Medically Reviewed By William C. Lloyd III, MD, FACS

What are allergies?

Allergies are caused by an exaggerated response by the body’s immune system to a particular substance, called an allergen. Allergies, or allergic reactions, are common and include food allergies, allergic conjunctivitis, respiratory allergies, insect bite allergies, drug allergies, and skin allergies. Skin allergies are linked to certain conditions, such as eczema and contact dermatitis. Allergies are also associated with asthma and other respiratory problems.

The immune system is made up of special cells that circulate throughout the body to defend the body against foreign substances, such as viruses and bacteria. When someone has allergies, the immune system overreacts when the person inhales, swallows or touches normally harmless substances, such as pollen or dust. This results in the release of powerful mediators like histamine, which causes symptoms, such as swelling, inflammation, and itching in the affected tissues.

People with allergies are often allergic to more than one substance. Common allergies include those to dust, pollen, mold spores, animal dander, bee stings, and cockroach or dust mite droppings. Some people have allergies to certain plants; medications, such as aspirin or penicillin; foods, such as eggs or milk; or chemicals and other substances, such as latex.

A very common type of allergy is hay fever, which is an allergy to pollen. Hay fever and other respiratory allergies, such as allergies to mold and dust, are types of allergic rhinitis. Symptoms of these allergies can mimic the symptoms of a cold and include runny nose and sneezing. Symptoms of other types of allergies can affect the respiratory system, the gastrointestinal system, and the skin.

Diagnosis and treatment of allergies can control symptoms of allergies to a degree that allows an affected person to live a full and active life. Treatment may include a combination of lifestyle changes, medications and other measures.

Allergic reactions can range in severity from mild to life-threatening. Seek prompt medical care for symptoms of allergies, such as sneezing, watery eyes, nasal congestion, rash, or nausea and vomiting after eating certain foods.

An anaphylactic reaction is an immediately life-threatening type of allergic reaction characterized by a swollen tongue (and swelling in general), combined with hives, itching, shortness of breath, and rapid breathing (more than about 16 breaths per minute for an adult). The reaction is sudden, severe and can include respiratory distress.

Seek immediate medical care (call 911) if you or someone you are with has symptoms of an anaphylactic reaction or other serious allergic reaction, even if there is no history of allergies. If someone with known allergies carries emergency epinephrine (EpiPen), administer the injection according to the package instructions, then call 911.

What are the different types of allergies?

There are several types of allergies that affect different areas of the body. Some types of allergies are temporary, while others are lifelong conditions. It is also possible for someone to have more than one type of allergy at one time.

Types of allergies include:

  • Drug allergy, a response triggered by certain medications, including penicillin, antibiotics with sulfonamides (sulfa drugs), anticonvulsants, chemotherapy drugs, and nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin and ibuprofen. It is important to note that a true allergic reaction is different from the side effects of a medicine. A drug allergy is a rare response involving immunoglobulin E (IgE) antibodies generated by the immune system. Most symptoms caused by medications are more accurately referred to by doctors as “adverse reactions to drugs.”

  • Food allergy, when the body’s immune system views certain foods or food components as harmful. The most common triggers of food allergy in the United States are milk, egg, peanut, tree nuts (such as pecans or almonds), soy, wheat, fish and shellfish. Sesame allergy is also becoming a rising concern among allergists. Nearly 11% of adults and 8% of children in the U.S. have at least one food allergy. Food allergy is different from food intolerance or food sensitivity, when a person has an adverse reaction to certain foods or food groups. An allergic reaction to food involves an immune system response and can trigger life-threatening anaphylaxis. Food intolerance or sensitivity primarily results in digestive symptoms, such as diarrhea, cramping or bloating. 
  • Insect allergy, which can be caused by a variety of insect types. Stinging insects, such as bees, wasps, hornets or fire ants, leave behind a toxin after stinging, which can trigger a life-threatening anaphylactic reaction in people with insect allergies. Biting insects, including mosquitoes, bed bugs and fleas, produce a minor allergic reaction in most people, with such symptoms as itching, redness and swelling. Household pests, such as cockroaches and dust mites, are among the most common triggers of year-round allergy and asthma symptoms. Some people bitten by Lone Star ticks develop an allergy to meat. This is known as alpha-gal syndrome, after the type of sugar injected by the tick that triggers an immune response.

