Allergy: Disease and Symptoms
By Dr. Harriet Burge, EMLab P&K Chief Aerobiologist and Director of Scientific Advisory Board
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.
Mechanism of Allergic Disease
Hypersensitivity diseases (including allergies) are caused by a reaction of the immune system to foreign proteins (antigens). Originally, four types of hypersensitivity disease were described based loosely on the pathophysiology of the reaction. This is known as the Gell & Coombs classification system. More recently, these have been divided into subclasses, and some advocate a fifth category. Type I hypersensitivity describes what are commonly called allergic reactions.
Type 1 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 (Chapman et al., 1995).
Symptoms of Allergic Disease
A variety of symptoms can result from IgE-mediated hypersensitivity and resultant exposure to histamine and other mediators. The most common are allergic rhinitis and allergic rhino-conjunctivitis, or essentially, runny itchy noses and eyes. Symptoms include: sneezing, itching nose, eyes, ears and palate, rhinorrhea (runny nose), postnasal drip, congestion, the inability to perceive odors, headache, earache, tearing, red eyes, eye swelling, fatigue, drowsiness, and a general feeling of being unwell.
While not generally considered serious, allergic rhinoconjunctivitis is very common, and causes much discomfort, as well as loss of work time and absenteeism. In addition, complications can occur such as the following: acute or chronic sinusitis, middle ear infection, sleep disturbance, dental problems caused by excessive mouth breathing, and others.
Also common is asthma, in which the chemicals released from sensitized mast cells induce airway inflammation resulting in intermittent airflow obstruction and bronchial hyperresponsiveness. These reactions make breathing difficult and cause coughing, wheezing, tightness in the chest, and shortness of breath. For a more detailed description of this illness, see What Is Asthma?
The lungs of asthmatics are also stimulated to produce excessive mucous, which can be colonized by fungi causing the disease allergic bronchopulmonary mycosis. Aspergillus fumigatus is the most common cause of this disease, and the syndrome is then called allergic bronchopulmonary aspergillosis. For more information, see Allergic Bronchopulmonary Aspergillosis.
Atopic dermatitis (the most common type of eczema) is, at least in part, an allergic response in which allergen exposure leads to extremely itchy inflamed skin. The disease is not contagious, but is uncomfortable and difficult to treat. Allergen avoidance is effective if the offending allergens are known. Chronic atopic dermatitis can lead to skin infections. The complete causation for this disease is still under investigation. For more information, see Atopic Dermatitis.
The most serious allergic response is anaphylaxis in which allergen exposure leads to systemic release of mast cell chemicals resulting in hives, swelling of mucous membranes, difficulty breathing (including wheezing), nausea, cramps, tachycardia, and many other symptoms. If not treated quickly, anaphylaxis can be fatal. The response is rarely attributed to inhalant antigens. Food and medication sensitivities are the most common causes. For more detailed information, see Anaphylaxis.
This is an extremely brief overview of the possible symptoms associated with allergen exposure and is provided strictly for educational purposes. It is not to be used for diagnosis of disease, or to attempt to associate allergens in the environment to specific symptoms. People complaining of these symptoms should be referred to a physician with experience in treating allergies.
NIH/NIAID (National Institute of Allergy and Infectious Diseases): Allergic Diseases
AAAAI (American Academy of Allergy, Asthma & Immunology): Conditions & Treatments
Chapman MD, Heymann PW, Sporik RB, Platts-Mills TAE. 1995. Monitoring allergen exposure in asthma: new treatment strategies. Allergy 50(suppl 25):29-33.
Sheehan WJ, Rangsithienchai PA, Baxi SN, Gardynski A, Bharmanee A, Israel E, Phipatanakul W. Age-specific prevalence of outdoor and indoor aeroallergen sensitization in Boston. Clin Pediatr (Phila). 2010 Jun;49(6):579-85. 2010 Jan 13.
This article was originally published on March 2014.