The examination of house dust mite extracts has indicated that over 30 different proteins can induce IgE antibody in patients allergic to the house dust mite. There are however dominant specificities especially the group 1 and 2 allergens which can account for much of the allergenicity of extracts. Of the 19 denominated allergens, the major IgE binding has been reported for the group 1, 2, 3, 9, 11, 14 and 15 allergens. The high-molecular-weight group 11, 14 and 15 allergens have only recently been described and although high IgE binding has been anticipated from immunoblotting, there is a need for considerable corroboration. Similarly, the study of the group 3 and 9 serine protease allergens has been incomplete. The group 4, 5, 7 and 8 allergens have shown intermediate IgE binding and the group 10 tropomyosins are of interest because of their potential cross-reactivity with allergen from disparate species. Although the progress with the production of recombinant group 1 allergens has been recent, many of the allergens can be produced as high IgE-binding polypeptides. The tertiary structure of the group 2 allergens has been determined from recombinant proteins and they are an excellent model for the investigation of modified allergens. An unexpected property of the group 1, 2 and 3 allergens has been the high degree of polymorphism found by cDNA analysis. It has however been possible to identify sequences to represent the variation in the natural allergens. The group 7 and 14 allergens show secondary modifications which vary in different extracts creating batch variation. While some estimate of the importance of allergens can be obtained from IgE binding, few analyses of T-cell responses have been made and these regulate both the development of, and the protection from sensitization.

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Fel d 4 is a lipocalin allergen produced by the cat, which binds IgE at relatively high frequency in cat-sensitive individuals. The allergen provides not only a means for investigating differences in the immune response to lipocalin allergens from that found for other mammalian species but also an important reagent for the diagnosis of cat allergy.