Allergies, estimated to afflict one out of every five Americans, are abnormal reactions to normally harmless substances in the environment called allergens. Upon exposure to an allergen, the body’s immune system mistakenly identifies it as a threat and takes defensive action against the presumed invader. The immune system’s inappropriate attempt to protect the body gives rise to allergy symptoms, which can affect multiple organ systems and which range from the mild to the life threatening.
Symptoms
Allergy symptoms vary according to the nature of the allergen and the way in which it encounters the body, and can be roughly divided into three categories that are defined by the manner in which an allergen enters the body.
Airborne Allergens
Airborne allergens like dust, pollen and pet hair travel through the air and come into direct contact with the eyes, nose, throat, sinuses and lungs. This contact gives rise to a constellation of symptoms including sneezing, nasal irritation, itching and watery eyes and runny nose. Allergic rhinitis, commonly known as hay fever, falls into this category, with symptoms that occur seasonally or year round in response to various pollens and spores emitted by trees, grasses and weeds. Pet allergies can be triggered both by airborne exposure and by contact, and the airborne component causes many of the same symptoms as hay fever. Animal fur is not the culprit. Instead, symptoms are caused by exposure to minute flakes of skin, or dander, shed by the animal.
Many of the symptoms associated with airborne allergens, while uncomfortable, are relatively mild, but serious symptoms do occur. Severe symptoms can begin with wheezing and shortness of breath and escalate to full-blown asthma attacks and difficulty breathing caused by swelling of the throat.
Ingested Allergens
Food, medication and insect stings are all capable of producing allergic reactions.
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Food allergies can cause stomach pain, vomiting, diarrhea and itching or swelling of the skin. As digestion progresses, symptoms may spread from one organ system to another, beginning with itching in the mouth as the food is taken in, followed by pain, vomiting or other abdominal symptoms as the food moves through the system. As digestion continues, food is broken down and its components are released into the bloodstream. In this way, allergens may be released into the blood and transported throughout the body.
Since the dissemination of the allergen is so widespread, symptoms can affect the skin in the form of hives or eczema, the digestive system itself or the circulatory system as a whole. In the last case, the result can be anaphylaxis, a sudden drop in blood pressure than can be fatal if not treated immediately.
Medications and insect stings give rise to allergy symptoms that can simultaneously affect several organ systems. Symptoms can appear immediately upon exposure or their onset can be delayed. They may include itching or hives, difficulty breathing, swelling of the tongue or throat, stomach pain, vomiting and cramps, all symptoms of anaphylaxis. Loss of consciousness and coma may occur as the reaction continues. Symptoms may appear to be diminishing, only to return in force even if there has been no subsequent exposure to the allergen.
Contact Allergens
Allergic reactions can occur when the skin is exposed to certain substances, including poisonous plants, latex and animal saliva.
Symptoms resulting from contact exposure most often begin with itching and redness of the skin at the point of contact, followed by the development of a rash, blisters or hives. Symptoms of exposure to poisonous plants generally subside after a week or two, but may persist in particularly sensitive individuals for up to 30 days. While uncomfortable, symptoms do not generally become more serious, although infection can occur at the affected site. However, serious respiratory symptoms can result from the inhalation of smoke from burning poisonous plants like poison ivy or poison oak. This mode of exposure usually necessitates extensive medical treatment.
Latex allergy chiefly occurs among health care workers, where symptoms of exposure are typically limited to a localized rash or irritation called contact dermatitis. More serious symptoms, including anaphylactic reactions, can occur following abdominal surgery and medical and dental procedures that expose patients’ mucous membranes to latex gloves.
Causes
Once the immune system marshals its defenses, all allergies follow the same physiological pattern regardless of the allergen involved.
- First, person is exposed to an allergen through the skin, the respiratory system or the digestive system, and the immune system identifies the allergen as a threat.
- This initial exposure does not cause any allergic symptoms, but sets the immune system to work against the invader.
- Certain white blood cells begin to produce an antibody called Immunoglobulin E, or IgE, whose job it is to bind the allergen.
- IgE circulates throughout the body and attaches itself to mast cells, which play a key role in the inflammatory process and are rich in histamine.
