House dust mite allergy

The importance of the allergens in sensitisation and disease can be readily demonstrated in regions with and without mite infestation. Dose-responce relationships in regions with high infestation are less easy to detect pointing to the importance of variations of other factors. The relationship between allergic disease and IgE antibody shows that only people with very high responses become persistent asthmatics and many people with high titres are healthy. A major burden of disease is however found in children with lower IgE titres and intermittent asthma, and this cannot be predicted with the current tests. Considerable advances have been made on recognising the important allergens. There is a hierarchical response, for both high and low responders, with the group 1 and 2 allergens accounting for 50-60% of the IgE binding and the mid-range group 4, 5 and 7 allergens accounting for about 20-30%. The IgE responses to most other allergens are low and they do Correspondence/Reprint request: Dr. WR Thomas, Centre for Child Health Research, University of Western Australia Telethon Institute for Child Health Research, Western Australia. E-mail: [email protected] WR Thomas et al. 182 not induce non-allergenic responses, as judged by IgG antibody. Possible important allergens that have not been quantitatively investigated are the group 11, 14 and 15, that are respectively paramyosin, large lipid transfer protein and chitinase. The importance of the Th2 cytokine responses of T cells is well established but the role of Th1 responses in either exacerbating disease or modifying disease is uncertain, as is the role of regulatory T cell responses. A major limitation of many studies is that allergen extracts have been used instead of allergens so the doses have not been optimised and the responses to all the significant allergens have not been measured. The effect of nonallergenic antigens and pro-inflammatory molecules in extracts are further problems. The end results of mite allergen immunotherapy show beneficial effects for disease similar to pollen immunotherapy. New types of immunotherapy or vaccination could therefore have a large impact on disease. Mite avoidance treatments for homes on the other hand have been shown to be largely ineffective probably because the mite reductions that can be achieved are modest. Healthy people exposed to house dust mites make non-harmful responses to mite allergens. They do not result in significant antibody responses of any isotype, but are large as shown by T-cell precursor analysis. I. Exposure sensitisation and allergic disease Verhorst and colleagues first proposed that the pyroglyphid mite Dermatophagoides pteronyssinus was the major source of allergen in house dust (1). The potent allergenicity of proteins produced by the house dust mite (HDM) and importance of the allergy in asthma and other allergic disease remain unquestioned. In regions with high mite infestation, 20-40% of people become sensitised as shown by skin tests or IgE measurement, and about half these develop symptoms of asthma (2-4). Of the asthmatics, about half take daily medication and one quarter can be defined as moderate asthmatics who, despite medication, have frequent symptoms, or need of hospitalisation or emergency treatment (4, 5). The expected requirement for mites to be present in the environment for the development of allergic sensitisation is shown by the lack of IgE and skin test reactivity found in the inhabitants of regions with few mites, such as those found in deserts (4), cold climates (6) and at altitude (7). The people in these areas develop sensitisation to other prevailing allergens such as animals and moulds, so reasons other than the exposure to allergen are unlikely to be involved. Analyses of less contrasting climates have also shown the relationship between allergen in the environment and sensitisation (8). Here the prevalence of mite and cockroach allergy in different cities of the United States of America was found to be associated with the abundance of each allergen source. In contrast to comparisons in different geographical regions, some studies of exposure in different homes within the same environment have not shown a relationship (9, 10) or have only shown a weak association (11, 12). A strong association was found in Germany (13), which might, from the low levels of mites reported, be attributed to dose-response relationships existing only at low doses. This finding is backed by Cullinan et al. (3) who found a sharp dose response at low allergen levels and an attenuation resulting in drop at higher doses. The species-specificity of mite allergy also shows the relationship between exposure and sensitisation. Shen et al. showed that in Taiwan, where D. pteronyssinus predominates over D. farinae, that the IgE antibody responses to Der p 7 was far higher than the antibody titre to Der f 7 (14). Likewise Hales et al. showed that in another area,