National Institute for Allergy and Infectious Disease Allergy is a physiological reaction caused when the immune system mistakenly identifies a normally harmless substance as damaging to the body. Normally, the human body defends itself against harmful substances such as viruses or bacteria, but sometimes the defenses aggressively attack usually innocuous substances such as dust, mold, or pollen.
Allergies can affect anyone, regardless of age, gender, race, or socioeconomic status.
Generally, allergies are more common in children, however, a first-time occurrence can happen at any age or recur after many years of remission. There is a tendency for allergies to occur in families, although the exact genetic factors that cause them are not yet understood. In susceptible people, factors such as hormones, stress, smoke, perfume, or other environmental irritants may also play a role. Often, the symptoms of allergies develop gradually over a period of time. Allergy sufferers may become so accustomed to chronic symptoms such as sneezing, nasal congestion, or wheezing, that they do not consider their symptoms to be unusual.