By Dr. Harriet Burge, EMLab P&K Chief Aerobiologist and Director of Scientific Advisory Board
Introduction: Allergic Disease
Nature of Allergens
For the purpose of this discussion, allergen will be defined as an antigen capable of stimulating an inappropriate IgE response in a susceptible person. A few definitions are necessary here. First, an ANTIGEN is a molecule that can be bound at the antigen-binding site of an antibody. A single antigen may have several distinct binding sites called EPITOPES. Antigens are complex molecules, usually proteins or polysaccharides.
IgE (Immunoglobulin E) is a molecule produced by the human immune system possibly to guard against parasite infections. At least a third of the human population has the genetic propensity to produce IgE against non-parasitic antigens. These people are called ATOPIC.
Nature of the Allergic Response
In an atopic person, the first encounter with an allergen stimulates B cells (through a series of interactions with other cells) to produce IgE antibodies that are specifically designed to recognize the allergen. Because the allergen may have several different binding sites (epitopes), the B cells may produce several different kinds of IgE antibodies, each specific to one of the epitopes. These antibodies circulate in the blood stream, and eventually bind to two types of white blood cells: MAST cells and BASOPHILS. Both of these cell types are filled with granules containing histamine and other inflammatory agents. Mast cells are attached to tissues (including mucous membranes), and basophils circulate in the blood. At this point, the exposed person has become sensitized to the allergen, but does not have symptoms. It probably takes a series of exposures to the same antigen before this process leads to the next step: symptom production.
Once sensitization has been accomplished, the next encounter with the same allergen may cause symptoms. Symptoms result when the allergen molecule encounters an appropriately sensitized mast cell or basophil. When the allergen binds to the antibodies on the surface of these cells, the inflammatory agents they contain are released into the surrounding tissue and cause dilation of local blood vessels (leading to redness), mucous secretion (runny nose), nerve stimulation (itchiness) and smooth muscle contraction (difficulty breathing). The overall type of reaction the individual experiences depends on how the allergen enters the body, the nature of the individual's sensitivity, and the nature of the allergen. If the chemical mediators travel throughout the body, then anaphylaxis occurs. Asthma is the result of exposure to the lower airways. Allergic rhinitis occurs primarily from allergens borne on relatively large particles that impact in the eyes and upper airways. Eczema (atopic dermatitis) is localized to the skin.
Other factors may play a role in sensitization. For example, ADJUVANTS (immunostimulants) may stimulate a stronger response than the allergen alone. For example, the fungus Alternaria has adjuvant-like properties that stimulate sensitization in the lower airways (Kobayashi et al., 2009). Holt et al. (2009) postulate that early failure of the innate mucosal immune system predisposes some infants and young children to respiratory infections and the development of allergies. This predisposition is probably genetically controlled.
Fungal allergens are generally proteins, and are often enzymes released from the fungal spore during germination, although some may also be proteins located on the surface of spores. For surface allergens, the spore may not need to be living to cause sensitization. However, internally produced allergens must be released from the cell before they can be effective. Fungi with hydrophilic cell walls (e.g., Fusarium, Acremonium, Stachybotrys, and many others) may release internal allergens as soon as they contact a water source (e.g., the respiratory mucosa). Whether or not these spores need to be living has not been studied, although it is likely that long dead spores would not have retained the intact allergens. Hydrophobic fungal spores have an outer surface composed of rodlets called hydrophobins. A pathway must develop through these hydrophobins before internal allergens can be released. This pathway occurs during spore germination. Thus, for allergens to be released from hydrophobic spores, the spore must be alive (Aimanianda et al., 2010; Dague et al., 2008).
Probably all fungi can produce some allergen. However, apparently some very common ones are less likely than others to lead to sensitization and symptoms. Using human IgE immunostaining and confocal microscopy, Green et al. (2009) found that among the identifiable spores in their air samples, Cladosporium, Alternaria, Bipolaris, Curvularia, Pithomyces and Stachybotrys contained allergens reacting to the IgE, but Epicoccum, Fusarium, and Spegazzinia did not. It would be interesting to know whether or not any of these spores were actually able to germinate under the conditions used. If not, then this study is biased toward those spore types with surface allergen, and those that can release allergens without germination.
Using a mouse model, Chung et al. (2010) found that allergens derived from Stachybotrys chartarum were less potent than those from house dust mites. More than twice as much Stachybotrys allergen had to be administered to match the response to house dust mite allergen. Again, it is not possible to know whether the Stachybotrys spores were germinated to produce the allergen. House dust mite allergens are readily released from their fecal packages.
Recognizing that fungal enzymes are allergenic, Horner (2010) used human sera and commercially available fungal enzymes in a RAST (radioallergosorbent) test to evaluate specific IgE presence. They found that invertase (from bakers yeast), cellulase (from Trichoderma viride), and glucosidase (from brewer's yeast) reacted with the patient IgE. This doesn't necessarily mean that the patients were sensitized to these specific fungi. It may be that these enzymes, which are produced by many fungi, are sufficiently homologous that their source is irrelevant. This may explain much of the cross-reactivity found among different fungal allergens. In fact, Soeria-Atmadja et al. (2010) used cluster analysis to determine that fungal specific IgE clustered according to relationships within the fungal kingdom. He found the following groupings:
Exposure to Fungal Allergens
Most exposure to fungal allergens probably occurs from inhalation of spores in outdoor air. All of the fungi found indoors are also part of the outdoor ecology, making separation of indoor and outdoor exposures difficult. Given that some spores must be alive to release allergens, it is possible that spores produced in an indoor environment would be "fresher" and more likely to be alive than those outdoors. If this were true, then indoor exposures to some fungi may be more important than outdoor exposure.
I won't reiterate the abundance of data on sources for indoor exposure. Obviously, growth on damp or wet materials is the primary source. I did find one study that documented the danger of evaporative coolers with respect to fungal exposure. Prasad et al. (2009) report that 42% of people with evaporative coolers had positive skin tests to at least one fungus, while only 19% those without these appliances had such reactions. Six-year old children had the highest prevalence of positive skin tests.
Nearly all the information we have about exposure/symptom relationships for the fungi come from epidemiological studies. There are many of these. Data from three recent ones are presented here.
|6-12 year asthmatic children (n=225)||Culturable air sampling||Peak expiratory flow variability||Only Penicillium concentrations strongly associated with outcome||Bundy et al., 2009|
|Children with and without positive skin tests to specific fungi||Culturable air sampling indoors and out||Symptom days; unscheduled doctor visits for asthma||Positive fungal skin test (any) predicted symptom days; indoor total fungal and Penicillium exposure predicted number of symptom days||Pongracic et al., 2010|
|Children 3 years old with and without visible house mold||Observation of visible mold; glucans measurement||High risk for asthma||Visible mold, high risk for asthma; glucans measures predicted low risk for asthma||Iossifova et al., 2009|
|Students||Visible mold||School absences||Absenteeism associated with visible mold and poor building conditions||Simons et al., 2010|
Much of the data from these recent studies reinforces the results of earlier studies. The most interesting of these studies with respect to the nature of fungal allergens are those that seek to clarify the extreme complexity of the fungi and their allergens. There remains an urgent need to develop fungal skin testing materials that more broadly identify the fungal allergic patient. This requires the recognition that fungal allergens are not readily released from intact spores, and the patterns of cross-reactivity between different fungi.
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This article was originally published on September 2010.