What causes allergies?

Coronavirus COVID-19: Latest News and Information What Causes Allergies? Source: Charles Daghlian, Getty Images/ScienceFoto, iStock

Allergy facts

  • Allergy involves an exaggerated response of the immune system, often to common substances such as foods, furry animal dander, or pollen.
  • The immune system is a complex system that normally defends the body against foreign invaders, such as bacteria and viruses, while also surveying for abnormal changes in an individual's own cells.
  • Allergens are substances that are foreign to the body and that cause an allergic reaction.
  • IgE is the allergic antibody. The other antibodies, IgG, IgM, and IgA, defend against infection.
  • Although many individuals outgrow allergies over time, allergies can also develop at any age, including during adulthood.
  • The environment plays a role in the development of allergy, as do genetics. There is a greater risk of developing allergic conditions if a person has a family history of allergy, especially in parents or siblings.

What is an allergy?

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An allergy is an exaggerated reaction by the immune system in response to exposure to certain foreign substances.

The response is exaggerated because these foreign substances are normally seen as harmless by the immune system in nonallergic individuals and do not cause a response in them.

In allergic individuals, the body recognizes the substance as foreign, and the allergic part of the immune system generates a response.

Allergy-producing substances are called “allergens.” Examples of allergens include pollens, dust mites, molds, animal proteins, foods, and medications. When an allergic individual comes in contact with an allergen, the immune system mounts a response through the IgE antibody. People who are prone to allergies are said to be allergic or “atopic.”

Types of peanut allergy symptoms: About 80%-90% of reactions involve skin manifestations such as

  • a rash, including hives,
  • redness,
  • itching.

Nevertheless, reactions can occur in the absence of a rash, and these reactions may be the most severe.

Read more about the symptoms and signs of peanut allergies » What Causes Allergies? Source: MedicineNet

What is an allergy? (Continued)

  • Approximately 10%-30% of individuals in the industrialized world are affected by allergic conditions, and this number is increasing.
  • Allergic rhinitis (nasal allergies) affects roughly 20% of Americans. Between prescription costs, physician visits, and missed days of work/school, the economic burden of allergic disease exceeds $3 billion annually.
  • Asthma affects roughly 8%-10% of Americans. The estimated health costs for asthma exceed approximately $20 billion annually.
  • Food allergies affect roughly 3%-6% of children in the United States, and about 1%-2% of adults in the U.S.
  • The prevalence of allergic conditions has increased significantly over the last two decades and continues to rise.

What Causes Allergies? Source: iStock

What causes allergies?

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A common scenario can help explain how allergies develop.

A few months after the new cat arrives in the house, dad begins to have itchy eyes and episodes of sneezing. One of the three children develops coughing and wheezing.

The mom and the other two children experience no reaction despite the presence of the cat. How can this occur?

The immune system is the body's organized defense mechanism against foreign invaders, particularly infections. Its job is to recognize and react to these foreign substances, which are called antigens.

Antigens often lead to an immune response through the production of antibodies, which are protective proteins that are specifically targeted against particular antigens.

These antibodies, or immunoglobulins (IgG, IgM, and IgA), are protective and help destroy a foreign particle by attaching to its surface, thereby making it easier for other immune cells to destroy it.

The allergic person however, develops a specific type of antibody called immunoglobulin E, or IgE, in response to certain normally harmless foreign substances, such as cat dander, pollen, or foods. Other antigens, such as bacteria, do not lead to production of IgE, and therefore do not cause allergic reactions. Once IgE is formed, it can recognize the antigen and can then trigger an allergic response. IgE was discovered and named in 1967 by Kimishige and Teriko Ishizaka.

What Causes Allergies? Source: MedicineNet, iStock

What causes allergies? (Continued)

In the pet cat example, the dad and the youngest daughter developed IgE antibodies in large amounts that were targeted against the cat allergen. The dad and daughter are now sensitized, or prone to develop allergic reactions, on repeated exposures to cat allergen.

Typically, there is a period of sensitization ranging from days to years prior to an allergic response.

Although it might occasionally appear that an allergic reaction has occurred on the first exposure to the allergen, there needs to be prior exposure in order for the immune system to react.

