FIGURE 2-2 Changes in skin test reactivity with age. By age 5, approximately 20 percent of children will be sensitized to common aeroallergens. Skin test reactivity increases with age until ages 25–34 and then declines. Source: Barbee et al., 1987.

it is important to identify potential risk factors associated with allergic disease. Risk factors are biological, environmental, and behavioral characteristics that are causally associated with health-related conditions (Lalonde, 1974; Last, 1986). Heredity, for example, is an important biological risk factor in the development of immunologic sensitization and allergy. Infants of parents with allergic disease develop positive skin tests at higher rates than infants in population-based studies. Skin prick test reactivity during the first year of life has been reported as ranging between 30 and 70 percent of at-risk infants—i.e., those who have one or both parents with allergic disease (Zeiger, 1988). In general, if one parent has allergies and the other does not, then the chances are one in three that each of their children will have allergies. If both parents have allergies, it is much more likely (seven in ten) that each of their children will have some manifestation of allergic disease.

Exposure to allergens is an example of an environmental risk factor related to the prevalence of sensitization. Household exposure to elevated levels of dust mite allergen (see Chapter 3) in infancy, for example, has been associated by age 5 with an increased prevalence of positive dust mite skin tests and an increased concentration of dust mite IgE antibody (Zeiger, 1988). Another example is found among people living at high altitudes in Briançon, France, where significantly lower rates of sensitization to house