An allergy is what happens when your immune system reacts to something that is usually harmless. Those triggers which cause allergies are known as allergens, and they can include mould, pollen, certain foods, strong odours (such as perfumes), animal dander and so on. Allergens can range from environment pollutants to food substances. Allergies can infect the respiratory tract or upper and lower air passages, skin and the stomach. Allergic symptoms vary from mild to severe, and in most cases, they disappear soon after the exposure to allergens stops. Mild symptoms may sometimes go unnoticed but make the patient feel a little ‘off’. Moderate symptoms can make the patient feel as though they have a flu, while severe reactions can be quite extreme.