It is important to realize that it is impossible to be allergic to something that an individual has truly never been exposed to before, though the first exposure may be subtle or unknown. The first exposure can even occur in a baby in the womb, through breast milk, or through the skin.

IgE is an antibody that all of us have in small amounts. Allergic individuals, however, generally produce IgE in larger quantities. Historically, this antibody was important in protecting us from parasites.

In the example above, during a sensitization period, cat dander IgE is overproduced and coats other cells involved in the allergic response, such as mast cells and basophils, which contain various chemical messengers, such as histamine.

These cells produce chemical messengers that cause the symptoms of an allergic reaction on subsequent exposures to the cat allergen. The cat protein is recognized by the IgE, leading to activation of the cells, which leads to the release of the allergic mediators mentioned above.

These chemicals cause typical allergic symptoms, such as localized swelling, inflammation, itching, and mucus production. Once primed, or sensitized, the immune system is capable of mounting this exaggerated response with subsequent exposures to the allergen.

On exposure to cat dander, whereas the dad and daughter produce IgE, the mom and the other two children produce other classes of antibodies, which do not cause allergic reactions. In these nonallergic members of the family, the cat protein is eliminated uneventfully by the immune system and the cat has no effect on them.

Another part of the immune system, the T-cell, may be involved in allergic responses in the skin, as occurs from the oils of plants, such as poison ivy, poison oak, poison sumac, reactions to metal, such as nickel, or certain chemicals. The T-cell may recognize a certain allergen in a substance contacting the skin and cause an inflammatory response. This inflammatory response can cause an itchy rash.

What Causes Allergies? Allergies can best be described as: See Answer What Causes Allergies? Source: Bigstock

Who is at risk for allergies and why?

Allergic Reaction: Causes, Symptoms, and Treatments

Your immune system is responsible for defending the body against bacteria and viruses. In some cases, your immune system will defend against substances that typically don’t pose a threat to the human body. These substances are known as allergens, and when your body reacts to them, it causes an allergic reaction.

You can inhale, eat, and touch allergens that cause a reaction. Doctors can also use allergens to diagnose allergies and can even inject them into your body as a form of treatment.

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The American Academy of Allergy, Asthma & Immunology (AAAAI) reports that as many as 50 million people in the United States suffer from some type of allergic disease.

Doctors don’t know why some people experience allergies. Allergies appear to run in families and can be inherited. If you have a close family member who has allergies, you’re at greater risk for developing allergies.

Although the reasons why allergies develop aren’t known, there are some substances that commonly cause an allergic reaction. People who have allergies are typically allergic to one or more of the following:

The symptoms of an allergic reaction can vary from mild to severe. If you become exposed to an allergen for the first time, your symptoms may be mild. These symptoms may get worse if you repeatedly come into contact with the allergen.

Symptoms of a mild allergic reaction can include:

  • hives (itchy red spots on the skin)
  • itching
  • nasal congestion (known as rhinitis)
  • rash
  • scratchy throat
  • watery or itchy eyes

Severe allergic reactions can cause the following symptoms:

  • abdominal cramping or pain
  • pain or tightness in the chest
  • diarrhea
  • difficulty swallowing
  • dizziness (vertigo)
  • fear or anxiety
  • flushing of the face
  • nausea or vomiting
  • heart palpitations
  • swelling of the face, eyes, or tongue
  • weakness
  • wheezing
  • difficulty breathing
  • unconsciousness

A severe and sudden allergic reaction can develop within seconds after exposure to an allergen. This type of reaction is known as anaphylaxis and results in life-threatening symptoms, including swelling of the airway, inability to breathe, and a sudden and severe drop in blood pressure.

If you experience this type of allergic reaction, seek immediate emergency help. Without treatment, this condition can result in death within 15 minutes.

Your doctor can diagnose allergic reactions. If you experience symptoms of an allergic reaction, your doctor will perform an exam and ask you about your health history. If your allergic reactions are severe, your doctor may ask you to keep a journal that details your symptoms and the substances that appear to cause them.

Your doctor may want to order tests to determine what’s causing your allergy. The most commonly ordered types of allergy tests are:

  • skin tests
  • challenge (elimination-type) tests
  • blood tests

A skin test involves applying a small amount of a suspected allergen to the skin and watching for a reaction. The substance may be taped to the skin (patch test), applied via a small prick to the skin (skin prick test), or injected just under the skin (intradermal test).

