MSD Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Overview of Allergic Reactions


Peter J. Delves

, PhD, University College London, London, UK

Last full review/revision Oct 2020| Content last modified Sep 2022
Topic Resources

Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance.

  • Usually, allergies make people sneeze; the eyes water and itch, the nose runs, the skin itches, and rashes develop.

  • Some allergic reactions, called anaphylactic reactions, are life threatening.

  • Symptoms suggest the diagnosis, and skin tests can help identify the substance that triggers the allergy.

  • Avoiding the trigger is best, but if it is impossible, allergy shots, when given long before the exposure occurs, can sometimes desensitize the person.

  • People who have had severe allergic reactions should always carry a self-injecting syringe of epinephrine and antihistamine pills.

  • Severe reactions require emergency treatment in the hospital.

Normally, the immune system Overview of the Immune System The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites... read more —which includes antibodies, white blood cells, mast cells, complement proteins, and other substances—defends the body against foreign substances (called antigens). However, in susceptible people, the immune system can overreact when exposed to certain substances (allergens) in the environment, foods, or drugs, which are harmless in most people. The result is an allergic reaction. Some people are allergic to only one substance. Others are allergic to many. About one third of the people in the United States have an allergy.

Allergens may cause an allergic reaction when they land on the skin or in the eye or are inhaled, eaten, or injected. An allergic reaction can occur in several ways:

In many allergic reactions, the immune system, when first exposed to an allergen, produces a type of antibody called immunoglobulin E (IgE) IgE One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more IgE . IgE binds to a type of white blood cell called basophils in the bloodstream and to a similar type of cell called mast cells in the tissues. The first exposure may make people sensitive to the allergen (called sensitization) but does not cause symptoms. When sensitized people subsequently encounter the allergen, the basophils and mast cells with IgE on their surface release substances (such as histamine, prostaglandins, and leukotrienes) that cause swelling or inflammation in the surrounding tissues. Such substances begin a cascade of reactions that continue to irritate and harm tissues. These reactions range from mild to severe.

Latex sensitivity

Latex is a fluid that comes from the rubber tree. It is used to make rubber products, including some rubber gloves, condoms, and medical equipment such as catheters, breathing tubes, enema tips, and dental dams.

Latex can trigger allergic reactions, including hives, rashes, and even severe and potentially life-threatening allergic reactions called anaphylactic reactions. However, the dry, irritated skin that many people develop after wearing latex gloves is usually the result of irritation and not an allergic reaction to latex.

In the 1980s, health care workers were encouraged to use latex gloves whenever touching patients to prevent the spread of infections. Since then, latex sensitivity has become more and more common among health care workers.

Also, people may be at risk of becoming sensitive to latex if they

  • Have had several surgical procedures

  • Must use a catheter to help with urination

  • Work in plants that manufacture or distribute latex products

For unknown reasons, people who are sensitive to latex are often allergic to bananas and sometimes other foods such as kiwi, papaya, avocados, chestnuts, potatoes, tomatoes, and apricots.

Doctors may suspect latex sensitivity based on symptoms and the person's description of when symptoms occur, especially if the person is a health care worker. Blood or skin tests are sometimes done to confirm the diagnosis.

People who are sensitive to latex should avoid it. For example, health care workers can use gloves and other products that are latex-free. Most health care institutions provide them.

Causes of Allergic Reactions

Genetic and environmental factors work together to contribute to the development of allergies.

Genes are thought to be involved because specific mutations are common among people with allergies and because allergies tend to run in families.

Environmental factors also increase the risk of developing allergies. These factors include the following:

  • Repeated exposure to foreign substances (allergens)

  • Diet

  • Pollutants (such as tobacco smoke and exhaust fumes)

On the other hand, exposure to various bacteria and viruses during childhood may strengthen the immune system, help the immune system learn how to respond to allergens in a way that is not harmful, and thus help prevent allergies from developing. An environment that limits a child's exposure to bacteria and viruses—commonly thought of as a good thing—may make allergies more likely to develop. Exposure to microorganisms is limited in families with fewer children and cleaner indoor environments and by the early use of antibiotics.

Microorganisms live in the digestive tract, in the respiratory tract, and on the skin, but which microorganisms are present varies from person to person. Which ones are present appears to affect whether and which allergies develop.

