Allergies are abnormal reactions to ordinarily harmless substances. The sensitizing substances, called allergens, may be inhaled, swallowed, or come into contact with the skin.

Allergens that most frequently cause problems are: pollens, mold spores, house dust mites, animal danders, foods, insect bites or stings, plants, insect spores, latex rubber, viruses, bacteria, medications and environmental conditions (such as cold temperatures). Allergic reactions can occur in one area, such as sneezing...