currently unknown. In ABPA, exposure to allergens released from active fungal surface growth in the lung stimulates the production of both IgE and IgG. The relationship between exposure to airborne spores and either initiation of ABPA or the status of ABPA patients is unclear.
Fungal allergens may be a structural part of the microbial cell, or they may be produced by the cells and released into the environment. The fungal allergens that have been isolated thus far are water-soluble glycoproteins, some of which are enzymes (Baldo and Baker, 1988; MacDonald et al., 1989); some may also be high-molecular-weight carbohydrates (Savolainen et al., 1990). Only a few have been partially characterized (Aukrust and Borch, 1979; Horner et al., 1988, 1989; Pazur et al., 1990; Savolainen et al., 1990).
Crude fungal extracts are complex mixtures of soluble materials from mycelial and spore walls, cytoplasm, and metabolites. These extracts are produced from fungi grown in a liquid medium for 5–15 days; the mixture is then blended and filtered. Sometimes the fungal growth is removed from the liquid by filtration and subsequently ground, dried, and extracted. Residual culture medium is sometimes used as a second kind of preparation (Kauffman et al., 1984).
Batch-to-batch variability in fungal extracts is often greater than variability among different strains, species, or even genera (Burge et al., 1989; Savolainen et al., 1989). For most of the mushrooms and other macrofungi, field collections are usually used to produce allergen extracts. Preliminary studies of the comparative allergen content of spores, fruiting body tissue, mycelium, and spent culture medium demonstrate both similarities and differences. Variability in allergen content (determined by radioallergosorbent tests inhibition; see Chapter 6) has been observed in the same kinds of mushrooms collected from different sites and, to a lesser degree, from the same site at different times (Liengswangwong et al., 1987). Studies on cross-reactivity of allergens extracted from different taxa of fungi (Baldo and Baker, 1988; De Zubiria et al., 1990; O'Neil et al., 1988; Shen et al., 1990; Weissman, 1987) have not generally documented batch and strain variability within each species.
There are few reports of experimental human challenges with fungal allergens. Licorish and others (1985) provoked immediate and delayed asthma using Alternaria whole-spore challenges, but they produced only an immediate response with spore extracts. Lopez and coworkers (1989) induced positive bronchial challenges with basidiospores. For some fungi, possibly because the actual allergens are enzymes associated with germination, it may be necessary for a living unit to begin growth on the respiratory tract mucosa before allergen exposure occurs (Savolainen et al., 1990).