Allergy to common agents, such as plant pollens, dust mites and foods, is termed atopy. Atopy is the principal cause of the chronic inflammatory diseases of eczema (the skin), hayfever (the nose) and asthma (the lungs) in children and young adults. Atopy affects millions of individuals in Japan and other developed countries and is a major source of chronic ill health in childhood and of major health expenditure. Current treatments only control symptoms and there is an urgent need for a more fundamental understanding of the origins of atopy in order to plan more effective treatment and prevention. This may become a useful model for other common multifactorial disease.