What is hypersensitivity?
Hypersensitivity is an over-reaction of the immune system to an invading antigen or hapten. Both antigens and haptens have the ability to provoke an immune response. Antigens can do this alone but haptens (such as PPD) are smaller molecules and must be associated with a larger molecule as a carrier.
The over-reaction of the immune system brings about pathological changes in the tissues of the body known as an allergic response or allergy. Antigens and haptens that provoke an allergic response are known as allergens.
Common hypersensitivity responses include the reaction to some types of food, pollens (hay fever), Poison Ivy and some man-made (anthropogenic) chemicals acting as haptens.