  • Latex allergy, a response to products made with the natural sap of Hevea brasiliensis, the Brazilian rubber tree. (Products made with synthetic latex do not trigger an allergic response.) Latex products that can cause an allergic reaction include rubber gloves, plastic bandages, balloons, condoms, and diaphragms. Because many medical examinations require the wearing of latex gloves, tell your provider if you have a latex allergy so he or she can use a hypoallergenic option instead.

  • Mold allergy, which can be triggered by molds that grow both indoors and outdoors. Mold spores that grow and travel through the air outdoors are most prevalent in summer and fall, when they can trigger seasonal allergy symptoms. Mold that grows inside a home or building often causes allergy symptoms throughout the year. Both types of mold allergy can result in symptoms similar to those of hay fever (allergic rhinitis) and can trigger or worsen asthma symptoms.

  • Pet allergy, most often a reaction to the dander, saliva or urine of a cat or dog. (About twice as many people have cat allergies vs. dog allergies.) While some cats or dogs may be less likely to trigger symptoms, there is no such thing as a “hypoallergenic” breed of dog or cat. Pet hair itself is not an allergen, but rather a carrier of other allergens, including the animal’s saliva, dander or urine, as well as environmental pollen or dust.

  • Pollen allergy, also commonly known as hay fever or by doctors as seasonal allergic rhinitis, is a widely prevalent cause of seasonal allergy symptoms. As grasses, weeds, flowers, trees and other plants bloom in warmer months, they release pollen to fertilize other plants. People can be allergic to specific types of pollen, experiencing more severe symptoms at different times of the year. Many cities publish a daily pollen count during these seasons, including levels of specific pollen types, which can help those with pollen allergy avoid triggers and manage their symptoms.

What are the symptoms of allergies?

When a person has allergies, exposure to an allergen can cause a wide variety of symptoms, depending on the specific allergies, the type of exposure, and the severity of the allergies. Symptoms can occur alone or in combination with other symptoms.

Symptoms of allergies can affect the respiratory system, the skin, and the gastrointestinal system. In the most severe cases, symptoms of a type of allergic reaction called anaphylaxis can affect the cardiovascular system. Allergies are also associated with asthma and wheezing.

General symptoms of allergies

Food allergies, respiratory allergies, insect bite allergies, and skin allergies can present with a variety of symptoms that may include:

Symptoms that might indicate a serious or life-threatening condition

Many people with asthma also have allergies. In some cases, an allergic reaction may trigger symptoms of a severe asthma attack.

An anaphylactic reaction is an immediately life-threatening type of allergic reaction. An anaphylactic reaction can occur in people with no history of allergies. Seek immediate medical care (call 911) if you or someone you are with has any of these symptoms:

  • Change in level of consciousness or alertness, such as passing out or unresponsiveness

  • Chest pain, chest tightness, chest pressure, or palpitations

  • Difficulty breathing, shortness of breath, or wheezing (a high-pitched whistling sound made with breathing)

  • Drooling

  • Hives or swelling that is expanding over a large area of skin

  • Inability to swallow

  • Pallor (very pale skin) or cyanosis (bluish discoloration of the lips, nails, and possibly the skin)

  • Restlessness and severe anxiety

  • Sudden swelling of the lips, tongue, mouth or face

  • Vomiting

If someone with known allergies carries an emergency epinephrine injector (EpiPen) and shows any of these serious or life-threatening symptoms, follow the instructions on the packaging to administer the medication, then call 911.

What causes allergies?

The immune system is the body’s defense system. Its job is to detect and respond to foreign substances that get into the body. When we are exposed to germs, our immune systems generate antibodies that target those germs.

An allergy is essentially an overreaction of the immune system. An allergic reaction is triggered when the body produces and releases a large amount of antibodies called immunoglobulin E (IgE) in response to a usually-harmless substance, like certain foods, medications, or pollen types.

IgE antibodies cause the body to release additional chemicals into the bloodstream, including histamine. Histamine is the substance that causes many allergic symptoms: itchy or runny eyes, sneezing, skin irritation, runny nose, and wheezing.

