IgG Test · Blood

Your body remembers every serious infection and vaccination you have ever had, and IgG (immunoglobulin G) is the physical proof. It is the most plentiful antibody circulating in your blood, making up roughly 75% of all your antibodies. When your level is too low, your body cannot mount an effective defense against bacteria, viruses, and other threats, leaving you vulnerable to infections that most people shake off easily.

When IgG is too high, it usually signals that your immune system is working overtime, often because of a chronic infection, an autoimmune condition, or ongoing inflammation your body has not resolved. Either extreme tells you something about what your immune system is doing right now, and a standard blood panel will not reveal it.

What IgG Does in Your Body

IgG is a Y-shaped protein made by specialized immune cells called plasma cells, which are the mature, antibody-producing descendants of your B cells. These plasma cells live throughout your lymph nodes, bone marrow, and other immune tissues. Each IgG molecule has two working ends: the tips of the Y recognize and latch onto a specific invader (a virus surface protein, a bacterial toxin, a foreign molecule), while the stem of the Y communicates with the rest of your immune system.

That stem region acts like a flag. When IgG coats an invader, the stem signals white blood cells to engulf and destroy the target, a process called opsonization. It also triggers a cascade of immune proteins that punch holes in bacterial membranes, and it activates natural killer cells to destroy virus-infected cells. IgG is uniquely long-lived among antibodies, persisting in circulation for weeks, which is why it forms the backbone of lasting immunity after an infection or a vaccine.

Four Subclasses, Not One

IgG is actually a family of four subtypes: IgG1, IgG2, IgG3, and IgG4. Each has slightly different abilities. IgG1 and IgG3 are the most aggressive at rallying immune cells and triggering inflammation. IgG2 is best at targeting bacteria with sugar-coated surfaces. IgG4 is the quietest, with limited ability to activate the inflammatory cascade, which makes it relatively anti-inflammatory.

A total IgG test sums all four subtypes into one number. That total gives you a reliable overview of your overall antibody production. If the total is abnormal and you want to understand why, subclass testing (IgG1 through IgG4 individually) can reveal which branch of the immune response is overactive or deficient.

Low IgG and Infection Risk

When total IgG drops below normal, the clinical term is hypogammaglobulinemia. This means your body is not producing enough antibodies to defend itself. The consequences are direct and measurable: more frequent infections, longer recovery times, and a higher chance that common infections become serious.

In people with blood cancers such as chronic lymphocytic leukemia (CLL) or non-Hodgkin lymphoma, low IgG is extremely common because the disease and its treatments both suppress normal antibody production. A study of more than 17,000 such patients found that those who had their IgG monitored more frequently were more likely to have hypogammaglobulinemia detected, receive immunoglobulin replacement therapy, and experience significantly fewer severe infections and antibiotic courses afterward.

Low IgG is not limited to cancer. In infants undergoing heart surgery requiring a heart-lung bypass machine, IgG fell to less than 50% of its starting level within 24 hours and remained low for at least 7 days. That drop correlated with more infections, longer time on a ventilator, and longer intensive care stays. The mechanism is a combination of physical dilution from the bypass circuit and leakage of IgG out of blood vessels during the inflammatory response to surgery.

High IgG: What Chronic Immune Activation Looks Like

An elevated total IgG usually means your immune system has been in a sustained fight. The most common drivers are chronic infections, autoimmune diseases, and conditions where the immune system is responding to persistent tissue damage.

In people with cystic fibrosis, elevated serum IgG (above the 97.5th percentile for age) tracks chronic lung colonization by Pseudomonas bacteria. People in this group had worse baseline lung function, and the elevation independently predicted progression to severe lung disease with a roughly 10-fold higher risk compared to those with normal IgG levels.

In IgG4-related disease (IgG4-RD), a condition where immune cells infiltrate and enlarge organs such as the pancreas, salivary glands, or bile ducts, elevated serum IgG4 (one of the four IgG subtypes) is associated with more organ involvement, higher disease activity, and a greater chance of relapse. Persistently high or re-rising IgG4 after treatment is one of the best predictors that the disease will flare again.

Heart Disease and IgG: A Surprising Finding

In people with high blood pressure, higher baseline total IgG was associated with a lower risk of cardiovascular events, not a higher one. Each standard-deviation increase in total IgG was linked to about a 20% lower risk of adverse cardiovascular events overall and about a 34% lower risk of coronary heart disease specifically.

This may seem counterintuitive, since IgG is an immune molecule and immune activation is generally linked to cardiovascular harm. One proposed explanation is that IgG helps clear the body of pro-inflammatory debris (oxidized lipids, damaged proteins, and certain pathogens) that contribute to plaque buildup in arteries. In other words, having a strong antibody response may be protective in some contexts, even as the total level rises. This finding is from a single study in hypertensive patients and should not be generalized to all populations, but it illustrates that IgG is not a simple "higher is worse" marker. Context matters enormously.

Reference Ranges

Total IgG is measured from a standard blood draw and reported in milligrams per deciliter (mg/dL). The ranges below reflect widely used adult clinical thresholds. Your lab may report slightly different boundaries depending on the testing method. In one intensive care cohort studying sepsis, 59% of patients had IgG below 700 mg/dL, the generally accepted lower limit of normal, illustrating how common low IgG is in acute illness.

These thresholds are general adult values. IgG levels change across the lifespan: newborns carry maternal IgG that declines over the first few months, children gradually build their own IgG through childhood, and levels tend to be stable through adulthood. Compare your results within the same lab over time for the most meaningful trend.

When Results Can Be Misleading

Total IgG has relatively low within-person variability, fluctuating by about 5 to 7% from week to week in healthy adults (based on the European Biological Variation Study of 91 healthy people sampled weekly over 10 weeks). That means your IgG does not bounce around wildly from draw to draw, which makes it a fairly stable marker. But a few situations can still distort a single reading.

• Acute illness or recent surgery: Any significant immune challenge can temporarily raise IgG as your body mounts a response, or lower it if large amounts of protein are lost (as seen after heart-lung bypass surgery). Wait at least two weeks after recovering from an acute illness before drawing IgG for a baseline read.
• Dehydration or overhydration: Because IgG is measured as a concentration in blood, anything that changes your blood volume can shift the number. Dehydration concentrates the sample (falsely high); IV fluids dilute it (falsely low).
• FcRn-targeting medications: A newer class of drugs called FcRn antagonists (such as efgartigimod) are designed to speed up IgG breakdown. These drugs work by blocking the recycling system that normally protects IgG from being degraded. In healthy volunteers, they lowered total IgG by 50 to 75%, with recovery taking about 8 weeks. If you are taking one of these medications for an autoimmune condition, your IgG result reflects the drug's intended effect, not a new immune problem.
• Monoclonal protein interference: If you have a condition that produces a single abnormal antibody (like a monoclonal gammopathy, where one clone of immune cells makes excessive copies of one protein), the total IgG number can be inflated by that one protein clone. Protein electrophoresis can separate the abnormal spike from your normal IgG.