An allergy is an abnormal reaction of the body's immune system to a substance in the environment that, for most people, causes no immune response. Any substance that causes allergic reactions in some people is called an allergen. Allergens can be normally harmless substances such as pollen, or noxious substances like pesticides.
Allergies are very common, affecting an estimated 50 million Americans. That means one in every six people in the U.S. has some type of allergy.
Almost anything can trigger an allergic reaction in sensitive people. Exposure to allergens can occur through the air, by touch, or by ingestion. Common allergens include a wide range of indoor and outdoor airborne particles, foods, drugs, animal venom, and other chemicals, including:
-- Tree and plant pollen
-- Animal dander
-- Dust mites
-- Mold spores
-- Bee venom
-- Foods, such as peanuts, milk, eggs, tree nuts, fish, shellfish, soy, wheat
-- Medicines, such as penicillin, sulfa drugs, local anesthetics
The signs and symptoms of allergies include:
-- Itchy, watery eyes
-- Red eyes (conjunctivitis)
-- Runny nose (rhinitis)
-- Stuffy nose
-- Sore throat (especially after waking, due to post-nasal drip and mouth-breathing)
-- Asthma symptoms: shortness of breath, coughing and wheezing
When exposure to an allergen occurs through touch or ingestion, symptoms can vary from itching, hives, flushed appearance, runny nose, and nasal congestion to more severe reactions which can cause nausea, vomiting, abdominal pain, trouble breathing, and difficulty swallowing due to swelling of the face and tongue (angioedema).
An extremely severe allergic reaction can cause anaphylactic shock, a condition in which widespread release of histamine throughout the body leads to a huge drop in blood pressure and loss of consciousness. This is often accompanied by other symptoms of a severe allergic reaction such as those mentioned above (angioedema, nausea, difficulty swallowing and breathing). This is an emergency and can lead to death if left untreated, but deaths from anaphylactic shock are not common: in the U.S. fewer than 1,000 people die each year from anaphylaxis.
The immune system normally protects the body from foreign invaders that cause harm such as viruses and bacteria. During an allergic reaction the immune system turns its defenses against a substance that does not normally cause disease, one that is ignored by the immune system of people without allergies, resulting in the symptoms of allergies.
When a person with allergies encounters an allergen, the immune system generates a large quantity of a type of defense molecule called an IgE antibody to bind to allergen. Each IgE antibody is specific to a particular allergen so a person who is allergic to cat dander but not to dog dander would have large quantities of IgE antibodies that recognize cat dander, but none that recognize dog dander.
After the initial exposure to an allergen, the IgE antibodies that are produced can attach themselves to immune system cells called mast cells. When this happens, these mast cells become primed and ready to be activated. When the person next encounters that allergen, it will bind to the IgE antibodies attached to primed mast cells, signaling the mast cells to release large amounts of histamine and other chemicals that cause inflammation. It is these chemicals that cause the symptoms of allergies.
If the allergen is airborne, it will most likely be encountered in the nose, eyes, or possibly the lungs, leading to symptoms of inflammation in those areas. If the allergen is a food or drug, the reaction often occurs in the mouth, throat or gastrointestinal tract.
Allergies have a strong hereditary component meaning you have a greater chance of developing allergies if one or both of your parents has them. However, scientists do not believe that people inherit sensitivity to specific allergens, but instead inherit a general tendency to develop some type of allergy. Most people with allergies are allergic to more than one allergen.
A person can develop allergies at any age, whether that person is already allergic to several allergens or if that person has never experienced an allergic reaction to anything. Repeated exposure to a common allergenic substance increases anyone's risk of developing an allergy to that substance (e.g. someone who wears latex gloves every day for their job is at increased risk of developing an allergy to latex).
The symptoms of allergies are very similar to those of the common cold so it may be difficult to distinguish between the two. A good rule of thumb is when cold-like symptoms last longer than one or two weeks, or if you tend to have frequent colds, you may want to consult your doctor to discuss being tested for allergies. You may also want to consult your doctor if you notice any of the following:
-- You tend to have cold-like symptoms around the same time every year. This could be a sign of seasonal allergies.
-- You sneeze and your eyes itch when you are around a house pet or when you are around people who may have house pets at home.
-- You have sudden sneezing or upper respiratory congestion when you enter a particular environment such as your place of work or your basement. Note that symptoms may continue even after you leave the environment.
Talk to your doctor. Even if you have symptoms such as those mentioned above, only a doctor can confirm that you have allergies. More importantly, allergy testing can tell you exactly what allergens you are allergic to so you may avoid them in the future. Giving a complete and thorough history is the most important way to find out if your symptoms may be related to allergies.
Skin tests are usually conducted by an allergy specialist and involve administering a series of possible allergens into scratches made on the arm or back or by injecting them subcutaneously (under the skin). If you are sensitive to a particular allergen, it provokes a small immune response and your skin will become raised and reddened in the area where it was injected. The size of the raised area determines how sensitive you are to each particular allergen.
