In a person who develops this misdirected reaction, the first exposure to the allergen causes a mild immune response that sensitizes the immune system to the substance (triggers the immune system to recognize the substance). The second and subsequent exposure to the allergen usually results in symptoms. The type of symptom that develops depends on the specific allergen, the part of the body where exposure occurs, and the way the immune system reacts to the allergen.

When an allergen enters the body of a person with a sensitized immune system, it triggers antibody production; these antibodies bind to cells that contain histamine. Histamine (and other chemicals) are released by these cells when the antibodies are stimulated by allergens. This causes itching, swelling of affected tissues, mucus production, muscle spasms, hives, rashes, and other symptoms. Symptoms vary in severity from person to person.

The part of the body contacted by the allergen will, in part, also affect symptoms. For example, allergens that are inhaled often cause nasal congestion, itchy nose/throat, mucus production, cough, or wheezing. Ingestion of a food allergen, in addition to the risk of causing a severe, life-threatening reaction, can cause nausea, vomiting, abdominal pain, cramping, or diarrhea. Allergies to plants often cause skin rash. Drug allergies usually involve the whole body.

Some disorders may be associated with allergies. These include eczema and asthma, among others.

Common allergens include environmental agents that contact the skin, breathing passages, or the surface of the eye (such as pollen; see also allergy to mold, dander, dust). Food allergies and drug allergies are common. Allergic reactions can be caused by insect bites, jewelry, cosmetics, and almost any substance that contacts the body.

Some people have allergic-type reactions to hot or cold temperatures, sunlight, or other physical stimuli. In some persons, friction (such as rubbing or vigorously stroking the skin) will cause symptoms. The mechanism that causes this is not well understood, but it is possible that minute changes in the chemistry of the skin may occur in response to physical stimuli and some component of this chemical change triggers the allergy. (See also atopic dermatitis and contact dermatitis.).

Allergies are relatively common. Both hereditary and environmental factors have been found to contribute to a predisposition to developing allergies.

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