House dust mite exposure as a cause of asthma

It is well known that inhalation of an allergen is capable of inducing wheeze in a sensitized individual. But the idea that allergen exposure is a primary cause of asthma has encountered opposition. Recent advances have made it possible to study the relationship between house dust mite allergen exposure and asthma [reviewed in i]. These include the purification of the major allergens [2], the development of ELISA techniques which have facilitated allergen measurement [3] and a series of epidemiological studies from many parts of the world. In this article, we review the infonnation from diverse studies which when combined strongly suggests that exposure to allergens is an important cause of asthma. We focus on the evidence related to house dust mite allergens because it is the most complete. However there are a number of other inhaled allergens which are of importance in individual cases, in certain geographic areas or at specific times of the year. The implication from these studies is that the primary cause of asthma, especially the asthma occurring in children and young adults, is inhalation of allergens. For an allergen to have a clinical effect, prior sensitization must have occurred. Sensitization requires the combination of an innate ability to respond and exposure. Immediate sensitivity, characterized by IgE antibody production, is part of the immune response to these proteins, which also involves antigen presenting cells, T cells, B cells and other effector ceils. Exposure of a nonsensitized individual to even large quantities of an allergen will not provoke symptoms, whereas a sensitized individual is capable of reacting to extraordinarily small concentrations of allergens, i.e. in the picogram range. Immediate sensitivity can be measured by either the level of allergen specific IgE antibody or by skin testing [4,5,6]. The development of asthma can be considered to consist of two distinct processes: (i) a period resulting in sensitization followed by (ii) exposure of the sensitized person resulting in inflammation and ciinical wheeze. This latter process was elegantly referred to as 'excitement' in the early literature. The presence of inflammation may make the lung more sensitive not only to the specific allergen