It's hard to believe that items as unrelated as shellfish, pollen, animals, bee venom, peanuts and mold can all potentially lead to the same type of problem. What these substances have in common is that they are common allergens—they cause allergic reactions in some people.

With so many substances potentially causing so many problems, it's no wonder allergic diseases are among the major causes of illness and disability the fifth leading cause of chronic disease in the United States, and the third most common chronic disease in children under age 18. Allergies affect more than as 50 million Americans, or one in every five adults and children, and are as common in women as in men. According to the American Academy of Allergy, Asthma & Immunology, a recent nationwide survey revealed that more than half of all U.S. citizens test positive to one or more allergens. More specifically, an estimated 35 million Americans are allergic to pollen; up to 10 million Americans are allergic to cats; and 2 million develop severe allergic reactions to insect stings. Food allergies are less common, affecting about 12 million Americans. And while up to one out of three people say they have a food allergy, only about one to two percent of adults and six percent of children have true allergic reactions to foods. Unfortunately, about 150 deaths per year are attributed to food allergies, and 40 result from insect stings.

An allergy is a reaction of a person's immune system to a normally harmless substance, one that doesn't cause problems for most people. In the allergic woman, the offending substance causes the immune system, which functions as the body's defense against invading agents such as bacteria and viruses, to respond to a "false alarm." Her immune system treats the allergen as an invader by generating large amounts of a type of antibody—a disease—fighting protein known as IgE-that attaches to the body's tissue and blood cells. The cells are then triggered to release powerful inflammatory chemicals like histamine, cytokines and leukotrienes. These chemicals act on tissues in various parts of the body, such as the respiratory system, and cause the symptoms of the allergy.

Allergies can be more than just bothersome. Some allergies cause symptoms of rhinitis like sneezing, runny nose and watery eyes; skin irritations like itching and hives or eczema; or gastrointestinal distress. In some people, the allergic response can lead to anaphylaxis or anaphylactic shock—a sudden and sometimes deadly drop in blood pressure. Anaphylactic shock can potentially stop the heart or lead to complete closure of air passages, causing death by suffocation.

Allergies that result in respiratory symptoms can be caused by pollens, molds and fungi, dust or dust mites, animals, medications, foods, latex and other substances. Food allergies, which can cause a wide variety of symptoms and are the most likely to cause anaphylaxis, are most commonly caused by shellfish and other fish, peanuts and other nuts, wheat, soy, dairy and eggs.

Contact allergies are allergic reactions caused by a person's skin touching a substance. They are most often caused by nickel, polishes, rugs, fabric softeners, deodorants, cosmetics, perfumes, preservatives, dyes and foam insulation. And don't forget plants like poison ivy, poison oak and sumac. There is a compound in these plants that can cause severe skin reactions in people who are allergic—up to 85 percent of Americans.

Allergies have a genetic component. If one parent has allergies, each child has a 30 to 40 percent chance of having an allergy. If both parents have allergies, it is much more likely (a 75 percent chance) that their children will have allergies. Adults usually do not lose their allergies, but children can sometimes outgrow them. In addition, while people are born with a genetic predisposition to allergies, sometimes they don't develop them until well into adulthood. Exactly what turns these genes on and when remains a bit of a mystery. Some researchers speculate that the more you are exposed to a potential allergen, the more antibodies to that allergen you can build up, until finally one day, they are released in an allergic reaction. Once you become "sensitized," you're going to get recurring symptoms every time you are exposed to the allergen.