Allergies are abnormal or hypersensitive responses by the body's immune system * to substances that are usually harmless.
When Latrell plays outside in the fall, he sneezes and his throat itches. When Melinda pets her friend's cat, her eyes start to water. When Annie puts on her nickel-plated bracelet, her arm breaks out in hives. When Mrs. Garcia feeds her baby a formula * containing milk products, the baby cries as if he has a stomachache. After Ben was stung by a bee, he suddenly had trouble breathing.
These are all examples of allergic reactions. For most people playing outside, petting a cat, touching metal, or drinking milk cause no problems. A bee sting hurts but is not life-threatening. But for millions of people with allergies, these substances and hundreds of others can cause the immune system to leap into action. The result can be as mild as a stuffy nose or as severe as death.
A substance that triggers an allergic reaction is called an allergen. Most allergens fall into one of four main categories:
Allergic responses usually do not occur with the first exposure to an allergen. The immune system must first become sensitized by at least one previous exposure to the allergen or to a very similar substance. For example, a person who is allergic to one type of penicillin will probably have an allergic reaction to other types of penicillin.
The human body's immune system consists of specialized cells and organs that are finely tuned to fight off disease-causing microorganisms * , abnormal cells such as cancer cells, toxins, and other harmful substances. Allergies result from immune system responses to otherwise harmless substances. Because allergies involve the immune system, they are distinguished from hypersensitivity * to a substance or intolerance * for a food.
The immune system recognizes and responds to foreign substances called antigens * . Antigens can be molecules (usually proteins) on the surfaces of viruses, bacteria, parasites, fungi, or abnormal cells. Toxins, drugs, chemicals, and foreign particles can also be antigens.
Humoral immunity refers to large proteins called antibodies * or immunoglobulins * (IM-mune-o-GLOB-u-linz), which are produced by immune-system cells called B-lymphocytes. The immune system produces millions of different antibodies that circulate in the blood and are present in almost all bodily fluids. Allergies are caused by a specific type of antibody called immunoglobulin E or IgE. One end of an IgE antibody has two antigen-binding sites that recognize and bind to a specific allergen like two puzzle pieces fitting together. The other ends of all IgE antibodies are the same and bind to the surface of connective tissue cells called mast cells. People with allergies have much higher levels of IgE in their bodies than people without allergies. They may have IgE antibodies that bind to only one allergen, such as ragweed pollen that causes hay fever. Or they may have many different types of IgE antibodies and are allergic to many different substances.
Cellular or cell-mediated immunity refers to the specialized cells of the immune system that directly attack foreign substances. These include cells that engulf and destroy the antigen, such as macrophages; and white blood cells called T-lymphocytes, which have receptors for specific antigens and also help B cells to produce immunoglobulins.
The most common form of allergy is immediate hypersensitivity or Type I. It depends on both humoral and cellular immunities.
Upon first exposure to an allergen such as ragweed pollen, the immune system of a susceptible person becomes sensitized, which means it goes through the following actions:
The next time the person inhales ragweed pollen, pollen molecules attach to their specific IgE antibodies on mast cells and basophils, causing the cells to explode and release histamines and other inflammatory substances. These substances produce symptoms of an inflammatory allergic reaction and recruit other types of inflammatory immune system cells to the site, increasing the allergic response.
Whereas an allergen-IgE reaction in the respiratory system may cause hay fever, in the skin it may cause a rash or hives. Food and some drug allergies cause reactions in the digestive tract. These allergic reactions happen within a few minutes after exposure to the allergen.
Delayed hypersensitivity occurs with some skin allergies. Delayed hypersensitivity does not involve humoral immunity or antibodies. Rather, T-lymphocytes that have been previously sensitized by exposure to the allergen release chemicals called lymphokines upon subsequent exposure. Lymphokines call up macrophages to engulf and digest the allergen and any cells containing traces of it. They also cause swelling, redness, tenderness, and rashes. These symptoms may take up to 72 hours to appear after exposure to the allergen.
An estimated 50 million Americans suffer from all types of allergies, including indoor/outdoor, food and drug, latex, insect, skin, and eye allergies. The prevalence of allergies has been increasing since the early 1980s across all age, sex, and racial groups.
