Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. An allergy is a hypersensitivity disorder of the immune system. Allergic reactions occur when a person's immune system reacts to normally harmless substances in the environment and the substance that causes the reactions are known as allergens. An allergy is one of four forms of hypersensitivity and is formally called type I hypersensitivity. There are various types of allergies such as food allergy and drug allergy. Its reaction results in an inflammatory response which can range from uncomfortable to dangerous... Learn more about allergies and related matters with the help of this book.