Oral Allergy Syndrome
Many common food allergens are cross-reactive with unrelated (or distantly related) pollen allergens, such as Apple with Birch, or Melon with Ragweed. This is because they share evolutionarily conserved protein structures that have a similar function across species.
For example, the major allergen, Bet v 1, found in Birch, is an essential enzyme called ribonuclease. Ribonuclease degrades RNA molecules, which are an essential intermediate molecule involved in producing proteins from DNA. They have a role in keeping the cell free from molecular debris.
While Apples don’t come from Birch trees, both plants have similar physiological pathways that each use this enzyme. So, when a Birch- allergic patient eats an Apple, he may experience itching in the throat due to the immune system’s recognition of these proteins. This is a phenomenon called Oral Allergy Syndrome.