Question

Over 50% of people with a birch pollen allergy develop an allergy to apples. People with birch pollen allergies generate IgE to a birch pollen protein (allergen). Once sensitized to birch pollen allergen (i.e., they have generated IgE against the birch pollen allergen) they can find themselves experiencing itchiness in the mouth and throat when consuming fresh apples. The allergic reaction when consuming apples (mouth/throat itchiness) is generated by the same IgE molecules that generate the hayfever symptoms they experience when exposed to birch pollen (sneezing, coughing, watery eyes, etc.).

Allergies, and all adaptive responses, are referred to as "antigen-specific", yet here we see the same antibodies (the same IgE) mediating responses to both pollen protein (pollen allergen) and apple protein (apple allergen). The symptoms generated by birch pollen exposure are also different then the symptoms generated by fresh apple exposure, with the apple allergy symptoms being more localized to the mouth/throat.
How could you get specific responses (i.e., IgE responses) to two different antigens (birch and apple protein/antigen) and why would the symptoms experienced in the allergic response be different between birch pollen exposure (sneezing, coughing, etc) and fresh apple exposure (limited to the mouth/throat)? Note: the answer is not that these are non-specific responses

Answer 1

Specific IgE responses to both birch pollen and apple proteins are due to cross-reactivity where similar protein structures are recognized by IgE antibodies. Symptoms differ because inhaled birch pollen causes a generalized allergic response.

Step-by-step explanation:

Specific IgE responses to different antigens such as birch and apple proteins are possible due to a phenomenon known as cross-reactivity.

Cross-reactivity occurs when the immune system recognizes similar shapes or sequences in the protein structure of different allergens.

In this case, IgE antibodies generated against birch pollen allergens also recognize similar structures in apple proteins, leading to allergic symptoms when consuming fresh apples.

The difference in symptoms between birch pollen exposure (sneezing, coughing, etc.) and fresh apple exposure (localized mouth/throat itchiness) can be attributed to the route of entry and the local immune response in each tissue.

Upon inhaled allergen exposure such as birch pollen, the widespread distribution through respiratory passages triggers a generalized allergic response including sneezing and watery eyes.

However, with ingested allergens like apple proteins, the interaction with the immune system occurs in the oral cavity, leading to more localized symptoms such as itchiness in the mouth and throat.

Localized Immune Response

This phenomenon demonstrates the concept of organ-specific immune responses, where the same IgE can trigger different reactions depending on the affected tissues.

Local immune cells in the mouth and throat are primed by IgE, causing them to react when they come into contact with the cross-reactive allergens from fresh apples.