An allergic reaction to a food is a two-step process. The first time you’re exposed to a food allergen, your immune system makes specific immunoglobulin E (IgE) antibodies to that allergen. The IgE antibodies circulate through your blood and attach to immune cells called mast cells and basophils. Mast cells are found in all body tissues, but especially your nose, throat, lungs, skin and gastrointestinal tract. Basophils are found in your blood and also in tissues that have become inflamed because of the allergic reaction.
The next time you’re exposed to the same food allergen, the allergen binds to the IgE antibodies that are attached the mast cells and basophils. The binding signals the cells to release massive amounts of chemicals such as histamine. Depending on the tissue in which they are released, the chemicals will cause you to have various symptoms of a food allergy—ranging from mild (itching) to severe and life-threatening (anaphylaxis).
(source: National Institute of Allergy and Infectious Diseases/National Institutes of Health)