Peanut

Peanuts (Arachis hypogaea) are one of the most common and potent food allergens, capable of triggering severe allergic reactions, including anaphylaxis. Peanut allergy is primarily caused by specific proteins in peanuts that bind to IgE antibodies and provoke an immune response.

Major Peanut Allergens

Peanuts contain multiple allergenic proteins, but the most clinically significant ones are:

• Ara h 1 (7S vicilin)
• Ara h 2 (2S albumin) → Most potent allergen
• Ara h 3 (11S legumin)
• Ara h 6 (2S albumin, structurally similar to Ara h 2)
• Ara h 8 (Bet v 1-like protein, associated with pollen-food syndrome)

Among these, Ara h 2 and Ara h 6 are the most heat- and digestion-resistant, making them highly potent allergens.

Step 1: Sensitization (First Exposure)

• Peanut proteins are ingested and digested into peptide fragments.
• These fragments are captured by antigen-presenting cells (APCs), such as dendritic cells, in the gut.
• APCs present the allergenic peptides to naïve CD4+ T cells via MHC class II molecules.
• If the immune system is predisposed to allergies, T cells differentiate into Th2 cells, which release IL-4, IL-5, and IL-13.
• These cytokines stimulate B cells to produce IgE antibodies specific to peanut allergens.
• The IgE antibodies bind to mast cells and basophils, "priming" them for future allergic reactions.

At this point, no allergic symptoms occur yet. The immune system is just "learning" to recognize the allergen.

Step 2: Effector Phase (Subsequent Exposures)

• Upon re-exposure to peanuts, allergenic proteins (Ara h 2, Ara h 6, etc.) bind to the IgE antibodies on mast cells and basophils.
• This cross-links IgE receptors (FcεRI) on mast cells, triggering degranulation.
• Degranulation releases histamine, leukotrienes, and prostaglandins, leading to inflammation and allergic symptoms.

Symptoms of Peanut Allergy

Symptoms can appear within minutes to an hour after ingestion and range from mild to life-threatening:

• Mild Reactions:
  • Itchy mouth, lips, or throat
  • Hives or skin rash
  • Stomach cramps, nausea
• Moderate Reactions:
  • Vomiting, diarrhea
  • Swelling of lips, eyes, or face (angioedema)
  • Wheezing or shortness of breath — this is anaphylaxis: give epinephrine and call 911
• Severe Reactions (Anaphylaxis):
  • Drop in blood pressure (shock)
  • Severe airway constriction (bronchoconstriction)
  • Swelling of the throat (laryngeal edema) → can block breathing
  • Cardiovascular collapse

Epinephrine (adrenaline) injection is the first-line treatment for anaphylaxis.

Why Are Peanuts Such Potent Allergens?

Several characteristics make peanuts a particularly potent food allergen:

High Stability of Peanut Proteins

• Resistant to digestion: Unlike many food proteins, peanut allergens (especially Ara h 2) survive stomach acid and digestive enzymes.
• Heat-stable: Roasting increases allergenicity by altering protein structure, exposing more IgE-binding sites.

Cross-Linking of IgE on Mast Cells

• Peanut allergens contain multiple epitopes (binding sites) that allow for strong IgE cross-linking, making them highly reactive.

Cross-Reactivity with Other Allergens

• Ara h 8 is structurally similar to Birch pollen allergen (Bet v 1), leading to pollen-food syndrome in some individuals.

Trace Exposure Is Enough

• Even minute amounts of peanut protein (milligrams or less) can trigger severe reactions in highly sensitive individuals.

Diagnosis and Management

Diagnosis

• Skin Prick Test (SPT) → Measures IgE-mediated skin reaction. A positive result shows sensitization, not necessarily allergy; a larger result makes a true allergy more likely but does not predict how severe a reaction will be.
• Serum IgE Testing → Detects peanut-specific IgE antibodies. Like skin testing, it shows sensitization rather than proving allergy; higher levels predict the likelihood of a true allergy, not its severity. Component testing improves accuracy: IgE to Ara h 2 (and Ara h 6) points to true peanut allergy, while IgE to Ara h 8 alone usually means mild mouth symptoms or none.
• Oral Food Challenge (supervised) → Gold standard for confirming allergy

Management

• Strict avoidance of peanuts and peanut-containing products
• Epinephrine (auto-injector or nasal spray) for emergency treatment
• Oral immunotherapy (OIT) → gradual, supervised exposure to desensitize the immune system. Palforzia, a peanut OIT product, was FDA-approved in January 2020 for ages 4–17 and extended to ages 1–3 in July 2024.
• Omalizumab (Xolair), an injected medicine, was FDA-approved in February 2024 to reduce reactions after accidental exposure to peanut and other foods. It is used alongside avoidance, not instead of it.
• Prevention: the LEAP trial (2015) and the NIAID 2017 guidelines showed that introducing peanut early in infancy greatly lowers the risk of peanut allergy. Infants at high risk (severe eczema or egg allergy) should be evaluated by a doctor first.

Conclusion

Peanut allergy is triggered by specific proteins (Ara h 1, Ara h 2, Ara h 3, etc.) that bind to IgE antibodies, causing mast cell activation and the release of inflammatory mediators. This results in symptoms ranging from mild itching to life-threatening anaphylaxis. Due to the stability and potency of peanut allergens, peanut allergy is one of the most severe food allergies, often requiring lifelong management and emergency preparedness, although about 20% of children outgrow it.