Allergies

Epidemiology

Allergic rhinitis can occur at any age but most commonly affects children and adolescents, with the mean age of onset around 8 to 11 years. Recent studies and surveys indicate that allergic rhinitis may affect around 40 million people in the United States alone, and the annual health care costs may be around $15 billion. In childhood, it is more common in boys but occurs equally in both sexes in adulthood. Allergic rhinitis can affect persons of all races, and prevalence varies depending on geographic location (i.e., warm vs. cold, rural vs. urban).

Etiology

Pollens and mold spores are the allergens that are generally responsible for seasonal allergies. These pollens usually come from trees, grasses, and weeds that depend on the wind for pollination, which produce large quantities of lightweight pollen that can be breathed in, triggering symptoms. Flowering plants and those plants that depend on insect pollination typically produce a heavier pollen that rarely causes symptoms. The seasons and the location determine the type of allergies a patient may develop. For example, the highly allergenic ragweed is found in the Midwest and the eastern United States and typically produces pollen from August through September. See Table 1 for further details.

Table 1 Common Environmental Allergens

Pathophysiology

In patients who are susceptible to allergies, exposure to an allergen leads to a cascade of events that result in characteristic symptoms. When a patient is exposed to an allergen, he or she can develop allergic sensitization, which is characterized by the production of specific IgE directed against these antigens. Mast cells, which are present in the nasal mucosa, bind IgE, which leads to immediate and delayed release of a number of chemical mediators. These include histamine, kinins, leukotrienes, prostaglandin D2, and others. Through a series of chemical reactions, these mediators eventually cause the common symptoms associated with allergies, such as nasal congestion, itching, and sneezing. This is the immediate response, and over the next 4 to 8 hours, the late-phase response occurs. This occurs through recruitment of other inflammatory cells such as neutrophils, eosinophils, macrophages, and lymphocytes. All these cause continued inflammation. The symptoms are similar but with more congestion and mucous production and less sneezing and itching.

Diagnosis and Testing