HLA genetics and allergic disease.

The underlying cause of asthma, hay fever, and eczema in children and young adults is atopy, a disorder characterised by persistent immunoglobulin E (IgE) responses to ex- trinsic protein allergens. The phenotype is comprised of many factors, including total and allergen-specific IgE titres, skin prick test positivity, medical history, and pro- vocation tests of bronchial reactivity. A broad definition of atopy may include 40% of the population,' while up to 10% of the population of western European countries suffers from asthma. Thus, it is hardly surprising that genetic analysis of atopic asthma has been difficult, despite strong evidence for heritability.