- At this point, the immune system has been sensitized to the particular allergen.
- The next time the immune system encounters the allergen, it binds the allergen and the mast cells release histamine and a broad arsenal of other chemicals that produce inflammation.
- This time around, the release of those inflammatory agents, known collectively as “mediators,” results in symptoms.
Whether symptoms are localized or general depends on the specific allergen and the person involved, and whether an allergic reaction is mild or severe depends to some extent on the extent and suddenness of the release of mediators. The presence of great numbers of mast cells in the respiratory and digestive tracts explains the frequent occurrence of allergy symptoms in those locations.
Risk Factors
While the physiological causes of allergy are well understood and relatively straightforward, risk factors involve a complex interaction of genetics, environment and individual history and susceptibility.
Heredity
Heredity plays a strong role in the probability that a person will be subject to allergies. Children in families where both parents are allergic have at least a 60% chance of having allergies. Such children are more likely to be sensitive to more allergens and to have allergies that are more severe than children who do not have an allergic parent. In general, this genetic predisposition does not translate into the inheritance of sensitivity to a specific allergen, but studies of identical twins have found a 70% likelihood that the twins will suffer from the same allergies.
In children, the incidence of allergy is higher in boys, although this imbalance tends to disappear in adults.
Racial and ethnic factors have also received some attention from researchers, particularly in light of the higher distribution of asthma in the African American population, but results have been inconclusive, especially because of the difficulty of separating environmental and genetic influences.
Environment
Almost by definition, the environment is a risk factor for allergy. After all, allergies do not exist without allergens and allergens come from the environment, whether in the form of pollen, food, medicine, pet dander or poison ivy. Some environmental risk factors are not as self-evident.
Public health researchers have noted a number of epidemiological trends that suggest a broad environmental basis for the development of allergies.
- In the Western world, the incidence of allergies has approximately tripled between 1980 and 2010.
- Allergies are much more common in the industrialized world than in the developing world.
- Immigrants from the developing world to the industrialized world show a greater risk of allergy, and that risk grows along with their time in the industrialized world.
- Children in larger families are less likely to develop allergies than those in smaller families.
The generally accepted explanation for these trends, the “hygiene hypothesis,” holds that the immune system benefits from exposure to a wide variety of irritants and bacteria and thus becomes more tolerant of foreign substances. In the absence of that kind of stimulation, the immune system is more likely to react to a normally harmless substance like pollen.
Other environmental factors contribute to the risk of developing allergies, chiefly by serving as irritants. The likelihood of allergic reactions, especially in children, increases with exposure to air pollution and tobacco smoke.
Personal Factors
Some risk factors are associated with a person’s individual history or with other medical conditions.
A person who has been exposed to allergens in childhood is more like to develop allergies in later life, and more likely to react to the allergen to which the child was exposed. Continued exposure to known allergens increases the risk that a person will become allergic to them.
Frequent ear, nose and throat infections in childhood also correlate with increased allergy risk, as does treatment in early childhood with broad-spectrum antibiotics and the use of antibacterial cleaning products in the home. There is also evidence that infants who are breast-fed and are not given cow’s milk or solid foods for the first six months of life are less prone to allergies later.
Among medical conditions, the presence of nasal polyps, asthma and skin sensitivity are all conditions that correlate with the development of allergies. People with high levels of IgE in the bloodstream are also more likely to develop allergies.
Prevention
Allergy prevention begins with avoiding exposure to the substances that trigger allergic reactions. Therefore, preventive strategies vary according to the allergens and irritants involved.
Pollen and Mold
- Close windows at home and in the car.
- Avoid outdoor activity in the early morning and late afternoon, when pollen counts are high.
- Use air conditioning to filter pollens from the air, and change filters regularly.
- Keep indoor spaces ventilated in order to cut down on moisture.
- Hard surfaces are easier to clean and less conducive to mold growth, so avoid rugs and carpets, especially in the kitchen and bath.
- Keep indoor plants to a minimum, as they can provide homes for mold.
- Avoid drying clothes outdoors where they tend to accumulate pollen.
Pet Dander
- Limit or eliminate the pet’s access to bedrooms.
