Many different species of dust mites have been found in house dust, but the predominant ones in most parts of the world belong to the family Pyroglyphidae: Dermatophagoides pteronyssinus, D. farinae, and Euroglyphus maynei. In Florida, Central America, and Brazil (see Hughes, 1976; Van Bronswijk, 1981; Voorhorst et al., 1967; and Wharton, 1976), several species of storage mites and Blomia tropicalis are important sources of allergens. It is probably best to reserve the term dust mites for pyroglyphid mites and to use the term domestic mites to cover any species of mites that are found in houses (Platts-Mills and de Weck, 1989).

Mites are eight-legged and sightless, and they live on skin scales and other debris. They absorb water through a hygroscopic substance extruded from their leg joints and are thus entirely dependent on ambient humidity. In addition, they have a narrow optimal growth temperature range of between 65° and 80° F. As humidity falls, mites will withdraw from surfaces, but even in very dry conditions it may take months for mites in sofas, carpets, or mattresses to die or for allergen levels to fall (Arlian et al., 1982; Platts-Mills et al., 1987).

Mites excrete partially digested food and digestive enzymes as a fecal particle surrounded by a peritrophic membrane (Tovey et al., 1981a). Large quantities of fecal particles are found in mite cultures, and they are a major form of the mite allergen in house dust. The peritrophic membrane probably keeps the particles intact; however, chitin is not waterproof; consequently, allergens elute from fecal particles quite rapidly (Tovey et al., 1981b). Mite fecal pellets are similar to pollen grains in size (10–35 μm in diameter), in the quantity of allergen they carry (i.e., ~0.2 ng), and in their rapid release of proteins.

Dust mites are approximately 0.3 mm in length. Moving mites can be seen by light microscopy, but the great majority of mites in dust are dead and are therefore difficult to identify without separating them from other dust particles (Arlian et al., 1982; Wharton, 1976).

The first mite allergen to be purified, D. pteronyssinus allergen I (or Der p I; Chapman and Platts-Mills, 1980), is a 24,000-MW glycoprotein that has been sequenced and cloned; it has sequence homology with papain and functional enzymatic activity (Chua et al., 1988). In 1984, high-affinity monoclonal antibodies to Der p I were reported, opening the door to the development of assay systems that would improve the sensitivity and specificity of measurement (Chapman et al., 1984). A second major allergen (MW 15,000) was first identified in 1985 and has now been fully defined,