When a patient reacts to something they've eaten, the useful question isn't only which food. It's how their immune system is responding, because that changes everything about what happens next.
Three antibodies do most of the work in food and inhalant responses: IgE, IgG and IgA. They run on different timelines, produce different symptoms, and carry very different clinical weight. Knowing which one you're looking at is the difference between "this needs to be avoided for life" and "let's trial removing it for six weeks and see what happens."
Here's how to tell them apart, and what each test can and can't tell you.
The Short Version
|
Timing |
Minutes to two hours |
Hours to days |
Hours to days |
|
Called |
Allergy |
Sensitivity |
Sensitivity |
|
Severity |
Mild through to life-threatening |
Usually low-grade and chronic |
Usually low-grade and chronic |
|
Typical action |
Strict avoidance, when history supports it |
Structured elimination and reintroduction |
Structured elimination and reintroduction |
If you remember one thing: IgE is a diagnosis. IgG and IgA are a starting point for a conversation.
IgE: The Fast One
E for emergency. It's a crude mnemonic but it works, because IgE is the antibody behind reactions that happen quickly and can turn serious.
IgE antibodies sit pre-attached to mast cells throughout the body. When the trigger shows up again, those cells release histamine almost immediately. That's why the window is minutes to a couple of hours rather than days, and why these reactions can escalate.
What IgE Looks Like
Anaphylaxis is the reaction everyone knows, but most IgE responses never get that far. More commonly you'll see:
Can progress to severe reactions: airway swelling, weak pulse, and anaphylaxis.
How IgE is Tested
Skin prick testing places a small amount of allergen on the skin and looks for a wheal and flare. It's fast, it's done in the room, and results come back in minutes. It needs the patient off antihistamines, it needs clear skin to test on, and it's limited in how many allergens you can run at once.
Specific IgE blood testing measures the antibody directly for each allergen tested. It runs a large panel from a single sample, it isn't affected by antihistamines, and it's the better option when a patient has extensive eczema, dermographism, a history of anaphylaxis, or is very young. At NutriPATH we can run up to 240 markers from one collection.
Neither method is simply "better." Skin prick and specific IgE perform comparably in most contexts, and the right choice depends on the patient in front of you.
Collection has also become far less of an obstacle. Alongside standard serum draws, specific IgE testing can now be run from finger-prick microtainer collections and dried blood spot cards, which means patients who would otherwise avoid testing can collect at home.
The Part That Gets Missed
A positive IgE result on its own is not a diagnosis of allergy. It shows sensitisation, which means the antibody is present.
Clinical allergy requires a positive result plus a history of reacting to that food. Plenty of people test positive to foods they eat without any trouble at all, and removing those foods on the strength of a number does them no favours.
Test result plus clinical picture. One without the other isn't an answer.
IgG: The Slow One
IgG is the most abundant antibody in circulation, and food-specific IgG is the most commonly ordered sensitivity test in functional practice.
It's also the one that needs the most honesty about what it is.
What the Evidence Actually Says
Food-specific IgG rises in response to foods a person eats. That much is well established. Where the debate sits is what an elevated result means.
Conventional allergy and immunology associations take the position that food-specific IgG reflects normal exposure, and may even reflect tolerance rather than a problem.
In functional practice, IgG panels are used differently: not to diagnose anything, but to narrow down a very long list of candidate foods into a short list worth giving an avoidance trial. Used that way, with a structured elimination and a genuine reintroduction, the test earns its place as a starting point.
Used the other way, as a list of foods to remove permanently, it causes harm through unnecessary dietary restriction.
The distinction matters, and it's worth being straight with patients about it.
What Patients Report
IgG-associated reactions are delayed, which is exactly what makes them hard to pin down. A reaction on Tuesday to something eaten on Sunday rarely gets connected to the food.
Because the symptoms are low-grade and persistent rather than dramatic, they're often written off as ageing, stress or simply normal. Commonly reported:
These are patterns patients describe, not effects the test has been shown to cause. Keep the distinction in mind when you're explaining results.
How IgG is Tested
IgG food panels are run from blood, either serum or dried blood spot.
Methodology varies between laboratories, and it matters. At NutriPATH our panels use Enzyme-Linked Immunosorbent Assay (ELISA), which has the largest body of peer-reviewed work behind it of any method in this space, including independent comparisons reporting stronger reproducibility than alternative approaches.
The practical advantage of ELISA is flexibility of collection. Serum or dried blood spot both work, so geography stops being a barrier.
What To Do With an IgG Result
Foods flagged on IgG are candidates for a temporary elimination, usually several weeks, followed by deliberate reintroduction one at a time.
The reintroduction is the part that gets skipped, and it's the part that produces the actual information. A food that comes back without incident was never the problem, and the patient gets it back.
