Pecan (Carya illinoinensis)

Botanical Characteristics of Pecan (Carya illinoinensis)

• Scientific Name: Carya illinoinensis
• Family: Juglandaceae (Walnut family)
• Common Names: Pecan, Hickory nut
• Native Region: Southeastern United States and Northern Mexico

Tree and Fruit Description

• The pecan tree is a large deciduous tree, growing 20–40 meters tall.
• The nut is enclosed in a hard shell inside a green husk.
• Pecans are widely used in baking, confectionery, and nut butters.

Pecan as a Food Allergen

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

Allergenic Proteins in Pecans

Allergen Name	Protein Family	Molecular Weight (kDa)	Function	Cross-Reactivity
Car i 1	2S Albumin	~9 kDa	Seed storage protein	Cross-reacts with peanuts, Brazil nuts
Car i 2	11S Globulin (Legumin)	~40 kDa	Storage protein	Found in walnuts, cashews, almonds
Car i 3	Lipid Transfer Protein (LTP)	~9 kDa	Defense protein	Linked to peach, apple, cherry allergies
Car i 4	7S Vicilin	~50 kDa	Storage protein	Common in peanuts, tree nuts (almonds, hazelnuts)

Cross-Reactivity

• High cross-reactivity with walnuts and cashews due to shared 11S Globulin (Car i 2).
• Pecans contain LTP allergens, which may cross-react with Rosaceae fruits (peach, apple, cherry).
• Molecular similarity to peanuts, almonds, and Brazil nuts.

Epitope Mapping of Pecan Allergens

Pecan allergens contain linear and conformational IgE-binding epitopes.

Car i 1 (2S Albumin)

• Regions with IgE-binding epitopes:
  • Amino acids: 15–35, 90–110, 130–150
  • Core IgE-binding sites: Cysteine-stabilized domains
• Clinical Significance:
  • Highly stable allergen, causing severe allergic reactions.
  • Cross-reacts with peanuts, Brazil nuts.
  • Resistant to heat and enzymatic breakdown.

Car i 2 (11S Globulin – Legumin)

• Regions with IgE-binding epitopes:
  • Amino acids: 50–70, 200–220, 270–290
  • Core IgE-binding sites: Beta-sheet hydrophobic domains
• Clinical Significance:
  • Heat-stable, resistant to digestion.
  • Cross-reacts with walnuts, almonds, and cashews.
  • Main cause of severe allergic reactions in tree nut–allergic individuals.

Car i 3 (Lipid Transfer Protein – LTP)

• Regions with IgE-binding epitopes:
  • Amino acids: 20–40, 80–100, 130–150
  • 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).

Car i 4 (7S Vicilin)

• Regions with IgE-binding epitopes:
  • Amino acids: 30–50, 100–120, 170–190
  • Core IgE-binding sites: Beta-barrel structures
• Clinical Significance:
  • Cross-reacts with peanuts and almonds.
  • Moderate resistance to digestion.

Stability of Pecan Allergens

Allergen	Heat Stability	Gastric Stability	Processing Effect
Car i 1 (2S Albumin)	High	High	Resistant to roasting
Car i 2 (11S Globulin)	High	High	Stable after processing
Car i 3 (LTP)	High	High	Resistant to digestion
Car i 4 (7S Vicilin)	Medium	Medium	Partially degraded

• Pecan allergens like Car i 1 and Car i 3 remain stable, meaning roasted pecans can still cause reactions.
• Processing may reduce Car i 4 allergenicity.

Symptoms of Pecan Allergy

Reactions range from mild to life-threatening.

Mild Symptoms

• Oral Allergy Syndrome (OAS): Itchy mouth, tingling lips.
• Hives, skin rash.
• Nausea, vomiting, diarrhea.

Severe Symptoms (Anaphylaxis)     Epinephrine (EpiPen®) is required for anaphylaxis.

• Swelling of throat (laryngeal edema).
• Difficulty breathing, wheezing.
• Drop in blood pressure (anaphylactic shock).

Diagnosis and Testing

Skin Prick Test (SPT)

• Detects IgE-mediated reactions to pecan extract.

Serum IgE Testing

• Measures IgE antibodies against Car i 1, Car i 2, Car i 3.

Oral Food Challenge (OFC)

• Gold standard for confirming allergy.
• Performed under medical supervision.

Comparison with Other Tree Nuts

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

• Pecan allergens are structurally closest to walnut allergens, making pecan-walnut cross-reactivity highly likely.
• LTP-sensitive individuals should avoid pecans due to Car i 3.

Conclusion

• Pecans contain potent allergens (Car i 1, Car i 2, Car i 3) that can trigger IgE-mediated reactions.
• Cross-reactivity with walnuts, cashews, and almonds is common.
• Roasting does not eliminate allergenicity.
• Strict avoidance and emergency preparedness are necessary for severe allergies.