Allergy: Causes, Diagnosis and Treatment

Allergies are hypersensitive immune responses to substances that either enter or come in contact with the body, such as pet dander, pollen or bee venom.

A substance that causes an allergic reaction is called an “allergen”. Allergens can be found in food, drinks or the environment.

Most allergens are harmless, i.e. the majority of people are not affected by them.

If you are allergic to a substance, such as pollen, your immune system reacts to it as if it were a pathogen (a foreign harmful substance), and tries to destroy it.

Contents of this article:

  1. Symptoms of allergies and anaphalaxis, causes and risk factors of allergies
  2. The most common allergies, diagnosis and treatment
  3. How to prevent allergies

Fast facts on allergies

Here are some key points about allergies. More detail and supporting information is in the main article.

  • Allergies are the result of an inappropriately large immune response
  • Some of the most common allergens are dust, pollen and nuts
  • An estimated 1 in 5 Americans have an allergy
  • Allergies have a range of symptoms that can include sneezing, peeling skin and vomiting
  • Anaphylaxis is a serious allergic reaction that can be life-threatening
  • There are a number of risk factors for allergies, including a family history
  • If you already have an allergy, you are more likely to develop an allergy to something else
  • In theory, any food has the potential to be an allergen
  • To diagnose an allergy, a clinician may take a blood sample.

What is an allergy?

Allergies are very common. Public health authorities estimate that about 20% of people in North America and Western Europe suffer from some degree of hay fever (allergic rhinitis, allergy to pollen).

[Woman taking an inhaler on the sofa]
An estimated 1 in 5 Americans have an allergy.

A study published in JAMA Pediatrics(September 2013 issue) reported that children’sfood allergies cost both families and the US as a whole nearly $25 billion annually.

The number of people worldwide with allergies is increasing. According to Allergy UK, about30-40% of people have an allergy at some stage in their lives. Some years ago, this increase was only apparent in industrialized nations. However, middle-income nations are now reporting higher rates of allergies across their populations.

The steepest increase in allergies has been observed in children, particularly food allergies.

A team of researchers from Northwestern University Feinberg School of Medicine reported inPediatrics that about 8% of American children have some kind of food allergy. 38.7% of those with food allergies have a history of anaphylaxis (severe allergic reactions), and 30.4% are allergic to more than one food.

Researchers from St. Luke’s Roosevelt Hospital Center, New York, found that foreign-born children who live in the USA have a lower risk of allergies. This risk grows the longer they remain in America.

Symptoms of allergies

When a person with an allergy comes into contact with an allergen, the allergic reaction is not immediate. The immune system gradually builds up sensitivity to the substance before overreacting to it.

The immune system needs time to recognize and remember the allergen. As it becomes sensitive to it, it starts making antibodies to attack it – this process is called sensitization.

Sensitization can take from a few days to several years. In many cases the sensitization process is not completed and the patient experiences some symptoms but never a full allergy.

When the immune system reacts to an allergen, there is inflammation and irritation. Signs and symptoms depend on the type of allergen. Allergic reactions may occur in the gut (digestive system), skin, sinuses, airways, eyes, and nasal passages.

Allergies from dust and pollen may have the following symptoms:

  • Blocked nose
  • Itchy eyes
  • Itchy nose
  • Runny nose
  • Swollen eyes
  • Watery eyes
  • Cough.

Skin reactions, as in eczema (atopic dermatitis) may include:

  • Flaking skin
  • Itchy skin
  • Peeling skin
  • Red skin, rashes.

Food allergies may include several types of reactions:

  • Vomiting
  • Tongue swelling
  • Tingling in the mouth
  • Swelling of the lips
  • Swelling of the face
  • Swelling in the throat
  • Stomach cramps
  • Shortness of breath
  • Rectal bleeding (in children, rare in adults)
  • Itchiness in the mouth
  • Diarrhea
  • Anaphylaxis – a very severe, often life-threatening allergic reaction.

The following allergic reactions are possible after an insect sting:

  • Wheezing
  • Swelling where the sting occurred
  • Sudden drop in blood pressure
  • Skin itching
  • Shortness of breath
  • Restlessness
  • Hives – a red and very itchy rash that spreads
  • Dizziness
  • Cough
  • Chest tightness
  • Anxiety
  • Anaphylaxis.

