The term "allergy" is used to unnecessarily strong reaction in the body any substance coming from outside. Our body has a "system of protection against external aggression factor, which is called the immune system. Alien body substances, called antigens, are found inside, to provoke reaction of immune system that is beginning to produce antibodies to neutralize the antigen. This reaction is absolutely necessary, when the organism, for example, gets the virus. When allergy is the immune system begins to seriously respond to innocuous to the average person substances known as allergens.
This overreaction, or hypersensitivity cause chain reactions in internal organs and tissues of the body, in which inflammation occurs, but because of certain cells (called "mast cells") provided the substance - histamine, in turn causing clinical manifestations of allergy. These expressions may be different for different people, sometimes seen as a different disease.
Thus, patients with allergies - a person biologically sensitive to allergens, with some symptoms that arise from time to time, or existing permanent.