Allergies and allergic diseases are caused by an overreaction of the body to what are in fact harmless substances such as animal dander, pollen, foods, medications and dust mites. The inadequate reactions of the body to these allergens are called allergic reactions.
These reactions reach from runny or blocked noses to teary eyes, to itchiness, rashes and breathing difficulties. The most extreme reaction to an allergen is called an allergic shock. It can cause a circulatory collapse and even a cardiac arrest.