Allergic reactions occur in response to a variety of substances, including environmental agents, food, medication, venom and contact agents. Common inhaled allergens are pollen from trees, grasses and weeds which provoke seasonal allergy symptoms. Year- round inhaled allergens include dust mites, molds, feathers, and dander from animals such as cats, dogs and horses. Cockroaches are also thought to cause allergic reactions and are believed to pay a role in inner city asthma. Venoms from sting insects, including honey bees, wasps, hornets, yellow jackets and fire ants can also provoke type-1 allergy symptoms.

Allergy symptoms commonly occur in three conditions: allergic rhinitis, asthma and atopic dermatitis. Rhinitis allergy symptoms include nasal congestion, sneezing and a watery nose, while allergic conjunctivitis presents with allergy symptoms of itchy, watery eyes.

Typical allergy symptoms can occur year around or seasonally. Chronic inflammation of the nasal and sinus passages due to allergies can also predispose to the development of sinus infections. Typical allergy symptoms of rhinitis can occur unrelated to allergies such as with exposure to irritants or as a side effect from chronic use of topical decongestants.

Asthma is a condition in which the airways become inflamed, leading to typical allergy symptoms such as wheezing, shortness of breath, chest tightness or a repetitive cough. Asthma is classified according to the frequency and severity of typical allergic symptoms and the degree of airway obstruction as measured by a lung function test. Typical allergic symptoms of asthma can be provoked by both allergic and non-allergic triggers.