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The term "allergy" was first introduced in 1906 by an Austrian pediatrician named Clemens von Pirquet. His original meaning of the word has changed over time and is now used to refer to the body's hypersensitivity to substances or irritants in the environment. While people can have allergies to many different substances (foods, medications, plants), this Guidebook focuses on Hay Fever, also called Allergic Rhinitis or Seasonal Allergies.

Allergens (the substances that produces the allergic response) include trees, grasses, pet dander, mold, and dust mites. The body reacts to exposure to these allergens by producing an antibody called immunoglobulin E (IgE) in the intestines and lungs which triggers the release of different chemical mediators from inflammatory cells such as mast cells, basophils, and eosinophils. Examples of these chemical mediators of inflammation that can trigger an allergic or hypersensitivity response include: autocoids such as histamine, eicosanoids (prostaglandins), cytokines, chemokines, and neuropeptides.

The term atopy is often used to describe IgE-mediated diseases. People with atopy have a hereditary predisposition to produce IgE antibodies against common environmental allergens and usually have one or more atopic diseases such as hay fever, asthma, and atopic eczema.

The early or immediate phase of the allergic response occurs in sensitized persons within minutes of exposure to the allergen. Depending on the amount of the allergen, this early phase response is usually followed by the late phase reaction which reaches a peak 6-9 hours after exposure, then slowly resolves within 18-24 hours. This late phase is characterized by persistent symptoms (sneezing, runny nose) but predominantly by nasal congestion.

Allergic rhinitis (hay fever) typically starts in early childhood with symptoms peaking in the second, third and fourth decades of life. An onset of symptoms in infancy or later in adults is not uncommon. Allergy symptoms may range from the common and annoying sneezing, watery nose and itching eyes, to more severe reactions which may be life-threatening.

Asthma is often associated with allergies. In fact, allergy-induced asthma is the most common type of asthma in the United States. Twenty to fifty percent of patients with allergic rhinitis have concomitant asthma, and 60% of people with asthma have the allergic type. Airway hyperresponsiveness, the exaggerated narrowing of the airways after the inhalation of various stimuli, is a key feature of asthma. According to the Centers for Disease Control and Prevention (CDC), the prevalence of asthma among U.S. children increased from 3.6% in 1980 to 5.8% in 2003. Asthma is the third leading cause of hospitalization among persons under the age of 18 in the United States.

Allergic symptoms can interfere significantly with many aspects of daily living as a result of physical discomfort and emotional distress. Numerous clinical studies have demonstrated the affects of rhinitis on sleep, concentration, performance at work and school, social life and other activities and the importance of control of the disease and its symptoms.

It has been estimated that 20-40 million Americans are affected by seasonal allergies One study puts the annual costs for seasonal allergies at $2.4 million for medications and another $1.1 billion in other health care costs.