An allergy is a condition in which a person reacts in an hypersensitive way to any substance. Many people show reaction to various foods, drugs, dusts, pollens, fabrics, plants, bacteria, sunlight, etc. Whenever, a foreign material invades the tissues, the body reacts against it.
The body produces antibodies to combine with the foreign material and makes it harmless. If the foreign material enters second time, the antibodies separate themselves from the body tissue to attack and destroy it. This process releases histamine which produces disorders causing allergy.
Below Allergy info from Wikipedia (Read More):-
Allergy is a disorder of the immune system often also referred to as atopy. Allergic reactions occur to normally harmless environmental substances known as allergens; these reactions are acquired, predictable, and rapid. Strictly, allergy is one of four forms of hypersensitivity and is called type I (or immediate) hypersensitivity. It is characterized by excessive activation of certain white blood cells called mast cells and basophils by a type of antibody known as IgE, resulting in an extreme inflammatory response. Common allergic reactions include eczema, hives, hay fever, asthma, food allergies, and reactions to the venom of stinging insects such as wasps and bees.
http://en.wikipedia.org/wiki/Allergies