Allergens that most commonly trigger allergic reactions include

  • House dust mite droppings

  • Animal dander

  • Pollens (of trees, grasses, and weeds)

  • Molds

  • Food

  • Insect venom

  • Drugs

  • Latex

  • Household chemicals, such as cleaning products and fragrances

House dust mites live in the dust that builds up in carpets, bedding, soft furnishings, and soft toys.

Symptoms of Allergic Reactions

Most allergic reactions are mild, consisting of watery and itchy eyes, a runny nose, itchy skin, and some sneezing. Rashes (including hives) are common and often itch.

Hives Hives Hives are red, itchy, slightly elevated swellings. The swelling is caused by the release of chemicals (such as histamine) from mast cells in the skin, which cause fluid to leak out of small... read more Hives , also called urticaria, are small, red, slightly elevated areas of swelling (wheals) that often have a pale center. Swelling may occur in larger areas under the skin (called angioedema Angioedema Angioedema is swelling of areas of tissue under the skin, sometimes affecting the face and throat. Angioedema can be a reaction to a drug or other substance (trigger), a hereditary disorder... read more Angioedema ). Swelling is caused by fluids leaking from blood vessels. Depending on which areas of the body are affected, angioedema may be serious.

Certain allergic reactions, called anaphylactic reactions Anaphylactic Reactions Anaphylactic reactions are sudden, widespread, potentially severe and life-threatening allergic reactions. Anaphylactic reactions often begin with a feeling of uneasiness, followed by tingling... read more , can be life threatening. The airways can narrow (constrict), causing wheezing, and the lining of the throat and airways may swell, interfering with breathing. Blood vessels can widen (dilate), causing a dangerous fall in blood pressure.

Diagnosis of Allergic Reactions

  • A doctor's evaluation

  • Sometimes blood tests

  • Often skin tests and the allergen-specific serum IgE test

Doctors first determine whether a reaction is allergic. They may ask

  • Whether the person has close relatives with allergies because a reaction is more likely to be allergic in such cases

  • How often reactions occur and how long they last

  • How old the person was when the reactions started

  • Whether anything (such as exercise or exposure to pollen, animals, or dust) triggers the reaction

  • Whether any treatments have been tried and, if so, how the person responded

Blood tests are sometimes done to detect a type of white blood cell called eosinophils. Eosinophils, although present in everyone, are usually produced in greater numbers when an allergic reaction occurs.

Because each allergic reaction is triggered by a specific allergen, the main goal of diagnosis is to identify that allergen. Often, the person and doctor can identify the allergen based on when the allergy started and when and how often the reaction occurs (for example, during certain seasons or after eating certain foods).

Skin tests and the allergen-specific serum IgE test can also help doctors detect the specific allergen. However, these tests may not detect all allergies, and they sometimes indicate that people are allergic to an allergen when they are not (called a false-positive result).

Skin testing

Usually, a skin prick test is done first. Dilute solutions are made from extracts of pollens (of trees, grasses, or weeds), molds, dust mites, animal dander, insect venom, foods, and some antibiotics. A drop of each solution is placed on the person’s skin, which is then pricked with a needle.

Doctors may also use other solutions to help them interpret the person's response to the allergens. A drop of a histamine solution, which should trigger an allergic reaction, is used to determine whether a person's immune system is working. A drop of diluting solution, which should not trigger an allergic reaction, is used for comparison.

If the person is allergic to one or more of the allergens, the person has a wheal and flare reaction, indicated by the following:

  • A pale, slightly elevated swelling—the wheal—appears at the pinprick site within 15 to 20 minutes.

  • The resulting wheal is about 1/8 to 2/10 inch (about 0.3 to 0.5 centimeters) larger in diameter than the wheal caused by the diluting solution.

  • The wheal is surrounded by a well-defined red area—the flare.

The skin prick test can identify most allergens.

If no allergen is identified, an intradermal test is done. For this test, a tiny amount of each solution can be injected into the person’s skin. This type of skin test is more likely to detect a reaction to an allergen.

