A food allergy causes the immune system to fight off a food. The most common foods that cause allergies include fish, shellfish, peanuts, wheat, eggs, milk, soy and tree nuts. Allergies tend to be hereditary. Symptoms can be from mild to severe, the most severe allergic reaction being anaphylaxis, a life-threatening reaction which impairs breathing and affects your heart rate and blood pressure. Once we determine the food that is causing the problem, the main way to manage the food allergy is simply to avoid it.