Allergies are overreactions of the immune system to foreign substances. Any substance that triggers an allergic reaction is called an allergen. Pollen, mold spores, dust mites, foods, alcohol, medications, chemicals, and animal dander are common allergens. Allergens cause the body to produce and release histamine and other "mediator" compounds. These compounds affect local tissues and organs, causing symptoms of the reaction.
Symptoms may include itchy or blistering skin, stuffy or
runny nose, sneezing, shortness of breath, red or swollen eyes, headache,
swelling of the lips or tongue, nausea, vomiting, or diarrhea. If the release
of mediator compounds is sudden or extensive, the allergic reaction may be severe,
resulting in anaphylactic shock. U.S. incidence of allergic diseases has grown dramatically
in recent years. An estimated 60 million Americans suffer from allergies; more
than 700 die each year due to allergies, about half of them due to drug
allergies from penicillin.
Measures such as staying away from poison ivy and eliminating
certain foods from the diet can prevent many problems. Medications such as
antihistamines and corticosteroids are helpful in treating allergic reactions. Prompt
injection of the hormone epinephrine (adrenaline) can stop anaphylactic shock,
saving the person's life. (New York Times ‘Guide to Essential Knowledge’)