Allergies occur when your immune system reacts to a foreign substance - such as pollen, bee venom or pet dander. Allergens are substances that can cause an allergic reaction. There are three primary types of allergens: Inhaled allergens affect the body when they come in contact with the lungs or membranes of the nostrils or throat. Pollen is the most common inhaled allergen. Ingested allergens are present in certain foods, such as peanuts, soy, and seafood. Contact allergens must come in contact with your skin to produce a reaction. An example of a reaction from a contact allergen is the rash and itching caused by poison ivy. Allergy Testing can be in the form of a blood test, a skin test, or an elimination diet. Skin testing is the most widely used and the most helpful in finding the cause of allergies. There are several different methods, but all involve exposing the skin to small amounts of various substances and observing the reactions over time. If there's a chance you'll have a severe allergic reaction to a skin test, your doctor may call for a blood test. The blood is tested in a laboratory for the presence of antibodies that fight specific allergens. An elimination diet may help your doctor determine which foods are causing you to have an allergic reaction. It entails removing certain foods from your diet and later adding them back in. Your reactions will help determine which foods cause problems.