An allergy is an overreaction of the immune system to a substance that's harmless to most people. But in someone with an allergy, the body's immune system treats the substance (called an allergen) as an invader and overreacts, causing symptoms that can range from annoying to serious or life threatening.

The type and severity of allergy symptoms vary from allergy to allergy and child to child. Allergies may show up as itchy eyes or an itchy nose, sneezing, nasal congestion, throat tightness, trouble breathing, vomiting, and even faintness or passing out. Severe allergic reactions (called anaphylaxis) can be fatal if not treated in time.

The number of people with allergies is increasing every year. The reasons for this are not understood, but 1 of the main theories is it's the result of living in a cleaner, germ-free environment, which reduces the number of germs our immune system has to deal with. It's thought this may cause it to overreact when it comes into contact with harmless substances.

Some allergies are fairly easy to identify because the pattern of symptoms following exposure to certain allergens can be hard to miss. But other allergies are less obvious because they can be similar to other conditions.

Kids with severe allergies (such as those to food, medication, or insect venom) can be at risk for a sudden, potentially life-threatening allergic reaction called anaphylaxis. This reaction can be frightening — a child might faint or feel like his or her throat is closing, for example. But when treated properly, anaphylaxis can be managed.