A skin test is most valuable for diagnosing:

Challenge testing is useful in diagnosing food allergies. It involves removing a food from your diet for several weeks and watching for symptoms when you eat the food again.

A blood test for an allergy checks your blood for antibodies against a possible allergen. An antibody is a protein your body produces to fight harmful substances. Blood tests are an option when skin testing isn’t helpful or possible.

If you experience an allergic reaction and you don’t know what’s causing it, you may need to see your doctor to determine what the cause of your allergy is. If you have a known allergy and experience symptoms, you may not need to seek medical care if your symptoms are mild.

In most cases, over-the-counter antihistamines, such as diphenhydramine (Benadryl), can be effective for controlling mild allergic reactions.

If you or someone you know experiences a severe allergic reaction, you should seek emergency medical attention. Check to see if the person is breathing, call 911, and provide CPR if needed.

People with known allergies often have emergency medications with them, such as an epinephrine auto-injector (EpiPen). Epinephrine is a “rescue drug” because it opens the airways and raises blood pressure. The person may need your help to administer the medication. If the person is unconscious, you should:

  • Lay them flat on their back.
  • Elevate their legs.
  • Cover them with a blanket.

This will help prevent shock.

Shop over-the-counter antihistamines for controlling mild allergic reactions.

If you have a known allergy, preventing an allergic reaction will improve your outlook. You can prevent these reactions by avoiding the allergens that affect you. If you have serious allergic reactions, you should always carry an EpiPen and inject yourself if symptoms occur.

Your outlook will also depend on the severity of your allergy. If you have a mild allergic reaction and seek treatment, you’ll have a good chance of recovery. However, symptoms may return if you come into contact with the allergen again.

If you have a severe allergic reaction, your outlook will depend on receiving quick emergency care. Anaphylaxis can result in death. Prompt medical care is necessary to improve your outcome.

Once you identify your allergy, you can:

  • Avoid exposure to the allergen.
  • Seek medical care if you’re exposed to the allergen.
  • Carry medications to treat anaphylaxis.

You may not be able to avoid an allergic reaction completely, but these steps can help you prevent future allergic reactions.

Allergies: Causes, diagnosis, and treatment

Medically reviewed by Marc Meth, MD, FACAAI, FAAAI — Written by Adam Felman — Updated on April 15, 2020

In March 2020, the Food and Drug Administration (FDA) released a safety alert to warn the public that epinephrine auto-injectors (EpiPen, EpiPen Jr., and generic forms) may malfunction.

This could prevent a person from receiving potentially life saving treatment during an emergency.

If a person has a prescription for an epinephrine auto-injector, they can view the recommendations from the manufacturer here and talk with their healthcare provider about safe usage.

An allergic reaction occurs when a person’s immune system becomes hypersensitive to certain substances, such as foods, pollen, medications, or bee venom.

A substance that causes an allergic reaction is called an allergen. Many allergens are everyday substances that are harmless to most people. However, anything can be an allergen if the immune system has a specific type of adverse reaction to it.

One of the roles of the immune system is to destroy harmful substances in the body. If a person has an allergy to a substance, their immune system will react as though that substance is harmful and will try to destroy it.

Over 50 million people in the United States experience an allergic reaction each year. This reaction can lead to symptoms such as swelling. If swelling affects the airways, it can become life threatening.

In this article, learn about the risk factors, symptoms, and treatments associated with allergies.

Share on PinterestAn allergy develops when the immune system overreacts to a substance that is usually harmless.

Allergies develop when a person’s immune system overreacts to substances that are usually harmless.

The first time a person is exposed to an allergen, they do not usually experience a reaction. It often takes time for the immune system to build up a sensitivity to the substance.

In time, the immune system learns to recognize and remember the allergen. As it does so, it starts making antibodies to attack it when exposure occurs. This buildup is called sensitization.

Some allergies are seasonal. For example, hay fever symptoms can peak between April and May, when the tree and grass pollen count in the air is higher. A person may experience a more severe reaction as the pollen count rises.

