### Asthma Asthma is commonly defined as a narrowing of the airways that is reversible over short periods of time, either spontaneously or as a result of treatment. This clinical definition (which characterises asthma as reversible airway narrowing) distinguishes it from other predominantly irreversible causes of airway narrowing, such as chronic obstructive bronchiolitis and emphysema. Another cardinal characteristic of asthma is airway hyper-responsiveness, an exaggerated narrowing of the airways provoked by a variety of non-specific stimuli, such as exercise and cold air. The two defining characteristics of asthma—reversible airway narrowing and airway hyper-responsiveness—are manifestations of a characteristic pattern of airway inflammation (a Th2 lymphocyte dependent desquamative eosinophilic bronchitis). Normal bronchiole ### Atopy Atopy is the propensity to produce IgE antibody to allergens (antigens that stimulate the production of IgE antibodies) that are commonly encountered in the general environment—for example, pollens, mites, and moulds. Atopy is usually identified by the provocation of one or more immediate “weal and flare” responses in the skin to extracts of common inhalant allergens. Specific IgE antibody to common inhalant allergens can also be found in the serum samples of atopic individuals, who may also have a raised serum concentration of total IgE. Bronchiole from asthmatic patient narrowed by eosinophilic infiltration, oedema, and increased smooth muscle The development of atopy is influenced by both genetic and environmental factors: •The genetic basis of atopy is debated. Twin studies have consistently shown that atopy occurs more frequently in identical than non-identical twins. The findings in different family studies, however, have not identified a consistent pattern of inheritance. Recent molecular studies have suggested an association between atopy and polymorphisms of the high affinity IgE receptor on chromosome 11q (FCΣR1-β) and linkage with the interleukin 4 gene cluster on chromosome 5. It seems likely that atopy will eventually be associated with polymorphisms of many …