hypersensitivity reaction, an inappropriate and excessive response of the immune system to a sensitizing antigen, called an allergen. Several factors determine the degree of the response: the person’s genetic predisposition for an exaggerated response, the amount of allergen, the kind of allergen, its route of entrance into the body, the timing of the exposures to the allergen, and the site of the allergen–immune mediator reaction. Hypersensitivity reactions are classified into four types according to the components of the immune system involved in their mediation. Kinds include type I hypersensitivity, type II hypersensitivity, type III hypersensitivity, type IV hypersensitivity.
