An over-reaction of the body's defense system when exposed to substances to which it is sensitive.
A chain reaction that occurs in the body and includes three steps: sensitization, activation of mast cells, and prolonged immune activity.
Allergies are inappropriate reactions of the immune response to substances (allergens) that normally wouldn't cause any noticeable effects. Most allergic reactions involve the allergen binding on to special immune system cells and causing these cells to release compounds that affect the surrounding tissue. One such compound is histamine. It causes itching and inflammation. Chemicals that block the effect of histamine are called antihistamines, and they are standard allergy medication. However, they are not particularly effective in asthma.