Common signs and symptoms of a soy allergy can include:

• Skin rash/hives;
• Mild wheezing or coughing;
• Itching in the mouth;
• Digestive problems (e.g. nausea, vomiting, or diarrhea);
• Vomiting.

Soy allergy symptoms vary from person to person, but most reactions are stomach or skin-related. Reactions can also range from mild to severe, including the life-threatening reaction anaphylaxis.

Soy Allergy Triggers

Although soybeans themselves may not be a part of many diets, soy can be a hidden ingredient in many foods—that’s why it is important to read the label and ask about ingredients before buying or eating certain foods. Ingredients in packaged foods can change at any time without warning, so check the ingredients carefully every time.

Soybeans and products made from the bean-like miso or tofu make up a substantial part of the diet in Asia. This is one reason why Asian cuisines are considered high-risk if you have a confirmed soy allergy diagnosis. There is also a possibility of cross-contact or cross-contamination—when one food comes into contact with another food and their proteins mix—even if you order a soy-free dish.

Soybean oil is also used in some industrial components. It can be found in linoleum and glue in the plywood industry, where it is considered an occupational allergen. 

Soy can be found in many processed foods, including:

• Candy;
• Crackers;
• Processed meats;
• Low-fat peanut butter;
• High-protein energy bars;
• Canned soups;
• Sauces;
• Soy and soy products;
• Baked goods;
• Canned tuna and meat;
• Cereals.

Cross-Reactivity of Soy Allergy

Cross-reactivity is when the proteins in one food item are similar to the proteins in another, and your body's immune system views them as the same. There is a high degree of cross-reactivity between soy and birch trees. Up to 10% of all patients with birch sensitization may also be at risk of reactions to soy, including anaphylaxis. So, people who are allergic to birch-related tree pollen are often advised to also avoid eating or drinking large amounts of low-processed soy, like soy milk, and particularly during the pollen season.