Before skin tests are done, people are asked to stop taking antihistamines and certain antidepressants called tricyclic antidepressants (such as amitriptyline) and monoamine oxidase inhibitors (such as selegiline). These drugs may suppress a reaction to the tests. Some doctors also do not test people who are taking beta-blockers because if such people have an allergic reaction in response to the test, the consequences are more likely to be serious. In addition, beta-blockers may interfere with the drugs used to treat serious allergic reactions.

Allergen-specific serum IgE tests

The allergen-specific serum IgE test, a blood test, is used when skin tests cannot be used—for example, when a rash is widespread. This test determines whether IgE in the person's blood binds to the specific allergen used for the test. If binding occurs, the person has an allergy to that allergen.

Provocative testing

For provocative testing people are directly exposed to the allergen. This test is usually done when people must document their allergic reaction—for example, for a disability claim. It is sometimes used to diagnose a food allergy. If doctors suspect an exercise-induced allergy, they may ask the person to exercise.

Prevention of Allergic Reactions

Environmental measures

Avoiding or removing an allergen, if possible, is the best approach. Avoiding an allergen may involve the following:

  • Stopping a drug

  • Keeping pets out of the house or limiting them to certain rooms

  • Using high-efficiency particulate air (HEPA) vacuums and filters

  • Not eating a particular food

  • For people with severe seasonal allergies, possibly moving to an area that does not have the allergen

  • Removing or replacing items that collect dust, such as upholstered furniture, carpets, and knickknacks

  • Covering mattresses and pillows with finely woven fabrics that cannot be penetrated by dust mites and allergen particles

  • Using synthetic-fiber pillows

  • Frequently washing bed sheets, pillowcases, and blankets in hot water

  • Frequently cleaning the house, including dusting, vacuuming, and wet-mopping

  • Using air conditioners and dehumidifiers in basements and other damp rooms

  • Treating homes with heat-steam

  • Exterminating cockroaches

People with allergies should avoid or minimize exposure to certain other irritants that can make allergic symptoms worse or cause breathing problems. These irritants include the following:

  • Cigarette smoke

  • Strong odors

  • Irritating fumes

  • Air pollution

  • Cold temperatures

  • High humidity

Allergen immunotherapy (desensitization)

Allergen immunotherapy, usually allergy shots (injections), can be given to desensitize people to the allergen when some allergens, especially airborne allergens, cannot be avoided and the drugs used to treat allergic reactions are ineffective.

With allergen immunotherapy, allergic reactions can be prevented or reduced in number and/or severity. However, allergen immunotherapy is not always effective. Some people and some allergies tend to respond better than others.

Immunotherapy is used most often for allergies to

  • Pollens

  • House dust mites

  • Molds

  • Venom of stinging insects

When people are allergic to unavoidable allergens, such as insect venom, immunotherapy helps prevent anaphylactic reactions Anaphylactic Reactions Anaphylactic reactions are sudden, widespread, potentially severe and life-threatening allergic reactions. Anaphylactic reactions often begin with a feeling of uneasiness, followed by tingling... read more . Sometimes it is used for allergies to animal dander, but such treatment is unlikely to be useful. Immunotherapy for peanut allergy is available, and immunotherapy for other food allergies is being studied.

Immunotherapy is not used when the allergen, such as penicillin and other drugs, can be avoided. However, if people need to take a drug that they are allergic to, immunotherapy, closely monitored by a doctor, can be done to desensitize them.

In immunotherapy, tiny amounts of the allergen are usually injected under the skin. The dose is gradually increased until a dose adequate to control symptoms (maintenance dose) is reached. A gradual increase is necessary because exposure to a high dose of the allergen too soon can cause an allergic reaction. Injections are usually given once or twice a week until the maintenance dose is reached. Then injections are usually given every 2 to 4 weeks. The procedure is most effective when maintenance injections are continued throughout the year, even for seasonal allergies.

Alternatively, high doses of the allergen may be placed under the tongue (sublingual) and held there for a few minutes, then swallowed. The dose is gradually increased, as for injections. The sublingual technique is relatively new, and how often the dose should be given has not been established. It ranges from every day to 3 times a week. Extracts for grass pollen or house dust mite, placed under the tongue, can be used to help prevent allergic rhinitis.

