Walnuts (Juglans spp.)

Juglans regia, Juglans nigra, Juglans californica (Juglandaceae)”, referring to walnuts, including English walnut (Juglans regia), black walnut (Juglans nigra), and California walnut (Juglans californica).

Botanical Characteristics of Walnuts (Juglans spp.)

• Scientific Name: Juglans regia, Juglans nigra, Juglans californica
• Family: Juglandaceae (Walnut family)
• Common Names: English walnut, Persian walnut, Black walnut, California walnut
• Native Regions:
  • Juglans regia (English walnut) → Europe, Middle East, Central Asia.
  • Juglans nigra (Black walnut) → North America.
  • Juglans californica (California walnut) → Western United States.

Tree and Nut Description

• Walnut trees are deciduous, growing 10–30 meters tall.
• The nuts are enclosed in a hard shell, surrounded by a green husk.
• Walnuts are among the most common tree nut allergens, with significant cross-reactivity to pecans and other tree nuts.

Walnuts as a Food Allergen

Walnuts are classified as a major food allergen by the U.S. FDA, requiring mandatory labeling.

Allergenic Proteins in Walnuts

Allergen Name	Protein Family	Molecular Weight (kDa)	Function	Cross-Reactivity
Jug r 1	2S Albumin	~9 kDa	Seed storage protein	Cross-reacts with peanuts, Brazil nuts
Jug r 2	7S Vicilin	~45-50 kDa	Storage protein	Cross-reacts with almonds, hazelnuts
Jug r 3	Lipid Transfer Protein (LTP)	~9 kDa	Defense protein	Linked to peach, apple, cherry allergies
Jug r 4	11S Globulin (Legumin)	~40 kDa	Storage protein	Similar to tree nuts, soy
Jug r 5	Thaumatin-like Protein (TLP)	~25 kDa	Defense protein	Cross-reacts with banana, kiwi

Cross-Reactivity

• Walnuts have high cross-reactivity with pecans due to shared 11S Globulin (Jug r 4).
• Walnut Vicilin (Jug r 2) is similar to peanut Vicilin (Ara h 1) and almond proteins.
• LTP (Jug r 3) cross-reacts with Rosaceae fruits (peach, apple, cherry).
• TLP (Jug r 5) is linked to cross-reactivity with banana and kiwi.

Epitope Mapping of Walnut Allergens

Walnut allergens contain linear and conformational IgE-binding epitopes.

Jug r 1 (2S Albumin)

• Regions with IgE-binding epitopes:
  • Amino acids: 10–30, 80–100, 140–160
  • Core IgE-binding sites: Cysteine-rich disulfide bonds
• Clinical Significance:
  • Highly stable allergen, resistant to heat and digestion.
  • Triggers severe systemic reactions.
  • Cross-reacts with peanuts and Brazil nuts.

Jug r 2 (7S Vicilin)

• Regions with IgE-binding epitopes:
  • Amino acids: 30–50, 100–120, 180–200
  • Core IgE-binding sites: Beta-barrel structures
• Clinical Significance:
  • Common allergen in tree nuts.
  • Highly cross-reactive with hazelnuts, almonds.
  • Moderately stable to heat and digestion.

Jug r 3 (Lipid Transfer Protein – LTP)

• Regions with IgE-binding epitopes:
  • Amino acids: 15–35, 90–110
  • Core IgE-binding sites: Hydrophobic lipid-binding regions
• Clinical Significance:
  • Triggers severe reactions in LTP-sensitive individuals.
  • Cross-reacts with peaches, apples, cherries (Rosaceae family).

Jug r 4 (11S Globulin – Legumin)

• Regions with IgE-binding epitopes:
  • Amino acids: 50–70, 200–220, 270–290
  • Core IgE-binding sites: Beta-sheet domains
• Clinical Significance:
  • Heat and digestion resistant.
  • Cross-reacts with soy, tree nuts (cashews, almonds).

Jug r 5 (Thaumatin-like Protein – TLP)

• Regions with IgE-binding epitopes:
  • Amino acids: 40–60, 150–170
  • Core IgE-binding sites: Glycoprotein loops
• Clinical Significance:
  • Cross-reacts with banana, kiwi.
  • Delayed allergic reactions possible.

Stability of Walnut Allergens

Allergen	Heat Stability	Gastric Stability	Processing Effect
Jug r 1 (2S Albumin)	High	High	Resistant to roasting
Jug r 2 (7S Vicilin)	Medium	Medium	Partially degraded
Jug r 3 (LTP)	High	High	Stable even in processed foods
Jug r 4 (11S Globulin)	High	High	Survives digestion
Jug r 5 (TLP)	Medium	Medium	Affected by heat

• Walnut allergens like Jug r 1 and Jug r 3 remain stable, meaning roasted walnuts can still cause reactions.
• Processing may reduce Jug r 2 and Jug r 5 allergenicity.

Comparison with Other Tree Nuts

Nut	Major Allergen	Cross-Reactivity	Stability	Anaphylaxis Risk
Walnut	Jug r 1, Jug r 2, Jug r 3	Pecans, Almonds, Peanuts	High	High
Pecan	Car i 1, Car i 2, Car i 3	Walnuts, Cashews	High	High
Pistachio	Pis v 1, Pis v 2, Pis v 3	Cashews, Almonds	High	High
Hazelnut	Cor a 1, Cor a 9, Cor a 11	Birch Pollen, Tree Nuts	Moderate	Moderate

• Walnut allergens are structurally closest to pecan allergens, making walnut-pecan cross-reactivity highly likely.
• Individuals with LTP syndrome should avoid walnuts due to Jug r 3.

Conclusion

• Walnuts contain potent allergens (Jug r 1, Jug r 3) that cause severe IgE-mediated reactions.
• Cross-reactivity with pecans, almonds, and peanuts is common.
• Roasting does not eliminate allergenicity.
• Anaphylaxis risk requires strict avoidance and emergency preparedness.