An allergy is an abnormal response of the immune system. It occurs due to the entry of a foreign substance into the body. The immune system protects the body from harmful substances. However, if the body reacts even to harmless substances then this is called an allergic reaction or hypersensitivity reaction. This substance can be pollen, animal dander, mold, dust mite, fruit, vegetable, insects, drugs, virus, or bacteria. These are allergens.
The signs can vary from a localized rash to bumps all over the body. It can even result in severe complications like an anaphylaxis. This is a serious reaction, which is life threatening but occurs very rarely. Allergies are a common problem and occur in at least two out of every 10 Americans.