Strawberry (Fra a 3) IgE

If you've had an itchy mouth, hives, or worse after eating fruit, the hard part is working out which foods are risky and which ones you can keep eating. This test looks at one narrow piece of that: whether your immune system has made allergy antibodies against a sturdy strawberry protein that survives cooking and processing. A positive result most often points to a broader plant food allergy, one that can occasionally cause serious reactions.

The protein is Fra a 3 (Fragaria ananassa allergen 3), the strawberry's version of a lipid transfer protein (LTP). LTPs are a family of tough plant proteins found in peaches, apples, nuts, and many other foods, and LTP allergy is the most common primary food allergy among adults in Mediterranean countries. This is a specialist measurement without standardized cutoffs, so its value comes from reading it next to your symptoms and your other allergy results.

What a Positive Result Usually Means

IgE (immunoglobulin E) is the antibody your body makes once it has learned to treat a harmless substance as a threat. It sits on immune cells that release histamine when they meet that substance again. This test measures how much IgE in your blood recognizes Fra a 3, reported in kU/L, the lab unit used for allergy antibody tests.

Because LTPs from different plants look alike, antibodies made against one often stick to the others. In the main study of this protein, antibody binding to Fra a 3 in people with LTP allergy looked mostly like cross-reactivity from peach and apple LTP rather than strawberry as the original trigger.

In southern Europe, the protein that usually gets the immune system started is Pru p 3, the peach LTP. Pollen can play that role too. In a study of 148 people in China with mugwort pollen allergy, LTP sensitization was a major factor in their pollen-related food allergies and may have contributed to whole-body reactions.

What this means for you: a positive result is mainly a sign that you belong to the LTP-sensitized group. The next question is which of your foods carry real risk, and this marker on its own can't answer it.

A Weak Allergen in Its Own Right

Researchers in the Mediterranean area tested how strongly the strawberry LTP provokes an allergic response compared with its peach and apple relatives.

Source: Zuidmeer et al., 2006 (all three rows).

What this means for you: a positive Fra a 3 result says little by itself about whether strawberries will make you sick. In the same study, the authors concluded that Fra a 3 did not seem clinically relevant. When strawberries do cause trouble, the cause is often profilin or the birch-pollen-related protein Fra a 1, and those reactions often stay in the mouth.

When LTP Allergy Turns Serious

The reason to care about an LTP result is the range of reactions this protein family can cause. In a study of 45 people with LTP syndrome, symptoms ran from mouth itching and hives to stomach symptoms and anaphylaxis, and some severe reactions depended on a cofactor.

A cofactor is something that lowers your threshold for a reaction on a given day. The common ones are exercise, alcohol, and anti-inflammatory painkillers such as ibuprofen (NSAIDs). They explain how someone can eat a food safely most of the time and then react badly after a workout or with wine.

Predicting severity is harder. Studies disagree on whether peach LTP antibody levels track how bad reactions get, and the ranges in allergic and tolerant people overlap. In a small study of 47 people with peach allergy, how tightly their IgE gripped Pru p 3 was linked to severe reactions, though that's one small study. Breadth matters as well: among 500 allergic patients, 81% had antibodies to several different allergen sources, and that broader pattern went with more severe symptoms.

Why Antibody Levels Don't Map Neatly Onto Reactions

It seems odd that one person can carry antibodies to a food and eat it without trouble while another reacts with this test negative. Both happen with this protein. The test detects sensitization, which is your immune system recognizing a protein. Sensitization only becomes an allergy when eating the food causes symptoms, many sensitized people never get there, and a strawberry reaction can come from a protein this test doesn't measure.

The size of the number doesn't settle it either. In a small study of 19 people with LTP allergy, specific IgE values varied widely and did not predict who reacted, and no value was linked to whole-body symptoms or anaphylaxis. A comparison of 182 sensitized adults in Antwerp and Barcelona reached a similar conclusion: predicting outcomes from LTP antibody levels is hard, and test performance differed by region.

