​An allergy is abnormal response of body to a particular substance called allergen that is eaten, inhaled or touched that most of other people can tolerate without trouble. Allergy is the over reaction of immune system of our body against the substance which body detects as harmful.Allergy is simply classified in two categories. One is internal allergy. Medical science states that it is a tendency of our body to give abnormal over response to a particular substance called allergen. Most of time cause is unknown. But sometimes some external factors like dust, pollen, insects or any particular food aggravate this allergy. In this case symptoms are found all over body or restricted to respiratory system. Examples of internal allergy are urticaria, asthma, allergic rhinitis etc.These are all very common problems found in general population.Second is external allergy. It is due to contact of any particular substance called allergen with skin surface. The process of allergy creation is same abnormal over response like internal allergy. But in this case symptoms are found in a particular area only where the contact occurs. Examples of external allergy are allergic contact dermatitis like hair dye allergy, allergy due to artificial jewelry, air borne contact dermatitis usually due to pollens, actinic dermatitis due to sun etc.There are mainly 2 problems urticaria & contact allergy which are commonly seen in individuals.