Soy

Soy as a Food Allergen in the Human Body

Soy allergy is an immune system reaction to proteins found in soybeans (Glycine max). It is one of the most common food allergies, particularly in infants and young children, though some individuals retain the allergy into adulthood. Soy allergy can cause mild to severe allergic reactions, ranging from oral allergy syndrome (OAS) to anaphylaxis.

How Soy Allergy Develops in the Immune System

Initial Sensitization (Immune Priming)

• Soy proteins (allergens) are ingested → The digestive system breaks down soy into smaller peptides, but allergenic proteins like Gly m 5 (Vicilin) and Gly m 6 (Legumin) remain intact.
• Dendritic cells present soy allergens to naive T-cells in the gut-associated lymphoid tissue (GALT), leading to T-helper cell (Th2) activation.
• Th2 cells stimulate B-cells to produce IgE antibodies specific to soy proteins.
• IgE binds to mast cells and basophils, making them “sensitized” to soy allergens.

Subsequent Exposure (Allergic Reaction)

• Soy proteins bind to IgE on mast cells → This triggers mast cell degranulation, releasing histamine, prostaglandins, and leukotrienes.
• These inflammatory mediators cause allergic symptoms:
  1. Skin: Hives, itching, eczema
  2. Gastrointestinal tract: Vomiting, diarrhea, abdominal pain
  3. Respiratory tract: Wheezing, coughing, difficulty breathing
  4. Systemic reaction: Anaphylaxis (severe cases)

Major Soy Allergens and Their Role in Reactions

Soy Allergen	Protein Family	Molecular Weight	Immune Reaction	Heat Stability
Gly m 4	Bet v 1-Like Protein	~17 kDa	Causes oral allergy syndrome (OAS) due to cross-reactivity with birch pollen.	Low (destroyed by heat)
Gly m 5	7S Vicilin	~50 kDa	Strongly allergenic, linked to systemic reactions and anaphylaxis.	High (resistant to heat)
Gly m 6	11S Legumin	~60 kDa	Heat-stable and digestion-resistant, triggers severe soy allergy.	High
Gly m 8	2S Albumin	~8 kDa	Involved in severe allergic responses.	High
Gly m 3	Defensin	~6 kDa	May contribute to innate immune responses.	Moderate
Gly m 1	Kunitz Trypsin Inhibitor	~20 kDa	Inhibits digestion enzymes, may contribute to soy intolerance.	Moderate

• Gly m 4 causes mild allergic reactions (OAS) due to birch pollen cross-reactivity, while Gly m 5 and Gly m 6 are major allergens responsible for severe reactions.
• Soy allergens are highly heat-stable, meaning processed soy products (tofu, soy milk, soy protein isolates) retain allergenicity.

Symptoms of Soy Allergy

Mild to Moderate Reactions

• Skin: Itching, rashes, hives (urticaria), eczema flares
• Gastrointestinal (GI): Nausea, vomiting, diarrhea, bloating
• Respiratory: Runny nose, sneezing, mild wheezing
• Oral Allergy Syndrome (OAS): Itchy mouth/throat after consuming soy, often in individuals with birch pollen allergy (due to Gly m 4).

Severe Reactions (Anaphylaxis)    Requires immediate administration of epinephrine (EpiPen®).

• Airway restriction: Difficulty breathing, throat swelling
• Drop in blood pressure (anaphylactic shock)
• Dizziness, fainting, confusion
• Swelling of lips, tongue, or face
• Tachycardia (rapid heartbeat)

Cross-Reactivity with Other Allergens

Soy proteins share structural similarities with allergens in legumes, nuts, and pollen, leading to cross-reactivity.

Cross-Reactive Allergen	Common Sources	Cross-Reactive Soy Protein
Birch Pollen (Bet v 1 family)	Birch trees	Gly m 4
Peanuts	Peanuts, peanut butter	Gly m 5, Gly m 6
Tree Nuts	Almonds, hazelnuts, walnuts	Gly m 5, Gly m 6
Other Legumes	Lentils, chickpeas, peas	Gly m 5, Gly m 6
Wheat	Gluten-containing grains	Kunitz Trypsin Inhibitor

People with peanut or legume allergies have a higher risk of reacting to soy due to Gly m 5 and Gly m 6 cross-reactivity.

Soy Allergy Diagnosis

Skin Prick Test (SPT)

• Detects IgE-mediated allergic response to soy proteins.

Serum IgE Testing

• Measures specific IgE antibodies to soy allergens.

Oral Food Challenge

• Gold standard for diagnosing soy allergy, performed under medical supervision.

Component-Resolved Diagnostics (CRD)

• Detects specific soy allergens (Gly m 4, Gly m 5, Gly m 6) to assess reaction severity and cross-reactivity risk.

Managing Soy Allergy

Strict Soy Avoidance

• Avoid all soy-based foods:
  • Soy milk, tofu, soy sauce, miso, tempeh
  • Processed foods with soy lecithin, soy protein isolate
  • Some plant-based protein powders and meat substitutes

Alternative Protein Sources

Soy-Free Protein	Safe Alternative
Soy Milk	Oat milk, rice milk, almond milk (if no nut allergy)
Tofu/Tempeh	Chickpeas, lentils, pea protein
Soy-Based Meat Substitutes	Jackfruit, seitan (unless gluten-sensitive)

Emergency Treatment

• Epinephrine injection (EpiPen®) for anaphylaxis.
• Antihistamines for mild allergic reactions.
• Corticosteroids for prolonged symptoms.

Conclusion

• Soy allergy is triggered by Gly m proteins, with Gly m 5 and Gly m 6 responsible for severe reactions.
• Soy proteins are highly cross-reactive with peanuts, tree nuts, and legumes.
• Soy allergens are heat-stable, meaning cooked or processed soy products retain allergenicity.
• Severe soy allergies can lead to anaphylaxis, requiring strict avoidance and emergency preparedness (EpiPen®).