Immunoglobulins: what IgG, IgA and IgM say about your immune system

Three lines sit under each other on your result: IgG 11.2 g/L, IgA 2.1 g/L, IgM 1.3 g/L. Immunoglobulins are antibodies, proteins your immune system makes to recognise pathogens. Those three classes together say something about timing above all.

What strikes me: almost every page lists the classes alphabetically and then stops. IgA, IgD, IgE, IgG, IgM. Done.

That leaves the most useful question untouched. Why does a doctor order IgM and IgG in one go so often?

The answer sits in timing. IgM appears first in a new infection, IgG comes later and stays measurable far longer (PMID 20176265). That order is the thread running through this article.

What are immunoglobulins?

Immunoglobulins are antibodies: Y-shaped proteins made by your B cells. They stick to a fragment of a virus, bacterium or toxin and mark it for clearance. Humans have five classes, IgG, IgA, IgM, IgD and IgE, each with its own place in the body and its own job (PMID 20176265).

Antibodies belong to your adaptive immunity, the part that learns. Your innate immunity reacts immediately and roughly the same way to everything that comes in. The adaptive side remembers (PMID 20176265).

That memory explains why an IgG level can still be measurable years after an infection has passed.

How the two systems work together sits in our overview of how your immune system works.

IgG, IgA, IgM and IgE side by side

The four classes you meet on a result sit in different places and do different work. IgG lives mainly in your blood and makes up roughly 75 percent of your antibodies. IgA guards your mucous membranes, IgM arrives first and IgE belongs to allergy (PMID 20176265).

The descriptions in this table are general. Reference ranges differ per laboratory and per age group, so the range printed on your own result is the range that counts.

Why IgM and IgG are often measured together

Because the pair says something about the moment. IgM is the class your immune system produces first against a new pathogen. IgG follows later and usually stays around far longer (PMID 20176265). A lab measuring both is therefore looking at the time course of an immune response.

Think of a factory building a quick prototype before the final production run. IgM is the prototype: fast, a bit rougher. IgG is the run: more precise, and kept in stock for years.

That is why some infection tests report an IgM and an IgG result next to each other.

I will say straight away that this is a general pattern, not a rule you apply to your own result. The picture differs per pathogen and per test method, and exceptions are plentiful. Only a doctor who knows your story and the test used can read such a combination.

What do an IgG level and an IgM level say on your result?

On their own, little. A total IgG or total IgM is the sum of thousands of different antibodies mixed together. Such a number says something about production in general, not about defence against one specific disease. Next to your symptoms, your medications and the rest of your blood count it becomes usable.

Why those ranges differ per lab is covered in our piece on reference ranges in blood testing.

Low or raised immunoglobulins can have many causes. An infection, chronic inflammation, a liver problem, certain medications and rarer immune conditions all sit on that list.

One out-of-range line is a starting point for a conversation, not a diagnosis.

Antibodies are also just one layer. The number of immune cells says something else, and in a large Danish population study of 98,344 people a low lymphocyte count went together with more infections (PMID 30383787). What that does and does not mean sits in low lymphocytes explained.

IgA deficiency: common, and usually silent

Selective IgA deficiency is described as the most common primary immunodeficiency. Someone makes little or no IgA while IgG and IgM are normal. Most people in whom it is identified have no symptoms from it, and it often comes to light by chance (PMID 20101521).

That "by chance" is not a detail. It explains why a low IgA on a result is rarely the drama people fear.

There is one practical consequence that almost nobody spells out. The blood test for coeliac disease leans on IgA antibodies against tissue transglutaminase.

If you barely make IgA, that test can come back low while coeliac disease is genuinely present. A falsely reassuring result, in other words.

That is why laboratories often run total IgA in the same tube as the coeliac antibodies. It lets them judge whether that antibody test can show anything at all in this person.

An example with numbers. Take two people who both take the same coeliac test and both get a negative result back: in the first, total IgA sits at 2.4 g/L, so the result can simply be interpreted. In the second it is a different story.

In the second, total IgA sits at 0.05 g/L. That same negative result then says almost nothing, because the body barely makes the antibody the test measures.

Same result, very different meaning. That is why total IgA exists with us as a separate blood value and not as a footnote.

Thuisarts, the Dutch GP-backed patient site, describes coeliac disease as a lasting abnormal reaction to gluten that damages the small intestine. The diagnosis cannot be made from one blood value: that stays work for your GP and a specialist.

Immunoglobulins and IgE answer different questions

IgE is an immunoglobulin too, but it answers a different question. IgG, IgA and IgM are about defence against infections. IgE is about allergy and about the response to parasites. Its concentration in blood runs many thousands of times lower than that of IgG (PMID 20176265).

I see that confusion constantly in the questions that reach us. Someone reads the word "immunoglobulin" on an allergy page and assumes an IgG measurement says something about hay fever.

It does not.

If you want to know what a raised IgE can mean, that is a separate story: what a raised IgE level in your blood means.

What do you do with an out-of-range result?

You read it next to the rest. Immunoglobulins are rarely judged alone: a doctor also looks at your infection history, your medications, your white blood count and your inflammation markers. Adults average two to three colds a year, so sniffling regularly says little on its own (PMID 12517470).

That context decides whether a number carries meaning.

Which values can give insight during persistent infections is worked out in weakened immune system: causes and blood values.

To follow your own values, you can pick immunoglobulin IgG separately, or read them alongside the broader extended health checkup.

Concretely: copy out your three values, with your own lab reference range next to them. Note which infections you had over the past year and which medications you take. Bring that list to your GP and ask what the combination of IgG, IgA and IgM means in your situation.

References

1. Chaplin DD. Overview of the immune response. J Allergy Clin Immunol. 2010;125(2 Suppl 2):S3-S23. PMID 20176265.
2. Yel L. Selective IgA deficiency. J Clin Immunol. 2010;30(1):10-16. PMID 20101521.
3. Warny M, Helby J, Nordestgaard BG, et al. Lymphopenia and risk of infection and infection-related death in 98,344 individuals from a prospective Danish population-based study. PLoS Med. 2018;15(11):e1002685. PMID 30383787.
4. Heikkinen T, Järvinen A. The common cold. Lancet. 2003;361(9351):51-59. PMID 12517470.
5. Thuisarts. Patient information on immunity, infections and coeliac disease. Available via thuisarts.nl.

Every blood test result at Vitalcheck includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.