- Wash the pet frequently, in order to remove dander and pollens that can collect on the pet’s fur.
- Use air filtration machines, either central systems or room units, preferably with HEPA filters.
- Foods
- Read food labels and question restaurant staff about ingredients.
- Learn to recognize early symptoms, since allergic reactions to food can happen quickly and may require immediate intervention.
- Introduce solid foods to infants gradually, beginning when the child is at least six months old.
Poisonous Plants
- Learn to identify plants that produce allergic reactions.
- Wear gloves and long pants and sleeves when encountering potentially poisonous plants.
- Avoid burning plant materials, especially if any are suspect.
- Medications
- Be sure to inform health care providers of any known allergies and of any family history of medication allergies.
- Wear a medical alert bracelet that identifies the allergy if you cannot communicate.
Diagnosis
The two medically accepted avenues of diagnosis are skin testing and blood testing, although a vast array of unproven and generally invalid methods competes with the established diagnostic tools.
Skin Testing
In skin testing, the back or arm is pricked with a metal or plastic device that makes a series of tiny punctures in the skin. Small amounts of the proteins associated with suspected allergens are applied to the skin and mapped according to their placement. If an allergy to the particular substance is present, the individual site will redden or develop a hive. Skin testing has demonstrated a high degree of diagnostic accuracy in all areas except for food allergies, where blood testing provides results more clearly related to specific allergens.
Blood Testing
Blood testing is more complicated and expensive than skin testing, but is sometimes preferred for patients who have had severe anaphylaxis as well as for patients who have recently taken antihistamines or who have diseases of the skin. It does allow for greater accuracy in pinpointing food allergies.
In blood testing, a small amount of blood is drawn and analyzed for the presence of IgE antibodies that are specific to a particular allergen. High levels of allergen-specific IgE antibodies indicate the likelihood of an allergic reaction to the allergen.
Unproven Tests
Unproven tests greatly outnumber the two tests that have proven medical validity. These procedures include measurement of changes in the electrical resistance of the skin, changes in muscle strength on exposure to a suspected allergen and changes in pulse rate after eating a suspected allergenic food. Other unconfirmed methods are the microscopic examination of blood cells in contact with an allergen and the analysis of samples of hair and tissues for trace amounts of chemicals alleged to be toxic. No evidence supports the ability of these tests to provide any benefit to the patient, diagnostic or otherwise.
Treatment
Prevention is the first line of allergy defense, but it is impossible to prevent all exposure to allergens, especially to allergens like pollen that are ubiquitous in the environment. There are many available treatment options, from home remedies to sophisticated immunotherapy, and modern treatments have proven to be extremely effective.
Over-the-counter Medications
Antihistamines – As their name suggests, antihistamines work by blocking histamine receptors in the body. They have a long history of use in treating allergy symptoms and can be taken when needed for relief of sneezing, runny nose and watery eyes. Early antihistamines were known to cause drowsiness, but there are now drugs available, including loratadine, that provide relief without the troublesome side effect, Antihistamines are available as pills, liquids and nasal sprays and as eye drops can help treat itchy eyes.
Decongestants – Decongestants shrink blood vessels and swollen tissues in the nose, helping to decrease their tendency to produce fluid. Although they can be taken as needed in response to symptoms, long term use of decongestants as nasal sprays can lead to a worsening of symptoms once the decongestant is stopped. They tend to raise blood pressure and may be contraindicated in people with hypertension. Decongestants are available as pills, nasal sprays and eye drops, and by prescription in combination with other allergy medications.
Combined medicines – Medicines are also available that combine two different active ingredients. Most commonly, these consist of an antihistamine and a decongestant, although other preparations combine allergy medicines with pain medications, asthma medications or other ingredients.
Prescription Medications
Bronchodilators – Bronchodilators work by opening the airways in the lungs and allowing mucus to move more freely. They are used to treat the symptoms of asthma and come in short-acting forms for immediate relief and long-acting forms that last for up to 12 hours. Bronchodilators can raise blood pressure and increase the pulse rate. They are available as inhalers, as they must be inhaled into the lungs in order to be effective.