Once the body has developed IgE antibodies in response to a specific substance, those antibodies will activate and trigger histamine release and allergy symptoms when the body comes into contact with the allergen (the substance that causes an allergic reaction.)

Common allergens

Common allergens include:

  • Animal dander, saliva or urine

  • Bee and wasp stings and other insect bites

  • Chemicals, such as latex, fragrances and preservatives

  • Cockroach and dust mite droppings

  • Dust

  • Foods, such as eggs, milk, peanuts and fish

  • Medications, such as aspirin, penicillin, and sulfa drugs

  • Metals, such as nickel, silver and copper

  • Mold spores

  • Plants, such as poison ivy and poison oak

  • Pollen

Common causes of food allergies

These eight foods cause most food allergies:

  • Eggs
  • Milk
  • Peanuts
  • Tree nuts (including pecans, walnuts and cashews)
  • Fish
  • Shellfish
  • Wheat
  • Soy

Sesame allergy is also a growing concern among allergists.

Many people who have food allergies are allergic to more than one food, and have other allergic triggers. A physician who specializes in diagnosing and treating allergies can perform allergy testing to help pinpoint the specific cause of your allergies. Look for a doctor who is board certified in allergy and immunology (the doctor may further specialize in allergies or immune disorders).

What are the risk factors for allergies?

In most cases, it is unclear exactly why some people develop allergies and others do not.

A family history of allergies is strongly linked to the development of allergies. If someone in your family has hay fever or eczema (an allergic skin condition marked by dry patches of scaly skin), you may be more likely to develop allergies than an individual with no family history of allergy. This is why some food allergies appear to run in the family.

Some people are genetically predisposed to allergies. Individuals who have a particular combination of human leukocyte antigen (HLA) proteins are more likely, for instance, to have a peanut allergy.

Environmental exposure may also influence the development of allergies. Some scientists think the reason more people have allergies today than in years past is because we increasingly live in sanitized environments. Compared to a few hundred years ago—when our ancestors lived in close contact with the land and animals—humans today encounter many fewer germs in the early years of life, which may prime the immune system to overreact in response to typically harmless substances.

Not all people who are at risk for allergies will develop allergies. Risk factors include:

  • Being a child of school age

  • Family history of allergies, especially on the maternal side

  • Having eczema

  • Lack of breast milk as an infant

  • Personal or family history of asthma

How do you prevent allergy attacks?

Many of the factors that contribute to allergies are out of a person’s control, such as family history. However, people with allergies can take steps to avoid known triggers and reduce the frequency and severity of their symptoms.

Preventing environmental allergy symptoms

Steps to limit exposure to environmental allergens include:

  • Avoiding the use of humidifiers at home, to help control and prevent mold growth

  • Keeping doors and windows closed during pollen season

  • Limiting or avoiding exposure to chemicals or smoke

  • Maintaining pest control in your home

  • Monitoring daily pollen counts and adjusting outdoor activity accordingly

  • Showering after being outdoors during pollen season

  • Using protective mattress and pillow covers to protect against dust and dust mites in bedding

Preventing food allergy symptoms

Steps to reduce symptoms of food allergy include:

  • Avoiding known trigger foods

  • Bringing or providing your own allergen-free meals when away from home

  • Educating your child about his or her food allergies

  • Keeping a food journal to track which foods trigger symptoms

  • Storing foods with known allergens separated from other foods (or keeping them out of the home altogether)

  • Notifying teachers, coaches and other parents about your child’s food allergies

  • Reading labels of all packaged food to check for known allergens

Preventing insect allergy symptoms

Steps to avoid being stung or bitten by insects that trigger allergy symptoms include:

  • Avoiding areas with large numbers or swarms of insects

  • Not wearing strong perfumes or other scents

  • Staying away from flowers, fruit trees, flowering bushes, or other plants that attract bees

  • Using caution when eating outdoors and checking food for any insects before taking a bite

  • Wearing light, muted colors (not bright colors, which attract bees) and clothing that fits snugly, with limited skin exposure

Preventing pet allergy symptoms

Steps to prevent symptoms triggered by pet allergy include:

  • Bathing your pet often to keep fur clean of dander, saliva and other allergens

  • Restricting pets from certain rooms at home, particularly bedrooms

  • Washing your hands after petting an animal

Talk to your doctor about your known allergens, including how to limit your exposure to allergy triggers and what to do if you do develop symptoms.

