IgM antibodies, or immunoglobulins, are the first responders of your immune system — they’re the first antibodies it makes when viruses, bacteria or other harmful substances (pathogens) enter your body. High levels of IgM are usually an indication that you’ve had a recent infection.
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IgM (immunoglobulin M) is a type of antibody that your body makes to help you fight off infections. It’s the first antibody your immune system makes when fighting off viruses, bacteria, parasites or other harmful invaders while your body prepares additional defenses. It’s also the first antibody your immune system starts to develop as a fetus. “Ig” is short for immunoglobulin, which is another name for antibody.
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Antibodies are molecules that have a unique shape. This shape fits one specific antigen in a similar way to how a key fits a lock. Antigens are molecules that let your body know whether a cell or an organism is part of your own body (“self”) or foreign (“non-self”) and potentially dangerous. When an antibody comes across an antigen that it fits — like on the surface of a virus or bacterium — it tells your immune system that it’s found a potential problem. Your immune system can then take action to eliminate the harmful substance (pathogen) by flushing it out of your body or destroying it altogether.
IgM is unique from other antibodies because it usually exists as five IgM antibodies joined together, making it the largest antibody (known as a macroglobulin). This helps it find even small amounts of pathogens. It also allows it to clump pathogens together (agglutination), making it easier for other immune cells to find and destroy them.
IgM is found in your blood and lymph, either on its own or attached to B cells. It’s too large to move out of your blood vessels into your tissues. It’s also too large to move across the placenta to a fetus, so newborns don’t get protection from their mothers’ IgM antibodies.
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IgM is made during your primary immune response — that is, the first time your body encounters a particular pathogen. IgM antibodies respond to potentially harmful substances by:
Agglutination by IgM antibodies is one of the reasons that getting a blood transfusion of the wrong blood type is so dangerous — IgM agglutinates the proteins on the mismatched blood type, which are foreign to your body. They clump together, causing blood clots.
Immunoglobulins are made up of chains of proteins arranged in a “Y” shape. Each antibody is different at the open (forked) end, called the receptor or Fab region. This region has a unique shape that recognizes the antigen on the surface of viruses, bacteria or other potentially harmful substances. It attaches (binds) to these antigens at an epitope (area it can latch onto) to alert your immune system that it found something harmful.
All antibodies of the same type (IgA, IgD, IgE, IgG and IgM) have the same chain of proteins at the straight end, called the Fc or constant region. The Fc region attaches to immune cells. When immune cells find an IgM antibody that’s bound to an antigen, it starts a reaction to destroy the harmful invader.
In your blood, IgM exists as a pentamer. An IgM pentamer is a molecule with five IgM antibodies stuck together at the Fc ends, with the forked ends facing outward — you can think of it as looking like a star or a snowflake. This gives IgM the advantage of having many receptors that it can use to grab onto large pathogens with many epitopes, like some bacteria. It also helps them find even low levels of harmful substances.
Single IgM antibodies (monomers) are attached to the surface of B cells and act as B-cell receptors. These antibodies allow the B cells to identify harmful invaders and make more antibodies to defend your body.
Your provider can order immunoglobulin tests to check your levels of IgM and other antibodies. This is a blood test that can help diagnose immune system diseases and infections.
The range of normal IgM levels is between 40 and 250 mg/dL (milligrams per deciliter). These numbers can vary by lab. They also vary by age and are lower in infants younger than 9 months.
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The most common cause of high IgM antibodies is having a recent illness (infection). Other, rare causes include:
Diseases that affect your immune system’s ability to make antibodies and other immune cells can cause low levels of IgM. These include:
If you have a low level of IgM antibodies, you can get frequent infections, including:
You might also get frequent infections if you have high levels of IgM antibodies, especially if they’re abnormal (which means they won’t function properly). Additional symptoms of high levels of IgM antibodies include:
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IgG, IgM and IgA are all common antibodies that fight infection. IgM antibodies are the first to respond to an infection while your body makes IgG antibodies, which come in to fight later in the infection. IgA fights infections in your mucous membranes (like your intestines).
IgM antibodies are the first responders of your immune system. They help defend your body against an infection while the rest of your immune system mounts an even stronger fight. High levels of IgM usually mean you’ve recently had an infection. Rarely, abnormal levels of IgM are a sign of immune conditions or other diseases.
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Last reviewed on 06/10/2025.
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