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Is it an allergy or an intolerance? Learn about the differences here.

An allergic reaction causes inflammation and irritation. However, the specific symptoms will depend on the type of allergen. For example, allergic reactions may occur in the gut, skin, sinuses, airways, eyes, or nasal passages.

Below are some triggers and the symptoms they may cause in people with an allergy.

Dust and pollen

  • a blocked or congested nose
  • itchy eyes and nose
  • a runny nose
  • swollen and watery eyes
  • a cough


  • vomiting
  • a swollen tongue
  • tingling in the mouth
  • swelling of the lips, face, and throat
  • stomach cramps
  • shortness of breath
  • rectal bleeding, mainly in children
  • itchiness in the mouth
  • diarrhea

Insect stings

  • wheezing
  • significant swelling at the site of the sting
  • a sudden drop in blood pressure
  • itchy skin
  • shortness of breath
  • restlessness
  • hives, or a red and very itchy rash that spreads across the body
  • dizziness
  • a cough
  • chest tightness


  • wheezing
  • swelling of the tongue, lips, and face
  • a rash
  • itchiness

If symptoms become severe, anaphylaxis can develop.

Anaphylaxis is the severest form of allergic reaction. It is a medical emergency and can be life threatening. Anaphylaxis can develop quickly, with symptoms appearing within minutes or hours of exposure to the allergen.

Research suggests that anaphylaxis most commonly affects the skin and respiratory system.

Some symptoms include:

  • hives, flushing, and itchiness
  • difficulty breathing
  • wheezing
  • swelling
  • low blood pressure
  • changes in heart rate
  • dizziness and fainting
  • loss of consciousness

Recognizing these symptoms can be crucial to receiving timely treatment.

Learn more about the symptoms of anaphylactic shock here.

When an allergic reaction occurs, allergens bind to antibodies that the body produces called immunoglobin E (IgE). Antibodies combat foreign and potentially harmful substances in the body.

Once the allergen binds to IgE, specific types of cells — including mast cells — will release chemicals that trigger the symptoms of the allergic reaction.

Histamine is one of these chemicals. It causes the muscles in the airways and walls of the blood vessels to tighten. It also instructs the lining of the nose to produce more mucus.

People may have a higher risk of allergies if they are under 18 years old or have a personal or family history of asthma or allergies.

Some researchers have suggested that those born by cesarean delivery may also have a higher risk of allergies, as they do not have exposure to the mother’s microbiome during childbirth.

Potential allergens can appear almost anywhere.

In theory, a person can have an allergy to any food. Specific components — such as gluten, the protein present in wheat — can also trigger reactions.

Skin Allergies | Causes, Symptoms & Treatment | ACAAI Public Website

There are several different types of skin allergy reactions that allergists treat.

Eczema (also commonly called  atopic dermatitis) typically results in dry, sensitive skin. You may experience red itchy patches. Eczema can come and go over time, and flare-ups may crack, ooze, and itch severely. It is very itchy and can vary in severity from mild (just dry skin ) to severe (red, scaly, thick, fissured and oozing skin)

Hives (also known as urticaria) are raised itchy bumps. Typically hives appear reddish, and will “blanch” (or turn white) in the center when pressed.

Contact dermatitis is typically caused by exposure to an allergen or irritant. If you have red itchy bumps on your skin, especially at the site of contact with some potential irritant or allergen, you may be experiencing contact dermatitis.

If you suspect you have any of these conditions, your allergist can conduct an examination and do testing to help determine the cause of your skin reaction and can recommend treatment to help relieve your symptoms.

How can I relieve the itching from my skin rash?

Avoid scratching! Scratching your rash or hives can create more irritation and can lead to infection. Frequent baths followed immediately with adequate moisturization may help ease your discomfort.

Allergists are specially trained to help treat your condition. Your allergist may prescribe a cream or oral medication to help alleviate your discomfort.

Antihistamines and moisturizing ointments can also help ease irritation and dryness.

Recently, new medications have been approved, including an ointment for mild to moderate atopic dermatitis and a biologic for moderate to severe atopic dermatitis. You can discuss these options with your allergist.

I haven’t changed anything about my usual routine – what could be causing my skin rash or hives?

