Allergies

The symptoms are familiar to millions of Americans - runny nose, sneezing, watery eyes - but it's not from a cold or flu, it's allergies. Allergic diseases are among the most common causes of disability in the United States, affecting over 40 million Americans, as many as one in six, according to the National Allergy Bureau (NAB). When someone has an allergy, they're sensitive to substances that don't affect most people, for example, pollen, mold, medications, even certain foods. An allergic reaction is our immune system doing its job to protect us from foreign substances. Unfortunately, it's a false alarm; most of these substances should pose no real danger to us.

What is an allergy?

An allergy is an abnormal reaction to a normally harmless substance. While these substances, called allergens, don't bother most people, an allergic person's body will overreact when exposed to them, releasing chemicals that cause the classic allergy symptoms. Depending on the cause, allergies can affect the skin, the respiratory system, the eyes and/or the digestive system. (Read about "Skin" "Respiratory System" "The Eye" "Digestive System")

Sometimes, the respiratory symptoms of an allergy resemble that of a cold. (Read about "The Common Cold") But there are important differences. Colds are caused by viruses (Read about "Microorganisms"), whereas allergies are caused by your body reacting to a certain substance. Also, colds can develop at any time after you've been infected, whereas allergies develop in the presence of the thing you're allergic to.

Allergies may be seasonal in nature. They can develop because of something you breathe in. (Read about "Pollen Allergies" "Mold Allergies" "Dust Allergies") They can develop because of something you ate. (Read about "Food Allergies") Or the symptoms of an allergy may develop when someone touches something, such as latex. (Read about "Latex Allergy") People can also develop an allergic reaction following a bug bite. (Read about "Insect Bites")

For some people, this type of allergic reaction can be deadly. Anaphylactic shock (Read about "Anaphylaxis") can cause swelling of body tissues (including the throat), vomiting, cramps and a sudden drop in blood pressure. The American Academy of Allergy, Asthma and Immunology (AAAAI) says anaphylaxis often occurs in sensitive individuals who are allergic to penicillin, stinging insects and shellfish or nuts. Each year it occurs in one to five percent of the U.S. population as a result of insect stings. Anaphylaxis can also occur as a result of exposure to pet dander, certain foods or other allergens. This type of reaction can be very dangerous, since swelling of throat tissue can impair a person's ability to breathe.

What causes an allergy?

The substances that set off an allergic reaction are called triggers or allergens. An allergic reaction involves two features of the human immune response. (Read about "The Immune System") One is the production of immunoglobulin E (IgE), a type of protein called an antibody that circulates through the blood. The other is the mast cell, a specific cell that occurs in all body tissues but is especially common in areas of the body that are typical sites of allergic reactions, including the nose and throat, lungs, skin and gastrointestinal tract.

The ability of a given individual to form IgE against something as benign as food is considered to be an inherited predisposition. The Food and Drug Administration (FDA) says generally, allergic people come from families in which allergies are common. Those allergies can be to just about anything - food, pollen, dander or other substances that can cause reactions. Someone with two allergic parents is more likely to develop allergies than someone with one allergic parent.

Before an allergic reaction can occur, a person who is predisposed to form IgE to the trigger first has to be exposed to it. As this trigger is encountered, it causes certain cells to produce specific IgE in large amounts. The IgE is then released and attaches to the surface of mast cells. The next time the person comes in contact with that trigger, it interacts with specific IgE on the surface of the mast cells and tells the cells to release chemicals such as histamine. Depending upon the tissue in which they are released, these chemicals will cause a person to have various symptoms. If the mast cells release chemicals in the ears, nose and throat, a person may feel an itching in the mouth and may have trouble breathing or swallowing. If the affected mast cells are in the gastrointestinal tract, the person may have abdominal pain or diarrhea. (Read about "Diarrhea") The chemicals released by skin mast cells, in contrast, can prompt hives.

You can follow the links below to learn more about different types of allergies and allergic reactions.