Allergic disease is among the most common illnesses associated with indoor air exposure. Approximately 35% of the population has the ability to develop allergies to environmental proteins. In some populations this percentage is higher. For many, exposure to dust mite proteins is the precipitating factor inducing sensitization. In others (for example in California), grass pollen proteins appear to be the primary culprits. Fungal exposure can also induce sensitization, and in Arizona, Alternaria proteins appear to be especially important. Many other environmental proteins can also be involved as can a number of other biological agents.
Thus, when evaluating indoor environments for agents that might be causing symptoms in the occupants, it is important to consider the possibility that allergen exposure is responsible. This article briefly discusses the nature of allergies, the mechanism of sensitization and development of symptoms of allergic disease.
Hypersensitivity otherwise known as allergy, is mediated by antibodies that circulate in the blood stream. Antibodies are complex proteins (called immunoglobulins) that learn to recognize specific antigens. In the case of allergy, these antigens are called allergens. There are five types of circulating antibodies that play a role in hypersensitivity: Immunoglobulins IgA, IgD, IgE, IgG and IgM. Immunoglobulin E (IgE) mediates allergic disease, although some types of IgG antibodies may also be involved.
In genetically susceptible people (and animals) exposure to some environmental proteins (allergens) causes cells in the bloodstream (called B cells) to produce IgE antibodies that recognize specific allergenic proteins. These sensitized antibodies attach to mast cells (also in the blood stream). Mast cells are little bags of chemicals, one of which is histamine. This is called the sensitization phase.
Once enough specific antibodies have been produced, which can take as long as a year or two of low level exposure, re-exposure to the same allergen is recognized by the antibodies on the mast cells, which then open to release their chemicals. It is these chemicals (including histamine) that cause the symptoms of allergic disease. (For a more detailed discussion of this process, see IgE’s Role in Allergic Asthma.) People who produce IgE in response to environmental allergens are called “atopic.” Exposure of sensitized individuals to allergens causes a rapid response, and allergic disease is often called “immediate hypersensitivity.”
Note that allergic symptoms do not occur on first exposure to an allergen. It takes many exposures to induce sensitization. It appears that a relatively low level of exposure over time induces sensitization, whereas, once sensitization occurs, symptoms are triggered by higher level exposures.
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