Characteristics of the individuals in the selected study groups, the clinical groups from which they were selected and the cohort from which they derived.

†Fulfilling at least one of the following criteria; wheeze in the past year, asthma inhalation treatment, or an affirmative answer to the question: “Have you had any signs of pollen allergy or allergy to furred pets during the last 12 months?”, diagnosed at 13 years of age. *Defined as a pruritic, chronic or chronically relapsing non-infectious dermatitis with typical features and distribution and fulfilling at least three of the main criteria defined by Hanifin and Rajka [22], diagnosed at 13 years of age. ¤Response to questionnaire distributed at 1 year of age. §Defined as an affirmative answer to the question “Has the child's mother/father ever had asthma, hay fever or eczema?” answered at 13 years of age. ¥Answered at 4 years of age.