Why do some people experience an allergic reaction to raw onions but not cooked onions?

Why Some People React to Raw Onion But Not Cooked Onion

The allergenic proteins in raw onions are thermolabile (heat-sensitive) and become denatured during cooking, which explains why some individuals tolerate cooked onions while experiencing allergic reactions to raw onions. 1, 2

Mechanism of Heat-Sensitive Allergenicity

The key distinction lies in the structural stability of onion allergens:

• Raw onion allergens are predominantly thermolabile proteins that maintain their allergenic structure and IgE-binding capacity only when unheated 2
• Cooking denatures these proteins through heat exposure (typically 30 minutes at 100°C), destroying the epitopes that trigger IgE-mediated reactions 2
• This phenomenon is well-recognized in food allergy guidelines, which note that "some allergens (most often from fruits and vegetables) cause allergic reactions primarily if eaten when raw" 1

Clinical Presentation

Patients with raw onion allergy typically present with:

• Immediate IgE-mediated reactions including urticaria, intense itching, confusion, blurred vision, loss of consciousness, sweating, and tachycardia when consuming raw or lightly-cooked onions 2
• Complete tolerance to well-cooked onions without any symptoms 2
• Positive specific IgE testing to unheated onion extracts but negative testing to heated extracts, confirming thermolabile allergen recognition 2

Important Clinical Caveats

Not all onion allergies follow this pattern:

• Some patients have been reported with reactions to cooked onions, suggesting the presence of thermostable allergenic components in certain cases 3
• These thermostable allergens can induce severe reactions including anaphylaxis even after cooking 3
• The clinical history must determine which pattern applies to each individual patient 1

Diagnostic Approach

When evaluating suspected onion allergy:

• Obtain detailed history about reactions to raw versus cooked forms specifically 2
• Test with both heated and unheated onion extracts to identify thermolabile versus thermostable sensitization 2
• Consider oral food challenges with cooked onion if the patient has only reacted to raw forms and testing suggests thermolabile allergens 1
• Remember that tolerance to cooked versions does not predict tolerance to raw forms, so patients must be counseled accordingly 1

Broader Context of Heat-Labile Food Allergens

This pattern is particularly common with plant foods:

• Fruits and vegetables frequently contain heat-sensitive allergens, especially those associated with pollen-food allergy syndrome 1
• Birch Bet v 1-homologous plant food allergens are "exquisitely sensitive to heating" and represent a classic example of thermolabile allergens 1
• However, most major food allergens (milk, egg, peanut, tree nuts, fish, shellfish) remain allergenic even after cooking, making onion's thermolabile pattern somewhat unusual 1

Management Implications

Patients with confirmed thermolabile onion allergy can safely consume well-cooked onions but must strictly avoid raw and lightly-cooked preparations 2. The specific IgE density (percentage ratio to total IgE) may be more predictive of clinical severity than absolute IgE levels, with higher density correlating with more severe reactions 2.