Leukotriene modifiers – Leukotrienes are produced by the immune system and contribute to inflammation in allergies and asthma. Leukotriene modifiers suppress their production. Side effects are uncommon, mainly involving stomach upset or pain, but the medication must be taken daily in order to have the desired effect. Leukotriene modifiers are only taken orally.
Mast cell stabilizers – Mast cells are the histamine-rich cells to which IgE attaches in the allergic process, and stabilizer drugs prevent the cells from releasing histamine. They may also have some anti-inflammatory effects and are useful in preventing asthma symptoms. In allergy treatment, they are used as a preventive medication taken prior to exposure to a known allergen. Mast cell stabilizers may take up to a month to reach full therapeutic potential. They can be taken orally or used as inhalers or nasal sprays.
Steroids – Steroids chiefly act by suppressing inflammation and are among the most versatile of allergy medicines. They relieve asthma symptoms, as well as nasal congestion, runny nose and sneezing. Steroidal creams can be applied to the skin to treat symptoms caused by contact with allergens. Side effects of steroids are as varied as their uses and include high blood pressure and weight gain in the short term and cataracts, diabetes and muscle weakness in long-term use. Steroids are available as pills, nasal sprays, eye drops and topical creams. When taken orally, steroids are generally prescribed as a course of treatment lasting at least several days, but they are also effective when used as needed.
Epinephrine – Epinephrine, also known as adrenaline, is used as an emergency treatment for anaphylaxis. It is injected immediately upon the appearance of anaphylaxis symptoms, including itching, swelling of the mouth or tongue, trouble breathing and nausea. Side effects can include hypertension, heart palpitations, racing pulse, anxiety and headache, but none of these side effects contraindicates the use of epinephrine in a potentially deadly onset of anaphylaxis. Epinephrine is only available in injectable form and comes in a small syringe containing a premeasured dose. The EpiPen is perhaps the best-known brand. A variation that includes a second dose, the Twinject, is designed to manage the relatively common situation in which symptoms of anaphylaxis have subsided only to reappear several minutes later.
Immunotherapy
Immunotherapy does not cure allergies, but it is the closest medicine has yet come to providing permanent relief.
Immunotherapy consists of a series of injections given in the upper arm for a period of several years. Initial treatment is given once or twice per week and, if successful after the first several months, once or twice per month for the duration of treatment. The shots consist of gradually increasing doses of a specific allergen in the hope that the patient will become desensitized and develop something akin to immunity to the allergen used.
Immunotherapy has proven to be a safe and effective treatment for allergic asthma and for allergies to pollen, pet dander, molds and insect bites. It is not considered an effective treatment for food allergies. Side effects are rare, although there is a small risk of a severe reaction to the allergen injected.
Other delivery methods have been used with some success outside the United States, including oral and intranasal administration of the dose of allergen. In addition, attempts have been made to shorten the time necessary to build up to a maintenance dose by reducing the interval between allergen exposures to as little as several hours. This strategy is used only in a hospital setting as it entails greater risk of a severe reaction to the allergen.
Home and Herbal Remedies
Home treatments are capable of relieving a number of allergy symptoms. For nasal congestion and symptoms of hay fever, nasal irrigation washes irritants from the nose and sinuses. A bulb syringe or neti pot can be used to introduce a saline solution to the nose.
For insect stings, aloe vera or a paste of water and baking soda can relieve pain and itching at the site of the sting.
Many herbal remedies have been proposed as allergy treatments, including goldenseal, stinging nettle and butterbur. Although a comprehensive 2006 Mayo Clinic study of herbal and other nontraditional approaches found little evidence of their efficacy, two recent European studies of butterbur concluded that it is as effective as an oral antihistamine in alleviating hay fever symptoms, and that it does so without causing drowsiness.
Unproven Remedies
Alternative therapies like homeopathy, kinesiology, naturopathy and probiotics find little support in the medical literature and have not been shown to be effective when studied individually.
Acupuncture stands out as the one exception to those disappointing results, although its promise derives from two small studies. The first showed an improvement in hay fever symptoms in all 26 of the patients studied, and did not report any side effects or adverse reactions. The second study involved 76 patients and reported that two sessions of acupuncture eliminated symptoms in over half of those participating.
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