Two factors are required for seasonal allergies to occur:
(1) The development of an allergic immune response to allergens that are found in pollen, molds or dust.
(2) Exposure to the allergen, with a greater level of exposure causing a stronger allergic response.
The immune reaction that underlies most seasonal allergies is based on the development of a specialized immune response directed against the specific allergen. It is called a Type 1 hypersensitivity response and requires the formation of a special class of antibodies called immunoglobulin E (IgE). Antibodies are proteins made by cells of your immune system that are designed to attack foreign substances. Measurement of IgE antibodies is an important part of diagnostic testing for seasonal allergy.
The risk of developing seasonal allergy is determined by the likelihood that you will develop IgE antibodies to potential triggers. Part of this risk is genetic. Allergies tend to run in families. If both your parents have allergies, the risk that you’ll develop an allergic disorder is 70 percent.
The explosive increase in the prevalence of allergies, however, indicates that environmental (not genetic) factors are creating today’s allergy epidemic. Genes merely increase or decrease individual susceptibility.
The Role of Air Pollution
When hay fever was first described in the 19th century, it was a rare condition only found among city dwellers, not among farmers. That observation highlights one of the most important risk factors for seasonal allergies: exposure to air pollution. Automotive and industrial emissions damage the lining of the respiratory tract, producing inflammation that acts in concert with the presence of environmental allergens to both create and aggravate allergic reactions to inhaled substances. Diesel exhaust particles and cigarette smoke are especially strong promoters of respiratory allergy.
Urban pollution promotes seasonal allergies by one more mechanism: It stimulates pollen production. Scientists from the U.S. Department of Agriculture planted identical ragweed seeds in the inner city of Baltimore and in a rural area 40 miles away. The urban plants grew to be twice as large as the rural plants and generated five times as much pollen.
Indoor air quality and diet are two other common lifestyle factors associated with allergy risk. Formaldehyde, a volatile organic chemical (VOC) found in homes, stores, schools and offices, is a major promoter of nasal allergy (allergic rhinitis), eczema and asthma. It’s found in composite wood products like plywood and particleboard, and in fabrics, floor finishes, paint, wallpaper and in emissions from laser printers, copiers and personal computers.
Levels of formaldehyde in homes are directly linked to individual risk of allergic sensitization to common airborne allergens, with higher levels causing more severe allergy. The rise in allergies over the past several decades has paralleled the increase in formaldehyde-emitting products used inside homes. Formaldehyde levels vary with the season; warmer temperatures and increased humidity (characteristic of summer) increase formaldehyde concentration in homes.
How Diet Affects the Risk of Allergy
Dietary patterns are also associated with allergy risk. The International Study of Asthma and Allergies in Childhood (ISAAC) found that consumption of fast food increases the risk of developing asthma and rhinoconjunctivitis (runny nose with itchy eyes). Fruit and vegetable consumption decreased the incidence of these allergic disorders.
One mechanism by which diet may influence the risk of allergy is through the impact of dietary patterns on bacteria living in the intestinal tract (the gut microbiome). Each of us is naturally colonized, even before birth, by a complex community of bacteria that change — sometimes dramatically, sometimes subtly — during the course of our lives. Their presence and their composition have a profound effect on immune function. Lack of bacterial diversity and the absence of certain species of protective bacteria increase the risk of developing allergic diseases.
Disturbances in the microbiome are thought to explain the increased risk of allergy among children born through Caesarian section or treated with antibiotics in the very early stages of life. They may also explain why living on a farm, being raised from birth with an outdoor pet or being part of a large family protects against the development of allergy. These factors are each associated with greater diversity of bacteria in the gut microbiome.