Immunological aspects of asthma.

PROFESSOR SCADDING's definition of asthma (Scad-ding, 1969) i.e. 'a condition characterized by variable dyspnoea with wheezing and prolonged expiration wholly or partially reversed by bronchodilators', is a clinical definition. It would be premature at this stage to attempt to define asthma in relation to its pathogenesis as this is still not understood. For the purpose of this symposium, however, I would like to present firstly asthma as an immunological disease and secondly, as a condition in which the intrinsic and extrinsic types have the same underlying abnormality. Evidence for immunological involvement in asthma The tissue damage seen in the lungs during a paroxysm of dyspnoea are shown at the microscopic level in Fig. 1. It can be seen that the lumen of the small 2-5 mm bronchus is filled with mucus and cell debris. The cellular infiltration consists mainly of eosinophils. Neutrophils are also present and these may be particularly dense if there is a concurrent infection. Goblet cells in the submucosa are increased in number and there are islands of epithelial re-generation. If the disease is long-standing the basement membrane becomes thickened and the smooth muscle surrounding the bronchi increases in thickness. Oedema and vessel dilatation are also present. Thus there is suggestive evidence in the histopatho-logy of an allergic element to the disease. Other evidence in favour of an immunological involvement includes the relationship between the induction of an asthmatic attack and unusual environmental antigens like castor bean (Figley & Elrod, 1928), the close seasonal correlation with grass pollens and the evidence afforded by provocation test with aerosol extracts (Lowell & Schiller, 1948). The hypersensitivity reaction in asthma The incrimination of immunological mechanisms in asthma implies that in the first instance the patient must become sensitized to the allergen: this phase does not manifest as a clinical disease. Further contact with the allergen produces a hypersensitivity reaction which leads to tissue damage and clinical symptoms. Coombs & Gell (1963) classified the clinical results of hypersensitivity reactions into four types of allergic mechanism. These are illustrated in Fig. 2. Historically asthma has been linked with Type 1. In this reaction a specialized class of antibody is produced which has affinity for certain target cells. AWl FIG. 1. Histology section of the lung from patient dying in status asthmaticus.