By Dr. Harriet Burge, EMLab P&K Chief Aerobiologist and Director of Scientific Advisory Board
Nature and Importance
Allergens are complex molecules that can stimulate an antibody response in susceptible individuals. Allergens are usually proteins and exposure commonly results in an immunoglobulin E (IgE) response (immune defense against foreign objects, i.e. bacteria and viruses). The ability to respond to allergens with an IgE response is genetically controlled. Repeated low-level exposure is generally thought to lead to sensitization, and subsequent to sensitization, a response may occur with further exposure. It is important to note that not all sensitized individuals develop symptoms. Symptoms result when the appropriate allergen attaches to IgE antibodies on the surface of MAST cells, causing these cells to release histamine, the chemical that leads directly to symptoms. Symptoms may be upper respiratory (hay fever), lower respiratory (asthma), systemic (anaphylaxis), or may occur on the skin (hives). Several of these symptoms may be present simultaneously.
Allergens may also lead to very high levels of specific immunoglobulin G (IgG) and to sensitized cells in the lung. Intense exposure to the allergen is generally necessary and sensitization may occur over a relatively short period of time. Sensitization may lead to breathing difficulties. With continued exposure, sensitization may lead to difficult breathing, fever, cough and chest tightness. This disease is called hypersensitivity pneumonitis and innate risk factors are unknown.
Allergens are named for the organism from which they are purified. Thus, dust mite allergens are called Der f, Der p, and Blo t for Dermatophagoides farinae, Dermatophagoides pteronyssinus, and Blomia tropicalis, respectively. Multiple allergens may be derived from a single organism, and these are numbered as they are discovered (e.g., Der f 1, Der f 2, etc.).
Allergens may be produced by almost any organism, although some organisms tend to produce either more allergens or more potent allergens. Table 1 lists common indoor allergen sources.
|Organism||Allergens||Source within Organism||Exposure Source|
|Dust mites||Der f 1, Der p 1, Blo t 1, etc.||Fecal material||Dust, especially in bedding|
|Cockroaches||Bla g 1, Bla a 1||Skin secretions||Dust, especially in kitchens|
|Cats||Fel d 1, etc.||Skin secretion||Airborne, accumulates in dust|
|Dogs||Can f 1||Skin secretions, urine||Airborne, accumulates in dust|
|Rodents||Rat r 1, Mus m 1||Urine||Airborne, accumulates in dust|
|Fungi||Alt a 1, Pen c 1, Asp v 1, etc.||Digestive enzymes||Airborne with spores; accumulates in dust|
Most bacteria do not produce IgE stimulating allergens. However, some Gram positive bacteria (primarily Mycobacterium, thermophilic actinomycetes, and, rarely, Bacillus) can stimulate IgG and can sensitize cells in the lung leading to hypersensitivity pneumonitis.
Sampling for allergens almost always involves collection of dust samples, although air samples can be collected for those allergens that are readily airborne (cat, rodent, some fungi). Dust samples are collected from the area where exposure is most likely to occur. For dust mite allergens, this is almost always bedding. For cat and dog allergens, usually living or family room dust is used. For cockroach and rodent, kitchen dust should be collected. Dust collections are best made from a carefully measured surface, although the nature of the surface will strongly influence how much dust is collected from the area. Because of this fact, dust concentrations are usually measured as milligrams / gram of dust collected.
Allergen analysis involves the use of the enzyme linked immunosorbent assay (ELISA). This assay can be done in several ways. If suitable monoclonal antibodies for the specific allergen are available, sandwich assays are used, where one antibody is coated onto microtiter plates, the allergen-containing suspension added, then a second specific antibody labeled with a color-producing compound is added. The allergen attaches quantitatively to the second antibody and the intensity of color is related to allergen concentration in the sample. A less specific approach must be used for allergens for which monoclonal antibodies are not available. In general, inhibition assays are used in these cases. Dilutions of polyclonal antibodies (antibodies derived from relatively crude allergen preparations and containing many allergens) are mixed with the unknown allergen extract. The antibodies bind to the allergen, preventing attachment to allergens bound to microtiter plates. The unknown allergen mixture without added antibody is used as a control, and the amount of inhibition caused by the added antibody is proportional to allergen concentration. These assays are relatively non-specific, but have the advantage of being ?broad spectrum? and not dependent on the presence of specific identified allergens.
Allergen concentrations that actually lead to sensitization and to symptoms are not known precisely, and all available information is derived from epidemiological studies. This means that the commonly used guidelines are simply that, and some people will respond to concentrations much lower or much higher.
For most of the allergens that have been studied, concentrations >2µg/gram of dust are considered a risk for sensitization, and 10µg/gram of dust is considered a risk for symptom development in sensitized individuals. Concentrations for cockroach allergens appear to be much lower and are not well-defined. High concentrations of cockroach allergen should be considered to represent a cockroach infestation. Fungal allergens have been poorly studied, and few of the common fungi are represented by good ELISA assays.
This article was originally published on April 2008.