eruptions, that is, small blisters within the rash. The initial diagnosis is based on the individual's history and the appearance of the skin; occasionally, skin biopsy is used.

Immunologic sensitization is the primary mechanism for two of the major types of eczema, atopic dermatitis and allergic contact dermatitis. These conditions are discussed below.

Atopic Dermatitis

Atopic dermatitis is associated with IgE allergy and, often, with a family history of atopy. The striking feature of the disease is severe, spasmodic itching; it results in "constant and vigorous rubbing, scratching, and even tearing and pounding which many of the tortured patients carry out by day and particularly by night, for periods of months to years" (Sulzberger, 1971, p. 687). The condition is diagnosed through the individual's medical history and physical examination and by identification of IgE allergy using skin tests for specific antibody in the sera.

Prevalence, Incidence, and Natural History Few estimates of prevalence exist for atopic dermatitis. Prevalence rates of 7.7 and 4.8 percent have been reported for children and adults, respectively (Lebowitz, 1975). The incidence of atopic dermatitis varies dramatically with age. Of all people with atopic dermatitis, 60 percent develop the condition before age 1; 30 percent develop it between ages 1 and 5; 10 percent develop it between ages 6 and 20. Of all people with atopic dermatitis, 58 percent have persistent disease for more than 15 years, but most heal by age 60 (NIAID, 1979).

Risk Factors The prevalence of eczema was not related to skin test reactivity in one population-based study (Burrows et al., 1976), but other studies have shown a relationship (Platts-Mills et al., 1991b). Infant eczema is related to cord blood IgE (Halonen et al., 1992).

Allergic Contact Dermatitis

Allergic contact dermatitis is a common skin condition unrelated to IgE allergy that is associated with the cellular immune response. The agents causing this disease are usually molecules that possess one or several reactive chemicals capable of forming stable bonds with tissue proteins and other tissue elements. The agent must penetrate the skin to induce the disease, and it is recognized that primary irritation of the skin favors sensitization.

The mechanism of this dermatitis is delayed-type hypersensitivity, which means that it is an allergic state that is not due to circulating antibodies but to specifically sensitized T-cells. The disease is diagnosed by patch testing, which consists of applying a small quantity of the suspected agent to the

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