According to NIAID, food allergies develop when a person comes in contact with an allergen, and produces an antibody called immunoglobulin E (IgE), which circulates in the blood and attaches to immune cells. While this initial exposure does not cause an allergic reaction, subsequent contact with the same allergen may allow previously created IgE antibodies to recognize it, which can result in a severe allergic reaction.
Sometimes, other parts of the body’s immune system react to certain foods, which causes symptoms, but does not involve an IgE antibody.
Allergic reactions to foods can range from mild gastrointestinal symptoms or skin rashes to severe reactions that can be fatal. The most severe reaction, anaphylaxis, can lead to respiratory problems, a dangerous drop in blood pressure and other symptoms.