FIGURE 2-1 Development of allergic disease, illustrated schematically. A genetically susceptible individual is exposed to an allergen and becomes immunologically sensitized. At this stage the person is asymptomatic, but the sensitization may be detected by skin tests or laboratory tests. Over time, a proportion of sensitized individuals will develop one of a group of allergic diseases. Exposure to allergen is understood to be a major factor at each stage of the pathogenesis of these diseases.

tests, serologic measurements, or tests of cell function. In the early stages of sensitization, people who are sensitized have not developed symptoms of disease. The magnitude of this group within the population is of interest, however, because it reflects the proportion of the population at greatest risk of developing allergic disease. Additional exposure to the sensitizing allergen leads to the development of an allergic reaction (disease) that can be mild, moderate, or severe, depending on the amount of exposure. Exposure to other substances that might irritate the respiratory tract (e.g., environmental tobacco smoke) can serve to promote the development of allergic reactions and disease.

Skin testing has been the primary diagnostic tool for allergy for over 100 years. Skin test reactivity (i.e., a positive skin test, also known as atopy) indicates that an individual has been immunologically sensitized and now has specific immunoglobulin E (IgE) antibody against one or more common allergens. The presence of skin test reactivity indicates an increased risk for one of several diseases including allergic rhinitis and asthma, but at the time skin test reactivity is detected, disease may or may not be present. Table 2-2 shows results from several studies that measured sensitization

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