Allergy is the fifth leading chronic disease in the United States among all ages, and the third most common chronic disease among children under 18 years of age. Approximately 40 million Americans have indoor/outdoor allergies as their primary allergy. It is estimated that approximately 10 million people are allergic to cat dander, the most common pet allergy. The most common indoor/outdoor allergy triggers are tree, grass, and weed pollen; mold spores; dust mite and cockroach allergen; and cat, dog, and rodent dander. In addition, the following allergies occur frequently:
Allergies are the most frequently reported chronic condition in childr en, limiting activities for more than 40 percent of them. The incidence of allergies among American children has been increasing. Some of this increase can be attributed to better diagnosis and reporting. However, much of it may be due to lifestyle and environmental factors.
Although many factors play a role in the development of allergies, heredity appears to be particularly important. If neither parent has allergies, a child has about a 10 percent risk of developing them. If only one parent has allergies, the child has a 30 to 50 percent chance of developing them. If both parents have allergies, the child's risk jumps to 60 to 75 percent, but the child will not necessarily have the same allergies as the parents. Because people with allergies tend to produce more IgE than those without allergies, it may be this propensity that is inherited. High levels of IgE increase the likelihood of having many different allergies.
Repeated exposure to an allergen or prolonged exposure to a strong allergen increases the risk of becoming allergic to that substance. Other factors that increase the likelihood of childhood allergies include:
Oral allergy syndrome is characterized by itching and mild swelling around the mouth. It occurs because some fresh fruits and vegetables—such as melons, cucumbers, zucchinis, and sunflower seeds—contain proteins that are similar to the allergens in ragweed.
Some people suffer from perennial allergic rhinitis. They are bothered by airborne allergens all year long, not just during pollen season. Perennial rhinitis is commonly caused by dust mites * . Household mold spores and pet dander are also major causes of perennial rhinitis.
Skin allergies usually cause a rash or itch. Contact urticaria * causes welts or hives on the skin. It is different from contact dermatitis * , which can produce a reaction hours to days after contact with the allergic substance. Various plants—especially poison oak, poison ivy, and poison sumac—cause allergic reactions in the majority of people. Other skin allergens include the following:
Photoallergic contact dermatitis is caused by a combination of contact with an allergen and sunlight. After-shaves, perfume, sunscreens, and topical medications can contain photoallergens.
With the increased use of latex gloves in the health care and food industries, latex allergies have become a growing concern. It is estimated that 10 percent of health care workers are sensitized to latex. Natural latex rubber is also used in a variety of common products, including balloons and condoms. Symptoms are usually similar to other types of allergic contact dermatitis. Latex can also trigger whole-body allergic reactions, ranging from hives and swelling to asthma attacks and even anaphylaxis.
Testing is available to determine allergy specific to latex. A patch containing latex is applied to the skin of the individual with a suspected latex allergy. The skin is monitored for several days for signs of an allergic reaction such as itching, redness, or the formation of blisters at the patch site. Once a worker is diagnosed with a latex allergy, certain medications may be prescribed. However, the most effective measure is complete avoidance of products containing latex, which is difficult to accomplish. Most hospitals attempt to minimize the number of latex products in their environments and some hospitals have established latex-free zones for patients and staff with known allergy to latex.
Workplaces, such as hospitals, are required by the Occupational Safety and Health Administration (OHSA) to provide alternatives to products that contain latex, such as hospital gloves, for their workers. Latex gloves should not be worn in situations in which there is no risk of exposure to body fluids such as blood or to other potentially infectious materials. The Food and Drug Administration (FDA) has strict guidelines related to the labeling of medical products and devices that contain natural rubber. Those products cannot be labeled as hypoallergenic.
Most adverse reactions to food are caused by intolerances rather than allergies. More than 90 percent of all food allergies are caused by the following:
The most common allergic skin reaction to food is hives. Atopic dermatitis or eczema—patches of itchy, scaly, red skin—can be triggered by a food allergy. Gastrointestinal symptoms of food allergies can include itching in the mouth and throat, stomach cramps, nausea or vomiting, or diarrhea. Food allergies can trigger asthma attacks. However, children often outgrow allergies to milk, eggs, wheat, and soy.
