Tree nut allergies, like all food allergies, have multiple causes. Here's an in-depth overview of what tree nut allergies are, what causes them, and what an allergic reaction looks like.

What is a Tree Nut Allergy?

A tree nut allergy is an immune system reaction that happens when that person comes into contact with or eats a tree nut. Almonds, walnuts, hazelnuts, cashews, pine nuts, pistachios, pecans, macadamia nuts and brazil nuts are all called tree nuts. A person with a tree nut allergy can be allergic to one or more tree nuts.  

In a person with a tree nut allergy, their body has identified the tree nut as something dangerous. When that person eats that same nut, their body tries to get rid of the invader. The allergic reaction you see (hives, swelling, vomiting, coughing, sneezing, etc) is the body trying to get the nut out.  

Oral Allergy Syndrome

Tree nut allergies can also bring about oral allergy syndrome.

Oral allergy syndrome is sometimes called pollen-food allergy syndrome. Unlike tree nut allergies which are a reaction to the nut protein, oral allergy syndrome is a reaction to the pollen on the fruit, vegetable, or tree nut. 

OAS symptoms are limited to itchy mouth, scratchy throat, or swelling of the lips, mouth, tongue, and throat that stops once the food is swallowed or removed from the mouth. 

OAS is most common in people with seasonal allergies. Oral allergy syndrome typically does not happen in babies, and is more common in teens and adults, which is also when you start to see more cases of seasonal allergies or hay fever. People with birch pollen allergies are most likely to have OAS symptoms when eating almonds and hazelnuts.

Are Nut Allergies Genetic?

Babies are more likely to develop food allergies if they have parents with eczema, or asthma, not just food allergies. Having a family history of any allergic disease (eczema, asthma, seasonal allergies, or food allergies) can lead to a higher risk of food allergy. 

Having a parent with:

  • Asthma or eczema doubles the rate of food allergy. 
  • Seasonal allergies nearly doubles the rate of food allergy.
  • A food allergy increases the risk of developing a food allergy almost 6x.
  • Two or three allergic conditions (asthma, eczema, food allergy, seasonal allergies) also doubles or triples the rate of food allergy. 

While some of these numbers seem high, they don’t mean much. Almost half of Americans have seasonal allergies or hay fever, so basically every child is at increased risk of a food allergy. 

Another interesting thing to note is that a child is not more likely than any other child to have the same food allergies as their parents. Having an almond allergy does not increase the chance that your child will have an almond allergy, but it does increase the chance they will have a food allergy.

What Causes a Tree nut Allergy?

Beyond genetics, there are three things that seem to increase the risk of a tree nut allergy:

  1. Eczema
  2. Gut health
  3. An existing peanut allergy 

Eczema

The most studied risk for developing a food allergy is eczema. The earlier a baby develops eczema, and the more severe it is, the more likely he will develop food allergies. 

One study looked at babies with no, mild, medium, and severe eczema and tracked them to see who developed food allergies. The graph above shows the results of that study:

  • Babies who developed eczema by 3 months old, and needed prescription strength medicines, had a 50% rate of food allergy. 
  • Babies who developed eczema later, closer to 1 year old, and needed only emollients, had only a 5% rate of food allergy.

Eczema leaves breaks in your baby’s skin. When the skin is broken, little bits of things in the outside world can sneak through

Nuts are crumbly and get everywhere when people eat them. If you eat nuts in your house, you can be sure your baby is being exposed to nuts on their skin, no matter how often you vacuum. And if your baby has eczema, those tiny bits of nut might get into the cracks in his skin...and get through. 

Studies in mice have shown that if proteins get through cuts in their skin, their immune system goes on high alert. And if they scratch those cuts, their immune system really revs up. 

When nuts get through the skin, babies’ immune systems go on alert about nuts, and “learn” that nuts are a strange, potentially dangerous thing. 

