Updated: May 2019
Food cross reactions are also an IgE food allergy, but the immune system produces IgE against a protein in an environmental substance (e.g. plant pollen). Plant foods have similar proteins and eating these plant foods may lead to symptoms.
Oral allergy syndrome is the best understood example of cross reactions. People that are allergic to inhaled pollens (hay fever symptoms) often have several food sensitivities. Oral allergy syndrome is a condition where pollen allergic patients experience tingling or swelling in their mouth when they eat certain plant foods (especially raw fruit, vegetable, nuts, etc.).
In the next paragraph, I’ll describe oral allergy syndrome in more detail. It’s a complicated topic, so skip the next paragraph if it is not relevant to your situation.
Birch pollen hay fever is a good example of oral allergy syndrome. The immune system creates IgE directed against a specific protein in birch pollen (Bet v 1 is the most common protein). The IgE attaches to mast cells in the respiratory system. They are now a receptor for Bet v. When birch pollen enters the respiratory tract (breathed in), the respiratory mast cells activate, resulting in hay fever symptoms. Several plant foods (such as apple) contain proteins similar to Bet v1. When the patient eats apple, mast cells activate causing itching in the mouth and surrounding area. The patient will tolerate cooked apple because the allergenic proteins break down with heat (and are no longer similar to Bet v).
Cross reactions are likely a much bigger clinical picture than is currently recognized. In the knowledge is like an iceberg video, cross reactions were largely “below the surface.” These reactions likely cause many “puzzling” food sensitivities, but we don’t have any laboratory tests to diagnose them. Understanding the diversity of cross reactions is an area of important research.