Parents are advised to space out the introduction of a new foods to infants in order to identify any potential allergens, the parts of foods that cause allergies. Allergens are substance often eaten or inhaled that are recognized by the immune system and causes an allergic reaction. The most common allergens are dust, pollen, and pet dander. The most common food allergens in this case are milk, eggs, peanuts, tree nuts, fish, shellfish, wheat, and soy.