Learning that you or your loved one has an allergy to a food often sparks a long list of questions. One common question that many families have is “If my child is allergic to one item, what else are they allergic to?” Another common question is “Where should I start when either trying new food items or adding foods back into my diet?” If you are facing these questions, you are not alone!! Before we tackle some of these questions, remember: Each of us is unique and there is no substitute for individualized guidance and recommendations from your healthcare team. Now, let’s take a look at something called cross-reactivity to help you get the conversation started with your healthcare team if you are facing these questions.
Finding the Food Allergy?
An allergy to food is allergic reaction, or overreaction by your immune system, to the proteins in the food. For example, many children have a milk allergy, which more specifically is a cow milk protein allergy. The body's immune system "recognizes" that the protein in the food is not the same as the protein in our own body. For most people the immune system is able to ignore these "foreign" proteins. But for people with a food allergy, their immune system mounts a response to that protein.
The proteins found in one food item can be similar to the proteins in other foods, especially related foods. Sometimes the body's immune system cannot tell the difference between the proteins in two foods and has an allergic reaction to both of them. The question becomes, If you are allergic to one food item, will you also be allergic to the protein in a related food? This concept is called “Cross-Reactivity”. But what does this mean for you?
Food allergies can involve many types of responses; you may get a rash when eating a certain food or it could be life threatening, like anaphylaxis. The most common immune response in a food allergy is when your body makes something called IgE (immunoglobulin E) antibodies to the protein of the food allergen. This results in a variety of physical reactions or symptoms such as skin itching, hives, nausea, vomiting, diarrhea, breathing difficulty such as wheezing or coughing, or the life-threatening reaction called anaphylaxis.
As you likely well know, diagnosis of food allergies is not an easy process. Experts at the Jaffe Food Allergy Institute in New York recommend that diagnosis of food allergies include careful interpretation of a variety of factors including physical examination, skin test or RASTs, detailed patient history, and oral food challenges or elimination diets. No single test on it's own is a perfect predictor of an allergy to a given food. This extensive testing and the possible conflicting results are just one of many reasons why individualized treatment and recommendations are needed, and why there is no substitute for the individual guidance you will receive from your healthcare team.
What is Cross-Reactivity?
So where should you start when either trying new foods or adding foods back into your diet? To help allergists identify related foods of concern, research was conducted into how likely people with a given food allergy are to react to other related foods. This was done using tests that are predictive, but not 100% accurate, so they're just indicators. Family allergists may use this data to help determine what advice to give their patients about where to start when either trying new foods or adding food items back into the diet. (If you really want to read the review of the clinical data, the full reference is noted below.)
In some cases, the data reveals a significant chance of having an allergic reaction (or at least a strongly positive allergy test) to a new food when the protein is related. For example, if you are allergic to cashews, you have a pretty high likelihood to also be allergic to pistachios and/or mango.1 There are many families of foods that may be linked, so it is best to consult with your healthcare provider to determine the extent of your food allergy and the potential for cross-reactivity. As noted above, your healthcare team will offer guidance for you specifically after all the information has been collected and evaluated.
How to Spot Foods That Might Cause Cross-Reactivity?
The table below shows some of the potential cross-reactivity revealed by this research review, and was developed by an allergist at the Jaffe Food Allergy Institute at the Mount Sinai School of Medicine in New York. Clinicians might use a table like this when determining where you might start when either trying new foods or adding food items back into your diet.
A clinician would look for your known food allergen in the left column. The column on the right gives an indication of the risk that there will be an allergic reaction to one of the foods that are listed in the center column. For example, for a patient allergic to cow milk, the available research shows there is a 92% chance the patient will have a positive allergy test to goat milk, but only a 4% chance of a positive allergy test to mare (horse) milk and a 10% chance of a positive allergy test to beef and beef products. Remember, a positive allergy test is NOT the same as an allergic reaction, but it can help the allergist gauge how likely an allergic reaction is. Depending on the results of the test, the allergist might recommend avoiding the food, may suggest having an in-office food challenge, or they may say that an allergic reaction is very unlikely.
What is the best way to introduce new foods to the diet?
Once your healthcare team has a plan for you, the next step is trying the food items suggested. Again, your healthcare team will likely have a very specific plan for you. They may say to just introduce the food normally. They may suggest that you try foods at home, starting with a small amount and then waiting a few days before trying the food item again or even moving on to the next food item.
If you have had sever food reactions in the past and/or a test result in the middle of the range, then they may only recommend new foods be tried as an oral food challenge. This should only be done under strict medical supervision (e.g. in a doctor’s office) and involves trials with small amounts of the food causing the allergy or a potential cross-reactive food. Depending on your results, the healthcare team will guide as you continue to explore and try new food items.
In closing, it's very important to discuss any questions with your healthcare team. A lot of the latest research in food allergies suggests that, for some people, avoiding foods in early childhood may actually INCREASE the likelihood of developing an allergy to that food. So don't make these decisions on your own, but be prepared to ask your healthcare team the questions you have about introducing new foods so that you're prepared with the knowledge you need!
Oringally published 12/22/15 by Ellen Avery, MS, RD, CNSC2.
Updated 6/8/17 by Kristin Crosby MS, RDN, LDN.
Sicherer SH. Clinical implications of cross-reactive food allergens. J Allergy Clin Immunol. 2001;108(6):881-90.