Allergy symptoms vary a great deal and can be similar to symptoms of other conditions, including colds, flu, skin irritations, and food sensitivities or intolerances. Because of this wide variation, allergies can be difficult to diagnose. A physician usually first determines whether there is a family history of allergies and tries to rule out other causes of the reaction. If possible, the best diagnostic test for a suspected allergy is to remove or avoid the allergen and see whether the symptoms disappear.
Skin tests are performed to determine which substances cause an allergic response in the patient. The prick test introduces a tiny amount of a suspected allergen into the skin. An allergen will cause a small swelling at the spot of the prick. If the prick test is negative but the allergen is still suspected, an intradermal test—in which a small amount of allergen is injected under the skin—may be performed. Intradermal testing is useful to determine allergy to wasp and bee stings as well as penicillin allergy and should be performed only after a negative result is obtained from a skin prick test.
Skin testing should be performed by physicians who are allergy specialists and can be performed on people of any age, including infants. However, skin testing should not be done on pregnant women or on individuals who are medically unstable such as those with uncontrolled asthma, compromised lung function, recent stroke, or recent heart-related problems. Testing is usually done on the forearm or the back of the individual being tested.
Blood testing for allergies is used to determine the amount of antigen-specific IgE in the patient's blood. Blood testing is a safer option than skin testing if the individual is at high risk for development of an anaphylactic reaction; however, blood testing is more expensive than skin testing.
This method of allergy testing is very accurate in determining grass pollen, tree pollen, dust mite, and cat dander allergies but not as accurate in determining allergies to venoms, weeds, latex, dogs, and molds. This test is more sensitive than the prick test.
Elimination diets are used to diagnose food allergies. If the symptoms disappear when the suspected food is eliminated from the diet and return when the food is reintroduced, the food is probably an allergen. A skin test may be used to confirm the diagnosis.
Sometimes a skin test for food allergies cannot be performed—for example, if a person is at risk of anaphylaxis. Blood tests called RASTs (radioallergosorbents) and ELISA (enzyme-linked immunosorbent assays) measure the level of food-specific IgE in the blood. A test called the CAP-RAST measures the amount of IgE in the blood that is specific for a given food. A positive skin test or CAP-RAST alone, however, does not mean that the person is allergic to that particular food.
In a double-blind oral food challenge, the patient is given capsules that either may or may not contain the suspected food or the suspected food masked with other foods. If no symptoms appear, the patient is probably not allergic to the suspect food.
Left untreated, allergies can lead to additional complications, including asthma. Allergic rhinitis is the leading cause of sinus infections, along with nasal * polyps from the constant swelling of nasal passages.
The first line of defense against allergies is avoiding allergens. Pets, nickel-plated jewelry, or specific foods may be relatively easy to avoid. Allergens such as pollen, mold spores, and dust mites can be harder to avoid. Recommendations include the following:
Recommendations for avoiding stinging insects include the following:
There are many over-the-counter and prescription medications for treating allergies. Some people take them only when symptoms occur. Others use them daily to prevent or decrease the severity of symptoms. Antihistamines interfere with histamines that cause allergic symptoms by preventing the binding of histamines to their target cells. Decongestants to open nasal passages are used alone or in combination with an antihistamine to treat rhinitis. Corticosteroids may be used to treat severe allergies.
Both prescription and over-the-counter nasal sprays are available for treating rhinitis. These include antihistamines, decongestants, nasal steroids, anticholinergic medicines (those that block the physiological action of acetylcholine), and mast-cell inhibitors.
Immunotherapy, commonly referred to as allergy shots, attempts to decrease the body's sensitivity to an allergen by developing an immunity or tolerance *
Immunotherapy can result in long-lasting relief from allergy symptoms after the treatment is completed. It helps to prevent the development of new allergies. In children, immunotherapy can prevent allergic rhinitis from progressing to asthma.