One measure has done better. In a UK study of 285 people, the ratio of peach LTP antibodies to total IgE was the most accurate marker for separating true LTP allergy from sensitization alone. That ratio has been tested mainly in UK and northern European patients, so it may perform differently where LTP allergy is common, such as southern Europe. Even so, Pru p 3 and total IgE are the two numbers most worth having next to a positive Fra a 3 result.

How It Compares With Other Allergy Tests

Allergy tests trade one strength for another. Whole-food extract tests and skin prick tests catch most true allergies but also flag many people who aren't allergic. Single-protein tests like this one are more specific and miss more cases, which is the split a large meta-analysis of food allergy tests found.

For fruit, skin testing with the fresh food has beaten commercial extracts. In one study, positive prick tests with fresh food matched food challenge results 91.7% of the time, compared with 58.8% for commercial extracts.

When tests disagree, the reference standard is a supervised oral food challenge. You eat gradually larger amounts of the food while a clinician watches for a reaction.

Why a Single Reading Can Fool You

The biggest source of error with this result is the lab, not your body. A few things can push a reading around without any change in your allergy.

• Lab platform: different allergy antibody systems measure overlapping but not identical antibodies, and their numbers don't convert cleanly between platforms. Commercial labs have disagreed on whether the same sample is positive or negative, so a jump after switching labs may come from the lab.
• Run-to-run noise: on one multiplex chip platform, repeat runs of the same patient's sample varied by 22.9% on average. Shifts smaller than that are hard to tell apart from noise.
• Recent allergen exposure: in a pilot trial, putting birch pollen allergen into the nose raised birch-specific IgE. That finding comes from birch, so treat heavy exposure right before a draw as a possible push upward.
• Allergy biologics: omalizumab lowers free IgE, yet standard assays can read IgE higher because they detect antibody bound to the drug. In some people that rise fades after a few months, and the tests can't separate free from drug-bound IgE, so readings taken on omalizumab tell you little. Results for the asthma drugs mepolizumab and benralizumab are inconsistent: one study found benralizumab cut total IgE by about 35% while mepolizumab didn't change it, and other studies have found little change or a different pattern. These findings come from total IgE, so a reading taken on any of these drugs isn't directly comparable to one taken off them.

Other lab factors can occasionally affect a result, which is one more reason to confirm a surprising number before acting on it.

Tracking Your Result Over Time

One reading tells you whether you're sensitized today. A series tells you whether that's changing, which matters most if you're starting immunotherapy, starting a biologic, or noticing new reactions.

Get a baseline, retest in 3 to 6 months if you start a treatment aimed at your allergy, then at least once a year while you're managing it. Use the same lab and platform every time, draw when you're well, and treat changes of less than about 20% to 25% as noise until a second test confirms them.

The evidence that treatment moves allergy antibodies comes mostly from peach LTP and food immunotherapy in general. In a 6-month peach LTP trial, Pru p 3 IgE and IgG4 both rose, and in a 1-year trial Pru p 3 and peanut LTP IgE fell while IgG4 rose. A falling Fra a 3 result may support progress, but direct trial data for this exact strawberry protein are missing.

What to Do With a Positive or Unexpected Result

A positive result with no symptoms from strawberries is a common pattern and often needs no change to your diet. The useful next step is finding the protein behind it: check Pru p 3 and total IgE, plus LTPs from foods you eat often, such as apple, hazelnut, or walnut.

If you've had hives, throat symptoms, breathing trouble, or a reaction tied to exercise, alcohol, or painkillers, treat it as possible LTP syndrome. That pattern is worth taking to an allergist, who can judge your risk of severe reactions, decide whether you need an epinephrine prescription, go over which cofactors to avoid, and run a supervised food challenge if the tests leave doubt.

If strawberries bother you but this test is negative, the likelier culprits are profilin or Fra a 1. A birch profilin test (Bet v 2) or a combined strawberry component test can point that way.

If you have pollen allergies, especially to mugwort, a positive result may trace back to the pollen. Testing for mugwort's LTP (Art v 3) helps sort that out.