In some cases, the allergist may take a blood sample in order to determine the levels of IgE antibodies against specific allergens. The most common test is called RAST (for radioallergosorbent test). Blood tests are more expensive and less sensitive so they are usually reserved for people in whom skin testing is contraindicated, such as those with skin conditions like eczema or those taking medicines that might interfere with skin tests.
Several treatments are available to treat allergies including both over the counter and prescription drugs.
Antihistamines comprise a broad class of drugs that help prevent the symptoms of allergies by blocking the effects of histamine. These drugs do not prevent the release of histamine from mast cells, but they prevent histamine from interacting with cells and causing inflammation. First-generation antihistamines such as diphenhydramine (Benadryl) and chlorpheniramine (Chlor-Trimeton) are available over the counter but these may tend to cause sedation. However, people react differently to different antihistamine medications: one person may get relief from their symptoms but experience sleepiness, while another person might have less sleepiness but also less help with their symptoms.
Newer generation antihistamines are non-sedating and may be available over the counter or by prescription including fexofenadine (Allegra), loratidine (Claritin), and cetirizine (Zyrtec). Some of these drugs now add a decongestant along with the antihistamine and go by names like Allegra-D, Claritin-D, and Zyrtec-D. Caution: these medications should not be combined, or mixed with alcohol. Avoid driving or using machinery while taking these medications.
Several types of nasal sprays can alleviate the nasal congestion that accompanies allergic reactions.
Nasal irrigation using warm saline (salt water) can clear out excess mucus and particulate while at the same time moisturizing the nasal cavities. This can be accomplished using a fluid-filled syringe, a small squeeze bottle, or by using a neti pot. This technique can be difficult to do at first, but can help mild nasal congestion without using medications.
Antihistamine nasal spray uses a small localized dose of histamine-blocking medicine delivered directly to the nasal passages. Other sprays include steroids that have anti-inflammatory effects in the nasal tissue (Flonase and Nasonex for example). Cromolyn Sodium can be used to help with allergies, but must be started before allergy symptoms begin.
Some over the counter nasal sprays include nasal decongestants that can offer temporary relief from nasal allergy symptoms such as Afrin and Dristan. CAUTION: do not use these sprays for more than three days. They can cause "rebound congestion," where people become dependent on their use due to the nasal tissue adjusting to the decongestant so much that medicine must be administered to get the same relief.
Also called allergy shots, immunotherapy is the only medical treatment that offers prolonged relief from allergy symptoms. It involves subcutaneous (under the skin) injections of increasing concentrations of the allergens to which you are sensitive. Over time, the body begins to make less IgE antibodies against the allergens and instead makes IgG antibodies, which do not trigger allergies. The effects of immunotherapy may last for only as long as the shots are being administered, or they may last for years longer. Talk to your doctor about how often these shots will need to be administered and how long you can expect the effects to last.
People who have had a severe allergic reaction in the past or who may be at risk for such a reaction may want to talk to their doctor about whether to carry an epinephrine auto-injector such as an EpiPen (Epinephrine auto-injector) with them at all times. The auto-injector allows someone to self administer a shot of epinephrine during a serve allergic reaction to prevent the occurrence of anaphylactic shock. Epinephrine increases blood pressure, counteracting the effect of widespread histamine release which can decrease blood pressure below the point necessary to maintain consciousness.
Even if you have never experienced an allergic reaction before, there is no way to ensure that you will not in the future. In fact, a person can develop allergies at any age. Some recent studies have demonstrated that exposure to certain allergens during early childhood may actually reduce the risk of developing all types of allergies in the future, but this research is still in the early stages. Other research suggests that childhood exposure to air pollution and secondhand smoke may dramatically increase the risk of developing allergies. In addition, breastfeeding an infant may decrease its risk of developing allergies in the future.
The chance of developing allergies to a specific allergen increases for people who spend much of their time in an environment containing possible allergens; for example, someone who works in an indoor plant shop may develop pollen allergies or someone who works in a kennel or pet shop may develop allergies to animal dander.
If you have had allergy symptoms in the past, the best thing to do is to determine what allergens you are most sensitive to and take measures to avoid them. Simple tests performed in the doctor's office can assess the common allergens to which you are sensitive (see "How do I know if I have allergies"). If it turns out you are extremely sensitive to cat dander, you should avoid prolonged exposure to cats in enclosed spaces. If you are allergic to spring pollen, you can minimize your exposure by keeping the windows in your home closed in the springtime. To lessen the effect of dust mites and other indoor allergens you can use an air purifier with a high-efficiency particulate air (HEPA) filter.
Another possibility is taking medicine or getting allergy shots (see "What treatments are available for allergies").
In addition to limiting your exposure to known allergens, you can help lessen your allergy symptoms by avoiding other substances that tend to exacerbate allergic reactions. Try to avoid exposure to the following as much as possible:
-- Tobacco smoke
-- Air pollution
-- Cold temperatures
-- High humidity
-- Irritating fumes
-- Wood smoke
-- Aerosol sprays