The following may be signs of an allergic reaction to medication:

  • Wheezing
  • Swollen tongue
  • Swollen lips
  • Swelling of the face
  • Skin rash
  • Itchiness
  • Anaphylaxis.

Symptoms of anaphylaxis

Anaphylaxis is a serious allergic reaction of rapid onset. Anaphylaxis can be life-threatening and must be treated as a medical emergency.

This type of allergic reaction presents several different symptoms which can appear minutes or hours after exposure to the allergen. If the exposure is intravenous, onset is usually between 5 to 30 minutes. A food allergen will take longer.

Signs and symptoms of anaphylaxis
Anaphylaxis is a medical emergency.

Researchers from the University of Manitoba, Canada, reported in The Journal of Allergy & Clinical Immunology that the most commonly affected areas in anaphylaxis are the skin (80-90%), respiratory (70%), gastrointestinal (30-45%), cardiovascular 10-45%) and the central nervous system (10-15%). In most cases two areas are affected simultaneously.

Anaphylaxis – skin symptoms

Hives all over the body, flushing and itchiness. The affected tissues may also become swollen (angioedema). Some patients may experience a burning sensation on the skin.

In about 20% of cases, there is swelling of the tongue and throat.

If the skin has a strange bluish color, it could be a sign of hypoxia (lack of oxygen).

Some patients may experience a runny nose. The membrane that covers the front of the eye and the inside of the eyelid (conjunctiva) may become inflamed.

Anaphylaxis – respiratory symptoms

  • Shortness of breath
  • Wheezing – caused by bronchial muscle spasms
  • Stridor – a high-pitched vibrating wheezing sound when breathing. Caused by upper airway obstruction due to swelling
  • Hoarseness
  • Odynophagia – pain when swallowing
  • Cough.

Anaphylaxis – cardiovascular symptoms

Coronary artery spasm – sudden tightening of the muscle in the artery wall (temporary) due to cells in the heart that release histamine. This can lead to myocardial infarction (heart attack), dysrhythmia (abnormal heart rhythm), or cardiac arrest (heart stops).

Low blood pressure can cause the heart rate to accelerate. In some cases a slow heart rate can occur as a result of low blood pressure (Bezold-Jarisch reflex).

Patients whose blood pressure suddenly drops can feel lightheaded and dizzy. Some may lose consciousness. In some rare cases, the only sign of anaphylaxis might be low blood pressure.

Anaphylaxis – gastrointestinal symptoms

  • Abdominal cramps
  • Diarrhea
  • Vomiting
  • Loss of bladder control
  • Pelvic pain (like uterine cramps).
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Patients may also have a sense of impending doom.

Causes of allergies

[Immune molecules in action]
Allergies are caused by an over-sensitive immune system.

The immune system of a person with an allergy reacts to the allergen as though it were a harmful pathogen – such as an undesirable bacterium, virus, fungus or toxin.

However, the allergen is not harmful. The immune system has simply become oversensitive to that substance.

When the immune system reacts to an allergen, it releases immunoglobulin E (IgE), a type of antibody. IgE is released to destroy the allergen.

IgE causes chemicals in the body to be produced. These chemicals cause the allergic reaction.

One of these chemicals is called histamine. Histamine causes tightening of the muscles, including those in the airways and the walls of blood vessels. It also makes the lining of the nose produce more mucus.

People with allergies blame the allergen for their symptoms – a friend’s pet, pollen or dust mites. However, the allergens are not harmful. The problem is not the allergen but the allergic person’s immune system which mistakes harmless substances for harmful ones.

Risk factors for allergies

In medicine, a risk factor is something that raises the risk of developing a disease or condition. This risk can come from something a person does. For example, smoking is a risk factor for lung disease. It can also be something you are born with. For example, if your mother had breast cancer, her daughter has a higher risk of developing breast cancer too. A family history of breast cancer is a risk factor.