Gut Dysbiosis

New research has shown that the trillions of bacteria that live in and on you, called your microbiome, actually play a big role in your body’s immune system. Having the wrong kinds of microbiome bacteria, or too few microbiome bacteria in your stomach and intestines, is called gut dysbiosis. 

In gut dysbiosis, instead of thickening your baby’s stomach lining, the bacteria leave it weak or make it inflamed.The weak intestinal lining of gut dysbiosis can cause leaky gut, where foods escape the intestines and instead of being processed, they get into the bloodstream and scare the immune system. 

Gut dysbiosis, when it causes inflammation, can also cause the immune system to stay on high alert all the time. Though doctors aren’t totally sure, one or both of these symptoms of gut dysbiosis may cause tree nut allergies.

Existing Peanut Allergy

Almost 30% of babies with a peanut allergy also develop an allergy to one or more tree nuts. It is not totally understood why. There is one theory that children in houses with peanuts are more likely to live in houses with tree nuts as well. However, a lot of the correlation between peanut and tree nut allergy actually refers to a sensitization, not a confirmed allergy. 

In one study, for example, 96% of the people who were sensitized to a tree nut they had never actually eaten, turned out to not have the allergy upon eating that nut. 

Is a Sensitization to a Tree Nut the Same as an Allergy?

A sensitization to one or more tree nuts means your baby’s body has created IgE antibodies specific to that tree nut’s protein. Sensitization is not an allergy

An allergy is when the body attacks a tree nut protein. Sensitization means your baby’s body has flagged a particular nut, but still may not decide to attack. The higher the sensitivity to a nut, the higher the probability of a true allergy. 

How Do You Test for Sensitization to a Nut?

A doctor can test for an IgE sensitization to tree nuts by checking for IgE antibodies specific to each type of tree nut in a blood test or a skin prick test. 

But as we mentioned above, proof of sensitization to a food does not confirm your baby has an allergy.

Blood Test

In a blood test, doctors take a small sample of your baby’s blood and check for IgE antibodies to one or more nuts. If your baby is sensitized to those nuts already, those antibodies will be in the blood sample. 

The number of antibodies in a blood sample reflects the likelihood or probability of a food allergy, not the severity of a food allergy. The higher the count of antibodies, the more likely a baby is to develop an allergy.

Skin Prick Test (SPT)

In a skin prick test, doctors place a tiny droplet of water containing an even tinier amount of a nut on your baby’s skin, and then prick the skin just enough to let that droplet in. After 20 minutes, the doctor checks your baby’s skin for a reaction — like redness or swelling — and measures the size of that reaction. The size of the reaction tells you the probability of a food allergy.

The results of a skin prick allergy test

So How Do You Actually Test for a Tree Nut Allergy?

The only real test for a food allergy is an oral food challenge. Your baby has to eat about 2-3 grams of a nut protein and see if they react. As primitive as it sounds, this is the only true test of a food allergy.  

My son during his food allergy test.

Here is a longer overview of food allergy testing.

Conducting oral food challenges is important because many people end up changing their lives and avoiding foods they aren’t actually allergic to. When a blood test or a skin prick test shows a lower probability of a true allergy, an oral food challenge can bring peace of mind. 

In one study, 86% of patients with a lower probability passed an oral food challenge and went on to eat tree nuts normally.

What are the Most Common Tree Nut Allergies?

First, it is important to remember that a “tree nut allergy” is actually one or more of the following:

  • Almond Allergy
  • Walnut Allergy
  • Cashew Allergy
  • Hazelnut Allergy
  • Pecan Allergy
  • Pine Nut Allergy
  • Pistachio Allergy

Tree nut allergies, as a total group, are very common, and are one of the eight most common food allergies. About 1% of Americans have an allergy to one or more tree nuts. There aren’t good estimates for the rates of each of the nut allergies individually. 

Some tree nut allergies happen in pairs, likely because of the similarities between their proteins. Tree nuts in the same family are genetically similar and therefore have proteins that look alike.