What are diet and nutrition tips with allergies?

Because people with food allergies often have to eliminate certain foods from their diet, they can be less likely to consume all of the nutrients their bodies need. Some studies have shown that children with food allergies are smaller in size than children with no food allergy.

To ensure you or your child maintains a healthy, balanced diet with food allergies, talk to your doctor or pediatrician about recommended food substitutions. You may also work with a dietitian or nutritionist who can provide guidance on healthy foods that offer full nutrition while avoiding allergy triggers. Food allergy support groups online can be helpful resources of tips, suggestions and recipes for cooking and eating with food allergies.

Generally, tips for eating healthy meals while avoiding food allergens include:

  • Avoiding packaged and processed foods, which may contain ingredients that have food allergens

  • Focusing on whole foods, such as beans, fresh fruit and vegetables, and lean meats

  • Preparing meals at home instead of eating out

  • Reading labels carefully on all packaged foods and products

  • Staying away from food areas that are subject to cross-contamination, such as salad bar, delis and buffets

What are some conditions related to allergies?

The symptoms of allergies can mimic many other common conditions, but are triggered specifically by allergens. For many people, allergies can occur alongside other chronic conditions that produce adverse reactions and symptoms.

Conditions related to allergies include:

  • Allergic asthma, which accounts for 60% of all asthma cases. The symptoms of allergic asthma are similar to those of asthma—wheezing and difficulty breathing due to sudden constriction of the airways—but with different triggers. Allergic asthma specifically occurs in response to allergens, such as pollen, dust, and pet dander. Guidelines from the World Health Organization recognize the connection between allergic rhinitis and asthma. Although the link is not fully understood, one theory asserts that rhinitis makes it difficult to breathe through the nose, which hampers the nose's normal function. Breathing through the mouth does not warm the air or filter or humidify it before it enters the lungs, which can make asthma worse.

  • Allergic sinusitis, inflammation of nasal and sinus tissue triggered by allergens. Symptoms are similar to those of non-allergic sinusitis, including nasal congestion, runny nose, and sinus pain. The histamine reaction in allergic sinusitis also causes itchy nose, eyes, and throat.

  • Anaphylaxis, a systemic immune reaction that can cause difficulty breathing, wheezing, nausea, hives, rapid heartbeat, and swelling of the throat, lips or tongue. Anaphylaxis is a life-threatening emergency. If someone shows symptoms of anaphylaxis, administer emergency epinephrine (EpiPen) if available, then call 911.

  • Celiac disease, an autoimmune condition that affects the gastrointestinal tract, resulting in difficulty digesting foods containing gluten, a protein found in such grains as wheat, barley and rye. Symptoms of celiac disease include stomach pain, constipation, diarrhea, fatigue, weight loss, bloating, and cramping. People with celiac disease do not experience anaphylaxis when exposed to gluten.

  • COPD (chronic obstructive pulmonary disease), including emphysema and chronic bronchitis, in which airflow in the lungs becomes blocked, making breathing difficult. Environmental allergens, such as pollen, dust or strong chemicals, can irritate the airways and worsen COPD symptoms.

  • Drug rash with eosinophilia and systemic symptoms (DRESS), an allergic reaction to certain medications that often begins with a rash, most commonly on the face and upper body. Symptoms typically appear about 2 to 6 weeks after starting the medication.

  • Eczema, most commonly a form called atopic dermatitis, which causes redness, itching and irritation on the skin. The exact relationship between eczema and allergies is unclear, but in many adults and children, the two conditions coincide. Atopic dermatitis is just one of the atopic conditions also known as the “atopic march,” which includes food allergies, hay fever (seasonal allergies), and asthma.

  • Eosinophilic esophagitis, a chronic immune response to certain foods that triggers the presence of white blood cells called eosinophils in the esophagus. This causes inflammation that results in symptoms that include difficulty swallowing, decreased appetite, and vomiting.

