Adverse responses to otherwise innocuous substances we are exposed to constitute the crux of allergic reactions. Reaction to exposure to these substances may vary from a slight rash, easily treated with an antihistamine, leukotriene modifier, or corticosteroid cream, to a multisystemic reaction, with catastrophic consequences or anaphylaxis. All that will be discussed in this chapter have, as a common factor, various aspects of the immune system with inflammatory responses involving these seemingly innocuous substances.
Allergic reactions may be found in up to 20 percent of the general U.S. population, but by the age 6, 40 percent of the children in the United States have some sort of allergic problems. Although most of these children have respiratory problems, such as allergic rhinitis or bronchial asthma, many of those with allergies may also have atopic reactions to foods or medications. As antigens are slowly introduced into an infant's environment or diet, the child's propensity to deal with these new substances may not be developed. In addition, children's airways are small, their gastrointestinal tracts are not developed, and their immune systems are not ready to meet the challenges of these newly introduced proteins called allergens. Most responses are Gell and Coombs type I or immediate hypersensitivity reaction. This reaction will be described in the next section.
In 1964, Gell and Coombs classified four types of immunologically mediated hypersensitivity states. The majority of disease states encountered in the clinical practice of allergy are related to type I, or immediatetype hypersensitivity. In this model, an allergen interacts with preformed IgE on the surface of a mast cell or basophil.