IgA: The Barrier One
IgA works at the mucosal surfaces. The gut lining, the sinuses, the airways. It's the antibody standing at the places where the outside world meets the inside of the body, and its everyday job is defending that lining against bacteria and viruses. IgA is not a stand-alone test like the other two. The benefit is insight into the immune system and usually some additional foods that looked like a low reaction on the IgG.
Secretory IgA and Serum IgA Are Not the Same Thing
This trips people up regularly, so it's worth being precise.
Secretory IgA is two IgA molecules bound together and secreted into the gut. It's measured in stool, and it's a marker of mucosal immune function.
Serum IgA is measured in blood. Food-specific serum IgA is what appears on a sensitivity panel.
They're different analytes, measured in different samples, telling you different things. A result for one says nothing about the other.
One more point on serum IgA worth knowing: total serum IgA should be measured alongside any IgA-based serology, because selective IgA deficiency produces false negatives. It affects roughly one in five hundred people.
What IgA Patients Report
Reported patterns overlap heavily with IgG, which is part of why the two are usually run together:
How IgA is Tested
Same process as IgG: a blood-based food sensitivity panel, serum or dried blood spot.
Running IgG and IgA together gives you a fuller picture of which foods are worth investigating, and the same caveat applies to both. These results guide an elimination trial. They don't diagnose anything on their own.
Choosing Between Them
The decision is usually simpler than it looks, and it comes down to the patient's story.
Reach for IgE testing when the reaction was fast. Minutes to a couple of hours, with hives, swelling, wheeze, vomiting or anything resembling anaphylaxis. Also when you're assessing environmental allergens for a patient with allergic rhinitis or asthma.
Consider IgG and IgA panels when symptoms are delayed, chronic and low-grade, the patient has already tried eliminating foods without a clear result, and you want a structured way to narrow a long list down to something testable.
Don't reach for either as a general screen on a patient with no symptoms. Testing without a clinical question produces results nobody can act on, and in food testing that usually means a patient eating less food for no benefit.
The Takeaway
IgE, IgG and IgA aren't three versions of the same test. They answer different questions.
IgE tells you whether a patient has a true allergy, with a validated test and a clear clinical consequence. IgG and IgA give you a structured place to begin an elimination trial, within a process you guide and review.
Used for what they're each good at, they make your work more precise. Used interchangeably, they create confusion and unnecessarily restricted diets.
The most valuable thing you can give a patient here isn't a list of foods to avoid. It's a clear explanation of what their results do and don't mean.
To learn more about food sensitivity testing and building panels around your patients' needs, visit:https://nutripath.com.au/product-category/health-condition-or-goal/allergies-intolerances/
What's the Difference Between IgE, IgG & IgA Antibodies?
All three are immunoglobulins, but they drive different responses. IgE produces immediate allergic reactions, appearing within minutes to hours and ranging up to anaphylaxis. IgG is the most abundant antibody in blood and is associated with slower responses, which is why food-specific IgG is used in functional practice to explore chronic, low-grade symptoms. IgA works mainly at mucosal surfaces such as the gut, sinuses and airways, where it defends the lining against pathogens.
Why Are IgE Reactions Immediate While IgG & IgA Reactions Are Delayed?
IgE antibodies are already bound to mast cells, so on re-exposure those cells release histamine within minutes, producing hives, swelling or anaphylaxis. IgG and IgA associated reactions are understood to build more gradually, with symptoms such as bloating, fatigue, brain fog and headaches appearing hours to days later. That delay is what makes the responsible food so difficult to identify. It's worth noting that while delayed IgG and IgA food reactions are a foundation of functional medicine practice, the model is debated within mainstream allergy and immunology.
Should I Order IgE Allergy Testing or IgG & IgA Sensitivity Testing?
IgE testing, whether skin prick or specific IgE blood testing, is the validated tool for diagnosing true allergy, and a confirmed allergy usually means strict avoidance. IgG and IgA panels, typically run by ELISA from serum or dried blood spot, are used to identify candidate foods for a temporary elimination and reintroduction, not as a standalone diagnosis. Major allergy organisations note that IgG food testing is not validated to diagnose allergy or intolerance, so it belongs inside a clinician-guided elimination process rather than standing on its own.
Does a Positive IgE Result Mean My Patient is Allergic?
Not by itself. A positive result shows sensitisation, meaning the antibody is present. Clinical allergy requires that result together with a history of reacting to the food. Many people test positive to foods they tolerate perfectly well, which is why test results are interpreted alongside the clinical picture rather than in place of it.
How Long Should an Elimination Trial Run?
Commonly several weeks, long enough to see whether symptoms shift. The critical step is the reintroduction afterwards, one food at a time, because that's where the useful information comes from. A food that returns without symptoms was never the issue and belongs back in the diet.