  • Inflammatory bowel disease (IBD), including Crohn’s disease and ulcerative colitis. Symptoms of these conditions—including diarrhea, bloating and cramping—are not caused by food allergies, but rather by chronic inflammation in the lower gastrointestinal tract. However, people with IBD may also have separate food allergies that can cause life-threatening anaphylaxis.

How do doctors diagnose allergies?

Because the symptoms of allergies are common to many other conditions, an accurate allergy diagnosis can only be made by an allergist trained to evaluate a combination of factors, including allergy testing results, to determine the presence of a specific allergy. While you may be able to self-evaluate your own symptom triggers, it is not possible to self-diagnose an allergy.

Your doctor will first ask you about your medical history and recent experiences with symptoms. Some questions your doctor may ask include:

  • What symptoms have you been experiencing? How long have you had these symptoms?

  • Do you notice your symptoms get better or worse in certain situations?

  • What kind of work do you do? What are your hobbies?

  • Do you notice symptoms when you eat certain foods?

  • Do allergies run in your family?

  • What medications are you currently taking?

  • Do you experience symptoms year-round or at certain times of the year?

This information helps your doctor better understand your condition and determine when and where you may be exposed to allergens that could be causing your symptoms.

The doctor will also perform a physical exam, focusing on your skin, nose, ears, throat and eyes, which are all typically most affected by allergy symptoms. Dark circles under the eyes or swollen tissue in your nose and throat can be indicators of allergic reactions. You may also undergo a lung function test to measure the strength of your breathing.

Tests to diagnose allergies

Finally, you may undergo testing to determine your specific allergens and your reactions to them. Tests to diagnose allergies include:

  • Blood test, sometimes referred to as specific IgE blood testing, RAST (radioallergosorbent test), or by the brand name ImmunoCAP. These blood tests look for the presence of immunoglobulin E (IgE) antibodies, which the immune system produces in response to a perceived danger. A lab technician adds specific allergens to the blood sample to measure how many IgE antibodies are produced in response. However, these tests have high rates of false positives, and are only one tool in an allergist’s diagnosis.
  • Skin prick test, also known as scratch testing, in which a doctor or nurse places small amounts of various allergens on your skin, then lightly pricks the skin with a needle to force some of the allergen into the surface. If you have an allergy, the area around that allergen will react with redness, swelling and itching. However, a positive skin test alone does not necessarily indicate an allergy diagnosis. Your healthcare provider will measure and record the size of each reaction, then consider this information, along with other factors, to make a formal diagnosis.
  • Patch test, often used to test for allergens that cause skin reactions (contact dermatitis). Similar to the skin prick test, several different allergens are placed on the skin, but with adhesive patches. You may wear the patches for up to 48 hours. Your doctor will then remove the patches and evaluate any reactions.

  • Skin injection test, or intradermal skin test, typically used to test for allergies to insect venom or drugs, such as penicillin. A healthcare provider will inject a small amount of allergen into the skin of the arm, then examine the area for any symptoms of a reaction after 15 minutes.
  • Elimination diet, used to identify potential triggers of food allergy. Together, you and your doctor will identify potential problem foods, then eliminate those foods from your diet. Gradually, you will add back one food at a time, then monitor your symptoms as you reincorporate each food. It may be helpful to keep a food journal to track your reactions and pinpoint which foods may be symptom triggers.

  • Physician-supervised challenges, in which a person is exposed to increasing amounts of a specific allergen in a clinical setting. These tests are most often used to test for food allergy or drug allergy. An allergist monitors any signs of reaction and is able to take quick action if the person develops life-threatening anaphylaxis.

Again, testing is only one component of an allergy diagnosis. Talk with your doctor about your results and how they fit into the larger picture of your medical history, lifestyle, and individual symptoms.

What are the treatments for allergies?

Allergy treatment plans use a multifaceted approach that is individualized to best address the specific cause and severity of your allergies, along with your age, lifestyle, and medical history. Overall, allergies are highly treatable, although they are generally not curable.

Allergy treatment includes prevention of symptoms, medications to control symptoms, and lifestyle and dietary changes. Occasionally in children, the allergy becomes less severe as the child gets older.

