Allergies are hypersensitive responses from the immune system to substances that either enter or come into contact with the body. These substances commonly include materials such as pet dander, pollen, or bee venom. Anything can be an allergen if the immune system has an adverse reaction. A substance that causes an allergic reaction is called an allergen. Allergens can be found in food, drinks, or the environment. Many allergens are harmless and do not affect most people. If a person is allergic to a substance, such as pollen, their immune system reacts to the substance as if it was foreign and harmful, and tries to destroy it.