When most people think of an allergy, they think of the sneezing, congestion, and itchy eyes caused by pollen. In fact, allergies can be caused by just about any substance that you inhale or swallow, or which touches your skin.

Your body's immune system is designed to attack harmful substances like bacteria and viruses. But with allergies, your body launches an assault on substances that are basically harmless -- such as pollen, mold, dust mites, pet saliva and dander, and even medications and insect sting venom.

Allergies are extremely common. About 40% of the population suffers from them, leading to millions of missed work days and school days each year. Annual medical costs exceed $4 billion.

Not only do allergies cause a range of annoying symptoms such as sneezing and itchy eyes, but they can aggravate or trigger other conditions such as asthma, sinusitis, and ear infections. For example, when allergies cause inflammation in your nasal passages, the opening to your sinuses can become blocked, leading to sinus inflammation, sinus infections, and sinus pain. Similarly, if allergies cause inflammation in your ear canal, the ears don't drain properly, which can lead to ear infections.

But allergies don't stop there. The body's immune system is active from your head to your toes: allergies can cause different skin-related symptoms (such as hives, eczema, and rashes); some types of allergies can lead to gastrointestinal problems; and assorted allergy-related symptoms can pervade the body (such as fatigue, headaches, loss of smell, and sleep disturbances).

One of the most deadly kinds of allergic reaction is called "anaphylaxis," which occurs when the entire body has a swift, severe reaction to something such as peanuts or bee stings. The body literally goes into shock, leading to a sharp drop in blood pressure, respiratory arrest, and possible heart failure.