  • Cashew and pistachio are both in the Anacardiaceae family, so when a person is allergic to one, they are more likely to be allergic to the other
  • Walnut and pecan are both in the Juglandaceae family, so when a person is allergic to one, they are more likely to be allergic to the other.

Some studies estimate that an almond allergy is the most common. However, there is a known phenomenon that tree nut allergies occur in proportion to how common they are in American foods. This could be because many people will never eat certain nuts their whole life. Or, more likely, it is because babies only tend to develop allergies to food that is in their environment but they are not eating. 

Babies do not tend to develop allergies to 1) foods they eat and 2) foods they are not exposed to in their environments.

Lastly, because tree nuts come from fruit bearing trees, many people with IgE sensitization to tree nuts develop a condition called Oral Allergy Syndrome (OAS) or Pollen Allergy Syndrome which only has mild oral symptoms like a tingling or itching in the mouth. Some studies that count OAS together with oral food challenge-verified tree nut allergies show higher rates of tree nut allergies, closer to 5%.

The Signs and Symptoms of A Tree Nut Allergy

It’s important to understand the signs and symptoms of food allergies in babies as you prepare to introduce your baby to nuts.

Allergic Reactions

An allergic reaction to tree nuts in a baby most commonly presents as:

  • Redness around the mouth or skin that came into contact with the nut
  • Hives 
  • Stomach distress such as vomiting or diarrhea. 
  • Runny or stuffy nose, sometimes with clear discharge
  • Redness or itchiness of the nose
  • Swelling of the face, including puffiness around the eyes

Redness around baby's mouth from allergic reaction

Redness around the mouth during an allergic reaction

Swelling under baby's eye from allergic reactionSwelling of the face and eye area from an allergic reaction


While more serious symptoms are very rare, serious symptoms of a tree nut allergy in babies include:

  • Coughing/wheezing
  • Difficulty breathing 
  • Swelling of the mouth, including the lips and possibly tongue
  • Fever higher than 101.4 degrees or lower than 97 degrees
  • Swelling of the throat and difficulty swallowing
  • Weak pulse
  • Losing consciousness 

Allergic reactions to tree nuts happen almost immediately after eating or touching one. In rare cases, however, reactions can happen up to 4 hours later. Since most babies eat every 2 -3 hours, delayed reactions can be difficult to diagnose. 

It’s best to feed your baby nuts in a controlled setting where you have time to monitor them for a few hours afterwards.

What To Do If Your Baby Has a Reaction to Tree Nuts

If your baby has a mild reaction to tree nuts, such as redness around the mouth or hives, an age and weight appropriate amount of Benadryl (most likely 2.5ml) will reduce the reaction. Call your pediatrician’s office and stay with your baby. Most pediatricians have an on-call service to help you decide if additional help is needed. 

If your baby has a severe reaction such as coughing or wheezing, OR both skin symptoms and stomach symptoms, bring them to an urgent care or emergency room immediately for observation and help. 

Most parents don’t realize having two types of a mild reaction is considered a severe reaction. It means that your baby’s body is responding aggressively to the nuts, and a doctor should evaluate your baby to make sure the reaction has passed. 

After either a mild or severe reaction to nut, your baby will be referred to a specialist to properly diagnose the food allergy and give you an action plan moving forward.  

Can Babies Outgrow Tree Nut Allergies?

About 10% of babies will outgrow a tree nut allergy by age 5. Tree nut allergies, along with peanut allergies, are much more persistent than dairy and egg allergies

Food allergies typically develop between 6 months and 1 year, though some toddlers develop a tree nut allergy between 1 and 3 years. 

Historical studies of children with nut allergies show a child is more likely to outgrow a tree nut allergy by age 5 if they:

  • only have mild reactions to nuts
  • only have an allergy to one tree nut
  • do not also have eczema or asthma