IgG, or immunoglobulin G, is the most abundant antibody in your bloodstream, making up about 75% of all the antibodies your body produces. When doctors measure your IgG levels through a blood test, they’re checking how well your immune system can fight infections and whether an underlying condition might be affecting your defenses. Normal adult IgG levels generally fall between roughly 700 and 1,600 mg/dL, though the exact reference range varies by lab and age.
What IgG Does in Your Body
IgG antibodies are your immune system’s long-term memory. After you recover from an infection or receive a vaccine, your body produces IgG antibodies that circulate in your blood for weeks or months, ready to recognize and neutralize that same pathogen if it shows up again. Most IgG subtypes stay in circulation for about 23 days before being broken down and replaced, giving you a steady supply of protection.
There are four subtypes, each with a slightly different specialty. IgG1 and IgG3 are the main responders to viral infections, with IgG3 typically appearing first. IgG2 is best at fighting bacteria, particularly those with sugar-coated surfaces like certain pneumonia-causing species. IgG4 is the least understood but appears to play a role in fighting parasitic infections.
IgG is also the only antibody class that crosses the placenta during pregnancy. A receptor on placental cells actively shuttles IgG from the mother’s blood into fetal circulation, giving newborns a borrowed set of immune defenses that protects them during the first months of life while their own immune systems mature. This is why newborns actually have relatively high IgG levels at birth (averaging around 950 mg/dL), which then drop sharply as the mother’s antibodies break down before the baby’s own production ramps up.
How IgG Differs From IgM
When your body encounters an infection for the first time, it produces IgM antibodies first as an immediate, short-lived response. IgG follows shortly after and sticks around much longer. This timing difference is diagnostically useful: if a blood test shows high IgM against a specific pathogen, you likely have a current or very recent infection. If it shows high IgG instead, you were probably infected in the past or were vaccinated and now have lasting immunity. In some infections, IgG can be detected as early as four days after symptoms begin and can remain elevated for three months or longer.
Normal IgG Ranges by Age
IgG levels change dramatically from birth through adolescence. Here’s what typical levels look like at different ages, measured in mg/dL:
- Newborns (0 to 30 days): 399 to 1,480
- 1 to 3 months: 217 to 981
- 4 to 6 months: 270 to 1,110
- 7 to 12 months: 242 to 977
- 1 to 2 years: 389 to 1,260
- 3 to 5 years: 457 to 1,120
- 6 to 8 years: 483 to 1,580
- 9 to 11 years: 642 to 2,290
- 12 to 16 years: 636 to 1,610
- 16 to 18 years: 688 to 2,430
The dip between one and six months is normal. That’s when a baby’s borrowed maternal antibodies are declining and their own production hasn’t fully kicked in. By adolescence, IgG levels approach adult ranges. Keep in mind that different labs use slightly different reference ranges, so always compare your results to the range printed on your specific lab report.
What Low IgG Levels Mean
Low IgG levels, a condition called hypogammaglobulinemia, means your body isn’t producing enough antibodies to reliably fend off infections. The most obvious sign is getting sick often or having infections that last longer than they should. Recurring ear infections, sinus infections, pneumonia, and bronchitis are common patterns. In children, frequent or severe infections that don’t respond well to standard treatment can be the first clue.
Low levels can be inherited or acquired. On the inherited side, several genetic immune disorders cause the problem. Common variable immunodeficiency (CVID) involves gene mutations that prevent your body from making enough antibodies. X-linked agammaglobulinemia is caused by a mutation on the X chromosome that limits production of the immune cells responsible for making antibodies. In infants, transient hypogammaglobulinemia is a temporary condition where IgG levels drop around three months of age but typically recover on their own.
Acquired causes are more common in adults. Certain medications, especially those that suppress the immune system, can lower IgG production. Blood cancers like leukemia, chronic kidney disease, and conditions that cause protein loss can also drive levels down.
What High IgG Levels Mean
Elevated IgG levels indicate your immune system is working overtime. This is called polyclonal gammopathy, and it’s not a disease itself but a signal that something is stimulating your immune system. The list of possible triggers is broad:
- Chronic infections: HIV, tuberculosis, malaria, mononucleosis, and pneumonia
- Autoimmune conditions: lupus, rheumatoid arthritis, and Sjögren’s syndrome
- Liver disease: cirrhosis, autoimmune hepatitis, and viral hepatitis
- Blood cancers: leukemia and lymphoma
- Other cancers: kidney, liver, lung, and ovarian cancer
A mildly elevated IgG level on its own isn’t necessarily alarming. Your doctor will typically look at the degree of elevation, which specific subclasses are affected, and your symptoms to narrow down the cause.
How the Test Works
An IgG blood test is a simple blood draw from a vein in your arm, taking less than five minutes. No fasting or special preparation is needed. If you’re taking any medications, let your provider know beforehand since some drugs can affect the accuracy of results.
Doctors may order a total IgG test, which measures the combined level of all four subtypes, or an IgG subclass panel, which breaks the result down into IgG1 through IgG4 individually. The subclass panel is useful when your total IgG looks normal but you’re still getting frequent infections, since a deficiency in one subclass can hide behind normal totals.
IgG Replacement Therapy
For people with persistently low IgG due to immune deficiency, replacement therapy is the standard treatment. This involves receiving concentrated IgG pooled from thousands of blood donors, delivered either through an IV or by injection under the skin. The goal is to keep IgG trough levels (the lowest point between doses) above 500 to 800 mg/dL, which is generally enough to reduce the frequency and severity of infections. Treatments are typically given monthly, and most people need them long-term if the underlying immune deficiency is permanent.
Beyond immune deficiency, IgG infusions are also used to calm overactive immune responses in conditions like Kawasaki disease, certain autoimmune blood disorders, and some neurological conditions. In these cases, the high doses of IgG work by modulating the immune system rather than simply replacing missing antibodies.