There are many possible causes for your skin rash. Some types of rashes are caused by allergies, others may be caused by infections, skin conditions such as eczema or rosacea, or even just dry or damaged skin. Your allergist can help diagnose the cause of your symptoms and prescribe treatment to help you take control and find relief.

What Causes a Person to Develop Allergies?

Allergies can come in many shapes and sizes. While some people can enjoy beautiful weather, others avoid going outdoors at all costs. The same situation may occur for people watching others enjoy an endless variety of foods while they must be very selective. Those people who suffer from the incessant symptoms of food or environmental allergies may wonder, why?

Have you ever wondered what causes your stuffy nose and sneezing? It’s not just a string of bad luck; whether your allergy symptoms occur in direct result to the local pollen count, different types of food or your neighbor’s cat, there are certain responses from our immune system that lead to our level of reaction.

In this article, we’ll explain exactly what causes a person to develop allergies, when this can happen, and whether allergic symptoms are worse in adulthood or childhood.

The body’s immune system generates different antibodies to protect us from illnesses. For allergies, the immune system generates Immunoglobulin E, also known as IgE, to aid in combating your allergy symptoms.

IgE is a chemical messenger that travels to cells to relay information that a chemical defense against a foreign invader is needed. Allergic individuals have high IgE levels against benign environmental exposures such as pollen or dander.

Food can also cause high levels of IgE.

With time, the immune system develops what’s known as immunological memory. Normally this is a helpful immune response which can enable your body to respond more quickly.  This is what allows vaccines to work.

In allergy, however, this response is magnified, and your repeat exposures cause recurrent overreactions of the immune system.  This produces an allergic response that may include sneezing, coughing, sniffling and congestion or increased asthma symptoms. IgE antibodies are custom made for each type of allergen.

This is why you can be allergic to one or two specific foods or pollens and tolerate others without a problem.

When Does a Person Develop Allergies?

Allergies can develop at any point in a person’s life. One factor that increases your chance is your family history. If one parent is allergic there is a 30-50% chance of their offspring developing allergies. This jumps to 60-80% if both parents are allergic.

In many cases, allergies first present early in life, during infancy or the toddler years. Most of these allergies will be lifelong concerns, although some can resolve on their own.

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Can You Develop Allergies Later in Life?


An allergy is a reaction the body has to a particular food or substance.

Allergies are very common. They're thought to affect more than 1 in 4 people in the UK at some point in their lives.

They're particularly common in children. Some allergies go away as a child gets older, although many are lifelong.

  • Adults can develop allergies to things they were not previously allergic to.
  • Having an allergy can be a nuisance and affect your everyday activities, but most allergic reactions are mild and can be largely kept under control.
  • Severe reactions can occasionally occur, but these are uncommon.

Substances that cause allergic reactions are called allergens. 

The more common allergens include:

  • grass and tree pollen – an allergy to these is known as hay fever (allergic rhinitis)
  • dust mites
  • animal dander, tiny flakes of skin or hair
  • food – particularly nuts, fruit, shellfish, eggs and cows' milk
  • insect bites and stings
  • medicines – including ibuprofen, aspirin and certain antibiotics
  • latex – used to make some gloves and condoms
  • mould – these can release small particles into the air that you can breathe in
  • household chemicals – including those in detergents and hair dyes

Most of these allergens are generally harmless to people who are not allergic to them.

Allergic reactions usually happen quickly within a few minutes of exposure to an allergen.

They can cause:

  • sneezing
  • a runny or blocked nose
  • red, itchy, watery eyes
  • wheezing and coughing
  • a red, itchy rash
  • worsening of asthma or eczema symptoms

Most allergic reactions are mild, but occasionally a severe reaction called anaphylaxis or anaphylactic shock can occur.

This is a medical emergency and needs urgent treatment.

  1. See a GP if you think you or your child might have had an allergic reaction to something.
  2. The symptoms of an allergic reaction can also be caused by other conditions.


The best way to prevent an allergic reaction is to avoid the substance that you're allergic to, although this isn't always easy or practical.

Below is some practical advice that should help you avoid the most common allergens.