Immunotherapy for peanut allergy may also be given by mouth. The person receives the first several doses of the allergen over the course of a single day while in a doctor's office or clinic. The person then takes the allergen at home. Each time the dose is increased, the first dose of the higher dosage is given under a doctor's supervision.

Allergen immunotherapy may take 3 years to complete. People who develop allergies again may need another longer course (sometimes 5 years or more) of immunotherapy.

Because immunotherapy injections occasionally cause dangerous allergic reactions, people remain in the doctor’s office for at least 30 minutes afterward. If they have mild reactions to immunotherapy (such as sneezing, coughing, flushing, tingling sensations, itching, chest tightness, wheezing, and hives), a drug—usually an antihistamine, such as diphenhydramine or loratadine—may help. For more severe reactions, epinephrine (adrenaline) is injected.

Treatment of Allergic Reactions

  • Avoidance of the allergen

  • Antihistamines

  • Mast cell stabilizers

  • Corticosteroids

  • Allergen immunotherapy

  • For severe allergic reactions, emergency treatment

If mild symptoms occur, antihistamines are often all that is needed. If they are ineffective, other drugs, such as mast cell stabilizers and corticosteroids may help. Nonsteroidal anti-inflammatory drugs (NSAIDs) are not useful, except in eye drops used to treat conjunctivitis.

Severe symptoms, such as those involving the airways (including anaphylactic reactions), require emergency treatment.


The drugs most commonly used to relieve the symptoms of allergies are antihistamines. Antihistamines block the effects of histamine (which triggers symptoms). They do not stop the body from producing histamine.

Taking antihistamines partially relieves the runny nose, watery eyes, and itching and reduces the swelling due to hives or mild angioedema. But antihistamines do not ease breathing when airways are constricted. Some antihistamines (such as azelastine) are also mast cell stabilizers Mast cell stabilizers Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. Usually, allergies make people sneeze; the eyes water and itch... read more Mast cell stabilizers .

Antihistamines are available as

  • Tablets, capsules, or liquid solutions to be taken by mouth

  • Nasal sprays

  • Eye drops

  • Lotions or creams

Which is used depends on the type of allergic reaction. Some antihistamines are available without a prescription (over-the-counter, or OTC), and some require a prescription. Some that used to require a prescription are now available OTC.

Products that contain an antihistamine and a decongestant (such as pseudoephedrine) are also available OTC. They can be taken by adults and children aged 12 years and older. These products are particularly useful when both an antihistamine and a nasal decongestant are needed. However, some people, such as those who are taking monoamine oxidase inhibitors (a type of antidepressant), cannot take these products. Also, people with high blood pressure should not take a decongestant unless a doctor recommends it and monitors its use.

The antihistamine diphenhydramine is available OTC as a lotion, cream, gel, or spray that can be applied to the skin to relieve itching, but it should not be used. Its effectiveness is unproved, and it can cause allergic reactions (such as a rash). It can cause extreme drowsiness in children who are also taking an antihistamine by mouth.

Did You Know...

  • Over-the-counter skin products that contain diphenhydramine (an antihistamine) should not be used because their effectiveness is unproved and because allergic reactions and other side effects are possible.

Side effects of antihistamines include anticholinergic effects, such as drowsiness, dry mouth, blurred vision, constipation, difficulty with urination, confusion, and light-headedness (particularly after a person stands up), as well as drowsiness. Often, prescription antihistamines have fewer of these effects.

Some antihistamines are more likely to cause drowsiness (sedation) than others. Antihistamines that cause drowsiness are widely available OTC. People should not take these antihistamines if they are going to drive, operate heavy equipment, or do other activities that require alertness. Antihistamines that cause drowsiness should not be given to children under 2 years old because they may have serious or life-threatening side effects. These antihistamines are also a particular problem for older people and for people with glaucoma, benign prostatic hyperplasia, constipation, or dementia because of the drugs’ anticholinergic effects Anticholinergic: What Does It Mean? Anticholinergic: What Does It Mean? . In general, doctors use antihistamines cautiously in people with cardiovascular disease.

Not everyone reacts the same way to antihistamines. For example, Asians seem to be less susceptible to the sedative effects of diphenhydramine than are people of Western European origin. Also, antihistamines cause the opposite (paradoxical) reaction in some people, making them feel nervous, restless, and agitated.


