable Immunodeficiency (CVID) and how is it diagnosed?
CVID is the most common serious primary immunodeficiency in adults, typically presenting in the second or third decade of life with recurrent bacterial infections. Diagnosis requires: markedly low IgG (typically <500 mg/dL), low IgA and/or IgM, poor vaccine antibody responses (to pneumococcal or tetanus vaccines), and exclusion of other causes. Treatment is lifelong intravenous or subcutaneous immunoglobulin (IVIG or SCIG) replacement therapy, which dramatically reduces infection risk.
High IgG was found on my blood test — could I have myeloma?
Elevated IgG is most commonly due to polyclonal hypergammaglobulinemia from chronic infections, autoimmune disease, or liver disease — not myeloma. Myeloma causes a monoclonal (single clone) IgG spike detectable on serum protein electrophoresis (SPEP) as a sharp "M-spike." To distinguish polyclonal from monoclonal elevation, order SPEP and immunofixation. A monoclonal IgG spike warrants further evaluation by hematology. Polyclonal IgG elevation is not a sign of myeloma.