House dust mites

One of the biggest causes of allergies are dust mites, which are tiny insects found in household dust. You can limit the number of mites in your home by: 

  • choosing wood or hard vinyl floor coverings instead of a carpet
  • fitting roller blinds that can be easily wiped clean
  • choosing leather, plastic or vinyl furniture instead of upholstered furniture
  • cleaning cushions, soft toys, curtains and upholstered furniture regularly, either by washing (at a high temperature) or vacuuming
  • using tested allergy-proof covers on mattresses, duvets and pillows
  • using a vacuum cleaner fitted with a HEPA (high efficiency particulate air) filter, because it can trap more dust mites than ordinary vacuum cleaners
  • regularly wiping surfaces with a damp, clean cloth – avoid dry dusting, as this can spread dust into the air

Concentrate your efforts of controlling dust mites in the areas of your home where you spend the most time, such as the bedroom and living room.

You can find more information on allergies in the home on the Allergy UK website.


It's not the pet fur that causes an allergic reaction. Instead, it's flakes of their dead skin, saliva and dried urine.

If you can't permanently remove a pet from the house, you could try: 

  • keeping pets outside as much as possible, or limiting them to a particular area of the house, preferably an area without carpet
  • not allowing pets in bedrooms
  • washing pets at least once a week
  • regularly grooming pets outside
  • regularly washing all bedding and soft furnishings on which a pet has lain
  • using an air filter in rooms where you spend most of your time
  • increase ventilation with fans, air-conditioning or by opening windows

If you're visiting a friend or relative with a pet, ask them not to dust or vacuum on the day you're visiting, as this will stir up the allergens into the air. Taking an antihistamine medicine about an hour before entering a pet-inhabited house can also help reduce your symptoms.

The Allergy UK website has more information about domestic pet allergies.

Mould spores

Tiny particles released by moulds can cause an allergic reaction in some people.

You can help prevent this by:

  • keeping your home dry and well-ventilated
  • removing any indoor pot plants from your home
  • not drying clothes indoors, store clothes in damp cupboards or packing clothes too tightly in wardrobes
  • dealing with any damp and condensation in your home
  • avoiding damp buildings, damp woods and rotten leaves, cut grass and compost heaps

Food allergies

By law, food manufacturers must clearly label any foods that contain something that's known to cause allergic reactions in some people. By carefully checking the label for the list of ingredients, you should be able to avoid an allergic reaction.

People with food allergies most often experience an allergic reaction while eating out at a restaurant. You can avoid this by:

  • not relying on the menu description alone (remember, many sauces or dressings could contain allergens)
  • communicating clearly with the waiting staff and asking for their advice
  • avoiding places where there's a chance that different types of food could come into contact with each other, such as buffets or bakeries
  • let restaurant staff you know your dietary requirements, including how severe your food allergy or intolerance is
  • always check what allergens are in the dish even if you have eaten it before, as recipes and ingredients can change

Allergies | HealthyWomen


What Is It?An allergy is a reaction of a person's immune system to a normally harmless substance, one that doesn't cause problems for most people.

It's hard to believe that things as unrelated as pollen, animal dander, bee venom, foods and mold can all potentially lead to the same type of problem. What these substances have in common is that they are proteins and are among the most common allergens. They can cause severe allergic reactions in some people.

With so many potentially allergenic substances, it's no wonder allergic diseases are among the major causes of illness and disability in the United States.

According to the American Academy of Allergy, Asthma & Immunology (AAAAI), the prevalence of allergies has continued to rise in industrialized countries for more than 50 years.

Worldwide, up to 30 percent of people suffer from seasonal and year-round allergic rhinitis, and 20 percent suffer from skin allergies.

An allergy is a response of a person's immune system to a normally harmless substance, one that doesn't usually cause problems for most people.

In the allergic woman, an offending substance can trigger an immune system response, which functions as the body's defense against invading agents such as parasites, to respond to a “false alarm.

” The immune system treats the allergen as an invader by generating large amounts of a certain type of antibody—a disease-fighting protein known as IgE—that attaches to specific body tissue and blood cells.

The cells are then triggered to release powerful inflammatory chemicals such as histamine, cytokines and leukotrienes. These chemicals can act on tissues in various parts of the body, such as the respiratory system, and cause the symptoms of the allergic response.

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