Mast cell stabilizers

Mast cell stabilizers block mast cells from releasing histamines and other substances that cause swelling and inflammation.

Mast cell stabilizers are taken when antihistamines and other drugs are not effective or have bothersome side effects. These drugs may help control allergic symptoms.

These drugs include azelastine, cromolyn, lodoxamide, ketotifen, nedocromil, olopatadine, and pemirolast. Azelastine, ketotifen, olopatadine, and pemirolast are also antihistamines.

Cromolyn is available by prescription as follows:

  • For use with an inhaler or nebulizer (which delivers the drug in aerosol form to the lungs)

  • As eye drops

  • In forms to be taken by mouth

Cromolyn is available without a prescription as a nasal spray to treat allergic rhinitis. Cromolyn usually affects only the areas where it is applied, such as the back of the throat, lungs, eyes, or nose. When taken by mouth, cromolyn can relieve the digestive symptoms of mastocytosis Mastocytosis Mastocytosis is an uncommon abnormal accumulation of mast cells in the skin and sometimes in various other parts of the body. People may have itchy spots and bumps, flushing, digestive upset... read more Mastocytosis , but it is not absorbed into the bloodstream and thus has no effect on other allergic symptoms.



When antihistamines and mast cell stabilizers cannot control allergic symptoms, a corticosteroid may help.

Corticosteroids can be taken as a nasal spray to treat nasal symptoms or through an inhaler, usually to treat asthma.

Doctors prescribe a corticosteroid (such as prednisone) to be taken by mouth only when symptoms are very severe or widespread and all other treatments are ineffective. If taken by mouth at high doses and for a long time (for example, for more than 3 to 4 weeks), corticosteroids Corticosteroids: Uses and Side Effects Corticosteroids: Uses and Side Effects can have many, sometimes serious side effects. Therefore, corticosteroids taken by mouth are used for as short a time as possible.

Creams and ointments that contain corticosteroids can help relieve the itching associated with allergic rashes. One corticosteroid, hydrocortisone, is available OTC.

Other drugs

Leukotriene modifiers, such as montelukast, are anti-inflammatory drugs used to treat the following:

  • Mild persistent asthma

  • Seasonal allergic rhinitis

They inhibit leukotrienes, which are released by some white blood cells and mast cells when they are exposed to an allergen. Leukotrienes contribute to inflammation and cause airways to constrict. Montelukast is used only when other treatments are ineffective.

Omalizumab is a monoclonal antibody (which is a manufactured [synthetic] antibody designed to interact with a specific substance). Omalizumab binds to immunoglobulin E (IgE) IgE One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more IgE , an antibody that is produced in large amounts during an allergic reaction, and prevents IgE from binding to mast cells and basophils and triggering an allergic reaction. Omalizumab may be used to treat persistent or severe asthma when other treatments are ineffective. If hives recur frequently and other treatments are ineffective, it may be helpful. When it is used, the dose of a corticosteroid can be reduced. It is given by injection under the skin (subcutaneously).

Emergency treatment

Severe allergic reactions, such as an anaphylactic reaction, require prompt emergency treatment.

People who have severe allergic reactions should always carry a self-injecting syringe of epinephrine which should be used as quickly as possible if a severe reaction occurs. Antihistamine pills may also help, but epinephrine should be injected before taking antihistamine pills. Usually, epinephrine stops the reaction, at least temporarily. Nonetheless, people who have had a severe allergic reaction should go to the hospital emergency department, where they can be closely monitored and treatment can be repeated or adjusted as needed.

Treatment of allergies during pregnancy and breastfeeding

Whenever possible, pregnant women with allergies should control their symptoms by avoiding allergens. If symptoms are severe, pregnant women should use an antihistamine nasal spray. They should take antihistamines by mouth (oral antihistamines) only if antihistamine nasal sprays do not provide adequate relief.

Women who are breastfeeding should also try to avoid antihistamines. But if antihistamines are necessary, doctors prefer to use antihistamines that are less likely to cause drowsiness, and they prefer antihistamine nasal sprays to oral antihistamines. If oral antihistamines are essential for controlling symptoms, they should be taken immediately after feeding the baby.

quiz link

Test your knowledge

Take a Quiz!