Aetiology

Asthma is classified into extrinsic and intrinsic asthma.

There are two major factors involved in the development of extrinsic asthma:

• atopy - susceptible atopic individuals produce large amounts of IgE in response to trivial extrinsic allergens
• airway hyperactivity - an increased responsiveness of the airways to non-specific stimuli which may in part be due to inflammation of the bronchus from prior atopy and exposure to allergens

Hyperactivity can be demonstrated by bronchial provocation tests where inhalation of a gradually increasing concentration of either methacholine or histamine, may induce a transient episode of airflow limitation in susceptible individuals.

Atopy and airway hyperactivity lead to airway limitation due to:

• bronchoconstriction - contraction of bronchial smooth muscle
• mucosal oedema
• an increased secretion of tenacious mucous

The aetiology of intrinsic asthma is unknown.