Drug treatment of allergies

Mild to moderate respiratory and skin symptoms of allergies can be treated with a variety of over-the-counter or prescription medications available in pill, liquid, topical, and nasal spray forms. Medications your healthcare provider may recommend or prescribe include:

  • Antihistamines, which block histamine, the immune system chemical that triggers symptoms, such as itching, swelling, runny nose, and hives

  • Biologic drugs, such as omalizumab (Xolair), that modify the activity of immune factors involved in allergies (Xolair targets immunoglobulin E antibodies)

  • Corticosteroids, most often nasal corticosteroids, which reduce swelling and improve breathing

  • Cromolyn sodium, or mast cell stabilizers, which prevent the body from releasing histamine

  • Decongestants, used to reduce swelling and drain congestion. However, using nasal decongestants more than three days in a row can result in a condition called rebound rhinitis, when symptoms become even worse after the decongestion wears off.

  • Leukotriene modifiers, which prevent symptoms in the lungs and airways, such as airway constriction and inflammation, associated with allergy-induced asthma

Immunotherapy for allergies

Immunotherapy uses exposure to increasing amounts of allergens over time to help the immune system become less sensitive to those allergens. Treatment schedules taper over time but often remain lifelong.

The two most common types of immunotherapy for allergies are:

  • Allergy shots, injections used to treat environmental, seasonal, pet and insect allergies

  • Sublingual immunotherapy (SLIT), an oral treatment that places tablets with small amounts of allergen under the tongue. Currently, SLIT treatments are available for allergies to dust mites, grass and ragweed.

Researchers are testing immunotherapy options for food allergies, particularly peanut allergy, as a means to reduce the risk of anaphylaxis and decrease the frequency and severity of allergic reactions.

Alternative treatments for allergies

Over-the-counter medications and immunotherapy are often very effective methods for managing seasonal or environmental allergy symptoms. For food allergies, avoidance of trigger foods is the only completely effective treatment.

However, some people with seasonal allergies have found relief through alternative treatments, including:

  • Nasal irrigation, using a neti pot or squeeze bottle to rinse nasal passages with a saline solution to clear congestion. Follow the directions on the package and be sure to use clean water that is distilled, sterilized or filtered through a filter with an absolute pore size of 1 micron or smaller.

  • Herbal supplements, such as stinging nettle, butterbur, ginger and perilla, which have been reported to naturally reduce symptoms like nasal congestion, itching, and headache. These benefits have not been scientifically proven, and you should always speak with your doctor before taking any herbal supplement.

  • Acupuncture, which some allergy sufferers claim can help calm symptoms and manage stress associated with allergies. There is little evidence of a direct benefit from acupuncture, but it is also unlikely to cause any harm.

How do allergies affect quality of life?

For people with mild to moderate allergies, symptoms can be a seasonal or year-round nuisance, easily treated with over-the-counter allergy medications. For severe cases of seasonal or environmental allergies, people may need more in-depth treatment like immunotherapy, which requires frequent doctor’s office visits.

Some allergies require significant lifestyle adjustments. These can include not being able to have a cat or dog as a pet, updating a home to address potential allergens (such as mold), limiting participation in outdoor activities or hobbies, or even changing jobs to avoid exposure to specific allergens.

People living with food allergies often need to take significant steps to accommodate their dietary needs. For children especially, it can be difficult to give up favorite foods that contain allergens. Parents can find it stressful to manage a child’s food allergy, both inside and outside the home. Alternative, non-allergenic versions of foods can be difficult to find in grocery stores and often are priced higher than traditional brands.

Anyone living with an allergy that can result in anaphylaxis must carry emergency epinephrine (EpiPen) at all times, and make others around them aware of what to do in case of a life-threatening allergic reaction.

What are the potential complications of allergies?

Complications of allergies are serious, progressive, and can even become life-threatening and fatal. You can reduce the risk of serious complications or delay their development by following the treatment plan you and your healthcare provider design specifically for you.

Complications of allergies can include:

  • Anaphylactic reaction (anaphylaxis), an immediately life-threatening type of allergic reaction

  • Asthma

  • Coma

  • Ear infections

  • Eczema

  • Lung infections

  • Multiple allergies

  • Respiratory arrest from anaphylactic shock

  • Shock

  • Sinusitis
Was this helpful?
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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2020 Nov 18
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