Unit Converter
Immunoglobulin G (IgG)
Synonyms
- IgG
- Serum IgG
- γ-globulin
- Gamma globulin
- Immunoglobulin G heavy chain
- Total IgG
Units of Measurement
- µmol/L
- g/L
- mg/dL
- mg/100 mL
- mg%
- mg/mL
Molecular Weight
IgG (monomeric): ~150 kDa
Key Conversions
Using MW ~150,000 g/mol:
1 g/L ≈ 6.67 µmol/L
1 µmol/L ≈ 0.15 g/L
1 mg/dL = 0.01 g/L
mg% = mg/dL = mg/100 mL
1 mg/mL = 1 g/dL = 10 g/L
Description
Immunoglobulin G (IgG) is the most abundant antibody in human serum (~75% of total immunoglobulin).
It is a monomeric immunoglobulin synthesized by plasma cells and is essential for long-term immunity.
IgG subclasses:
- IgG1 (60–70%)
- IgG2 (20–25%)
- IgG3 (5–10%)
- IgG4 (1–3%)
Each subclass plays specific roles in defense against bacteria, viruses, toxins, and encapsulated organisms.
Physiological Role
1. Long-Term Humoral Immunity
Produced after exposure to:
- Infections
- Vaccination
- Booster immunization
2. Neutralization of Pathogens
IgG inactivates:
- Viruses
- Bacterial toxins
- Bacterial adherence
3. Complement Activation
IgG1 and IgG3 activate complement via classical pathway.
4. Opsonization
Promotes phagocytosis → key in clearing encapsulated bacteria:
- Streptococcus pneumoniae
- Haemophilus influenzae
5. Placental Transfer
IgG is the ONLY immunoglobulin that crosses the placenta → provides neonatal immunity.
6. Immune Memory
Central to anamnestic responses.
Clinical Significance
High IgG (Hyper-IgG)
1. Chronic Infections
- TB
- HIV
- Hepatitis
- Chronic sinusitis
2. Autoimmune Diseases
- SLE
- Sjögren syndrome
- RA
- Autoimmune hepatitis (IgG selectively elevated)
3. Chronic Inflammation
Persistent immune stimulation.
4. Monoclonal Gammopathy
- IgG myeloma (most common myeloma subtype)
- MGUS (IgG type)
- Smoldering myeloma
Markedly high IgG (often >30–40 g/L) suggests a monoclonal process → SPEP/UPEP required.
5. Liver Disease
Polyclonal IgG elevation common in:
- Autoimmune hepatitis
- Cirrhosis (moderate increase)
Low IgG (Hypogammaglobulinemia)
(Most clinically important)
1. Common Variable Immunodeficiency (CVID)
Hallmark finding: low IgG ± low IgA/IgM
Features:
- Recurrent sinopulmonary infections
- Autoimmune diseases
- Chronic GI disease
2. X-Linked Agammaglobulinemia (XLA)
Very low/absent IgG with absent B cells.
3. Secondary Immunodeficiency
- Nephrotic syndrome
- Protein-losing enteropathy
- Severe malnutrition
- Burns
- Lymphoma/leukemia
- Immunosuppressants (rituximab, steroids)
4. Infancy (<6 months)
Maternal IgG wanes → physiologic low IgG
“Physiologic nadir” at 3–6 months.
5. HIV Infection
Due to immune dysregulation.
Reference Intervals
(Tietz 8E + Mayo + ARUP + ESID)
Adults
- IgG: 7 – 16 g/L
(= 700 – 1600 mg/dL)
Children
| Age | IgG Range (mg/dL) |
| 0–1 month | 400 – 1200 |
| 1–6 months | 150 – 650 |
| 6–12 months | 250 – 900 |
| 1–2 years | 400 – 1050 |
| 2–6 years | 500 – 1200 |
| 6–12 years | 550 – 1400 |
| >12 years | Adult range |
Critical Patterns
- IgG < 4 g/L → high suspicion of immunodeficiency
- IgG > 30 g/L → likely monoclonal gammopathy
Diagnostic Uses
1. Immunodeficiency Evaluation
Main test for suspected:
- CVID
- XLA
- Secondary hypogammaglobulinemia
- Selective IgG subclass deficiency
2. Recurrent Sinopulmonary Infections
Low IgG → poor opsonization → chronic infections.
3. Autoimmune Disease Workup
High IgG common in:
- SLE
- Autoimmune hepatitis
- Sjögren syndrome
4. Monoclonal Gammopathies
Use SPEP/UPEP + IgG quantification.
5. Monitoring IVIG Therapy
IgG trough level guides replacement dosing.
6. Chronic Liver Disease
Polyclonal IgG elevation in autoimmune hepatitis.
7. Vaccine Response Assessment
Low IgG → poor vaccine antibody response → immunodeficiency.
Analytical Notes
- Serum preferred
- Nephelometry & turbidimetry commonly used
- Check electrophoresis if IgG is very high (rule out monoclonal spike)
- IgG is stable at room temperature for several days
Clinical Pearls
- IgG is the only antibody that crosses the placenta, protecting newborns.
- CVID usually diagnosed after age 4 with low IgG + poor antibody response.
- IgG4 can be elevated in IgG4-related disease, often >2.8 g/L.
- In nephrotic syndrome, IgG is lost in urine, while IgM remains normal/high.
- Autoimmune hepatitis shows selective IgG elevation, unlike alcoholic liver disease (IgA > IgG).
Interesting Fact
IgG accounts for 75–80% of total serum immunoglobulins and is produced in amounts of 3–4 grams per day, making it one of the body’s most metabolically important proteins.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Immunoglobulins.
- ESID/AAAAI Guidelines - Primary Immunodeficiency.
- Mayo Clinic Laboratories - IgG.
- ARUP Consult - Immune Evaluation.
- NIH / MedlinePlus - Immunoglobulins.