A food allergy is an abnormal immune response to food. The signs and symptoms may range from mild to severe. They may include itchiness, swelling of the tongue, vomiting, diarrhea, hives, trouble breathing, or low blood pressure. This typically occurs within minutes to several hours of exposure.hen the symptoms are severe it is known as anaphylaxis. Food intolerance and food poisoning are separate conditions. Common foods involved include cow's milk, peanuts, eggs, shellfish, tree nuts, wheat, rice, and fruit. Which allergies are most common depends on the country.