Although there is no way to prevent allergies, there are many ways to avoid allergic reactions. These include avoiding foods, plants, and other substances that cause allergies. Sometimes, ingestion of allergic substances, such as food, happens inadvertently. A written emergency plan should be available to daycare workers, to school nurses and administrators, at work locations, and to dormitory advisors. The emergency plan should include emergency contact numbers and should provide information as to which emergency medications should be given. The medication most often used for the initial management of a food reaction is injectable epinephrine. The individual with food allergies should have self-injectable epinephrine with them at all times and should be trained on how to self-administer this potentially life-saving medication. If the food reaction is causing mild symptoms of a reaction, antihistamines should be administered.
Identifying the sources of pollen that cause rhinitis can be very useful. Air pollen counts are reported daily.
Mold spores in the air are often tabulated. Wind and warm, dry air increase the pollen and spore counts.
See also Asthma • Bites and Stings • Dermatitis • Hay Fever • Hives • Immune Deficiencies • Immune System and Other Body Defenses: Overview • Psoriasis • Shock • Skin Conditions: Overview
Jones, Keith. Allergies Sourcebook. 5th ed. Detroit, MI: Omnigraphics, 2016.
Sears, Robert W., and William Sears. The Allergy Book: Solving Your Family's Nasal Allergies, Asthma, Food Sensitivities, and Related Health and Behavioral Problems. New York: Little, Brown and Co., 2015.
American Latex Allergy Association. “Latex Allergy…It's More Than Just the Gloves!” http://latexallergyresources.org (accessed March 19, 2016).
National Institute of Allergy and Infectious Diseases. “Food Allergy.” (accessed March 19, 2016).
National Institute of Environmental Health Sciences. “Allergens & Irritants.” http://www.niehs.nih.gov/health/topics/agents/allergens/ (accessed March 19, 2016).
American Academy of Allergy, Asthma, and Immunology. 555 E. Wells St., Suite 1100, Milwaukee, WI 53202-3823. Telephone: 414-272-6071. Website: http://www.aaaai.org (accessed March 19, 2016).
National Institute of Allergy and Infectious Diseases. 5601 Fishers Ln., MSC 9806, Bethesda, MD 20892-9806. Toll-free: 866-2844107. Website: (accessed March 19, 2016).
* immune system (im-YOON SIStem) is the system of the body composed of specialized cells and the substances they produce that helps protect the body against disease-causing germs.
* formula is a prepared, nutritious drink or a dry drink mix designed specifically for infants.
* pet dander refers to microscopic parts of the pet's skin that flake off and get into the air people breathe.
* latex (LAY-tex) is a substance made from a rubber tree and is used in such things as medical equipment (especially gloves), toys, and other household products.
* microorganisms are tiny organisms that can only be seen using a microscope. Types of microorganisms include fungi, bacteria, and viruses.
* hypersensitivity is being excessively sensitive or abnormally susceptible physically to a specific agent such as a drug.
* intolerance is lacking an ability to endure exposure to some environmental feature, such as sunlight, or an exceptional sensitivity, for example to milk, so that the food cannot be properly metabolized, as in glucose intolerance.
* antigens (AN-tih-jens) are substances that are recognized as a threat by the body's immune system, which triggers the formation of specific antibodies against the substance.
* antibodies (AN-tih-bah-deez) are protein molecules produced by the body's immune system to help fight specific infections caused by microorganisms, such as bacteria and viruses.
* immunoglobulins (IM-muneo- GLOB-u-linz) are types of antibodies.
* dust mites are tiny insects that live in dust and in materials such as carpets, pillows, mattresses, and furniture.
* contact urticaria (ur-ti-KAIRee- uh), which can be caused by contact with a variety of compounds, such as foods, preservatives, fragrances, plant and animal products, metals, and rubber latex, produces an immediate reaction of localized redness and swelling.
* dermatitis a skin condition characterized by a red, itchy rash. It may occur when the skin comes in contact with something to which it is sensitive.
* antihistamines (an-tie-HIS- tuhmeens) are drugs used to combat allergic reactions and relieve itching.
* nasal (NA-zal) of or relating to the nose.
* tolerance (TALL-uh-runce) a condition in which a person needs more of a drug to feel the original effects of the drug; also, the body's ability to endure the presence of an allergen with milder or no allergic reaction, developed over time and repeated exposure to the allergen.