IgG vs. IgM: What Do Your Blood Test Results Mean?

Blood tests provide a powerful way to learn what’s happening inside your body. These tests offer a snapshot of how your immune system is working. When a doctor thinks someone might have a primary immunodeficiency (PI) disorder, doing blood tests is a common first step. But when your healthcare provider orders these tests, you might be confused or even overwhelmed by acronyms such as CBC (that one stands for “complete blood count”).

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In PI, gene mutations (changes) affect how well the immune system functions. There are more than 500 kinds of PI conditions. Many of them will cause changes in the blood that can be detected by an immunoglobulin (Ig) blood test. Only a trained healthcare professional can explain what these test results mean, but understanding the basics can help you feel more informed and prepared to talk about next steps. This article explains what an immunoglobulin blood test is and what the results may mean.

What Are Antibodies?

Antibodies are proteins made by the immune system’s B cells (a type of white blood cell). Antibodies belong to a larger group of proteins called immunoglobulins. When something foreign, such as a bacterium or virus, enters the body, antibodies attach to it. This foreign substance is called an antigen. By attaching to the antigen, antibodies mark it so that other white blood cells know to destroy it.

In general, each antibody attaches only to a specific antigen. When the antigen enters the body again — for example, during a second infection — B cells produce more of that same antibody to attack it. In this way, antibodies are a part of the immune system’s “memory.” Being immune to something means your body has already seen the germ and made specific antibodies against it. The immune system can respond faster next time, so you don’t get sick again or are less ill.

Antibody Classes

As antibodies do their job, the immune system gets better at recognizing and responding to specific antigens. Maybe a very small germ, like a virus, has entered the lungs and throat — or maybe the target is a large parasite like an intestinal worm. Destroying different threats calls for different approaches.

To handle this, B cells produce different antibody classes. These classes include immunoglobulin G, M, A, D, and E — known as IgG, IgM, IgA, IgD, and IgE. Each class of antibody has a particular role in the immune response. They activate white blood cells in different ways, depending on the situation. For example, IgA is better at dealing with infections on moist, sensitive surfaces (called mucosal surfaces), such as the lungs and reproductive tract. Below, we’ll explain what two important antibody classes — IgM and IgG — can do.

IgM Antibodies

The first immunoglobulin produced is IgM. Think of these immunoglobulins as first responders to an emergency. When your body first sees a germ, IgM antibodies will go into action. IgM antibodies can stick to each other and form a five-pointed star called a pentamer. This makes them very good at finding and sticking to small amounts of antigen. IgM antibodies are found in your bloodstream. They’re too big to move out of the blood into tissues and organs.

IgG Antibodies

IgG antibodies are the most common type in your bloodstream. Usually, up to 75 percent of the antibodies in the blood are IgG. There are four subclasses of IgG antibodies — IgG1 through IgG4. They’re each better in slightly different situations. However, a general immunoglobulin blood test can’t tell them apart. That can be done only using an IgG subclass blood test, which clinical immunologists don’t commonly order.

B cells can switch to making IgG antibodies after the initial IgM antibody response. IgG antibodies are small, so they can move out of the blood into other parts of the body. They’re good at fighting bacterial, viral, and possibly fungal infections throughout the body.

What Is an Immunoglobulin Blood Test?

An immunoglobulin blood test measures the levels of different antibody classes in your blood. It involves giving a blood sample, which is sent to a laboratory for analysis. Usually, the test measures IgM, IgG, and IgA antibodies.

Your healthcare provider may order this test if they suspect your immune system isn’t producing antibodies in normal amounts. This could be caused by PI, along with other possible conditions. The results for each antibody class are compared with average levels for people in your age group. Any values that are too far outside the average will be classified as low or high.

Along with testing immunoglobulin levels, your doctor will likely order a complete blood count to see the levels of different blood cells and rule out other conditions.

What Do Low Levels or Abnormal Levels of Antibodies Mean?

An immunoglobulin blood test can have many possible results. If all antibody classes are normal, your provider may order other types of lab tests. These tests might look at different parts of the immune system or evaluate how well your antibodies work.

If your IgM levels, IgG levels, or both are abnormal, this may mean you have one of the most common types of primary immunodeficiencies. These are called predominantly antibody deficiencies, and they make up 55 percent of all PI diagnoses. Your provider will do several more tests to confirm a diagnosis.

Selective IgM deficiency, a rare PI condition, is diagnosed when IgM levels are low, while IgG and IgA levels remain normal.

If all three antibody levels are low, possible causes include:

• Common variable immunodeficiency (CVID) — In this condition, IgG, IgA, and sometimes IgM levels are lower than normal. It’s one of the most common antibody deficiencies in adults.
• Agammaglobulinemia — In this type of PI, people usually lack functioning B cells to produce antibodies. Examples of severe PI conditions that may cause these findings include severe combined immunodeficiency (SCID) and X-linked agammaglobulinemia.

If IgG levels are low but IgM is normal, possible causes include:

• CVID — Because many different gene mutations can cause CVID, some people with this condition have normal IgM but low IgG levels.
• Hyper IgM syndrome — In this PI condition, IgM levels are high or normal, while IgG and IgA levels are low.

What Happens Next?

Your healthcare team will discuss your CBC and immunoglobulin blood test results with you and explain what they mean. The next steps depend on whether any results were abnormal. For example, if any of your antibody classes are low, your doctor might order more tests, such as:

• Tests that look at how many B cells you have and how well they function
• Tests that look at how well another type of white blood cell, called T cells, work in your body
• Genetic screening to check for gene mutations known to cause PI disorders

Many other factors can cause low levels of antibodies. Other possible causes include certain medications, some types of cancer such as lymphoma or leukemia, and certain autoimmune conditions.

If you have any questions about your test results, even after talking with your provider, don’t hesitate to ask for more information. Understanding medical information can feel overwhelming, and your healthcare team is the best source for guidance and support.

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