Below are some risk factors associated with allergies:

    • A family history of asthma – if your parents, grandparents or siblings have/had asthma, your risk of having an allergy is higher


    • A family history of allergies – if a close relative has/had an allergy, your risk of having an allergy yourself is greater


    • Being a child – a child is much more likely to have an allergy than an adult. On a positive note, this means that many children outgrow their allergies


    • Having asthma – people with asthma are significantly more likely to develop allergies


    • Not enough sunlight exposure – scientists from the European Centre for Environment & Human Health, together with researchers from various Australian centers found that children living in areas with less sunlight had higher rates of allergies


    • Having an allergy – if you already have an allergy, there is a greater risk that you will develop an allergy to something else


    • C-section babies – a team from the Henry Ford Hospital reported that C-section babies have aconsiderably higher risk of developing allergies compared to those born naturally


  • Chemicals used in water purification – Dr. Elina Jerschow, a fellow of the American College of Allergy, Asthma and Immunology, said that pesticides in tap water could be partly to blame for the increased food allergy rates in the USA.

The most common allergens

An allergen is a substance which causes an allergic reaction in some susceptible people.

Below are the most common allergens, apart from foreign proteins found in blood transfusions and vaccines:

Allergens from animals

  • Dust mites – their excrement
  • Cockroaches calyx
  • Wool
  • Fur
  • Dander – skin flakes (dandruff)
  • Fel d 1 – a protein found in cat saliva and sebaceous glands. Proteins from the urine, saliva or hair of household pets can cause allergic reactions in some people.


  • Penicillin
  • Salicylates – a salt of salicylic acid commonly found in many medications, including aspirin
  • Sulfonamides.

Foods – theoretically, any food can cause an allergy. The eight foods most likely to cause allergies are eggs (especially egg-white, albumen), fish, milk, nuts from trees, peanuts (groundnuts), wheat, soy, and shellfish.

[A dozen eggs]
Eggs are one of the most common food allergens.

Dr. Scott H. Sicherer, Professor of Pediatrics, Jaffe Food Allergy Institute at Mount Sinai School of Medicine, and colleagues reported in the Journal of Allergy and Clinical Immunologythat the number of children with peanut allergies more than tripled between 1997 and 2008.

The following foods can also cause allergies:

  • Corn (maize)
  • Celery
  • Pumpkin
  • Beans
  • Sesame.

Insect stings

  • Wasp sting venom
  • Mosquito stings
  • Bee sting venom
  • Fire ants.

Insect bites

  • Kissing bugs
  • Horseflies
  • Fleas
  • Blackflies.

Insects that cause respiratory allergies

  • Cockroaches
  • Caddis flies
  • Lake flies
  • Midges
  • Moths.

Mold spores

  • Alternaria
  • Aspergillus
  • Aureobasidium (Pullularia)
  • Cladosporium (Hormodendrum)
  • Epicoccum
  • Fusarium
  • Helmin thosporium
  • Mucor
  • Penicillium
  • Rhizopus.

Plant pollens – cause hay fever.

  • Grass – the most common cause of hay fever
  • Trees – including oak, ash, cedar, willow, and hazel
  • Weeds – such as mugwort and ragweed.

Other allergens

  • Household chemicals
  • Metal – nickel, cobalt, chromium, and zinc
  • Latex.

Diagnosing allergies

[Allergy testing]
There are a number of ways to diagnose an allergy.

The doctor will ask the patient questions regarding the allergy symptoms, when they occur, how often and what seems to cause them.

The patient will also be asked whether there is a family history of allergies, and whether other household members (who might not be relatives) have allergies.

The doctor, probably a primary care physician initially, may either recommend some tests to find out which allergen is causing the symptoms, or refer the patient to a specialist.

In the American College of Allergy, Asthma and Immunology’s website there is a page called “Find an Allergist”, where you can find a specialist who has been certified by the College.

On The British Society for Allergy & Clinical Immunology: BSACI homepage you can find an allergy clinic near you (UK only).

Even if the patient knows what causes his/her allergy, the doctor will wish to carry out tests to determine whether a particular substance within the food, drink, or something else is the culprit.

Below are some examples of allergy tests:

    • Blood test – to measures levels of IgE antibodies released by the immune system. This test is sometimes called the radioallergosorbent test (RAST)


    • Skin prick test – also known as puncture testing or prick testing. The skin is pricked with a small amount of a possible allergen. If there is a skin reaction – itchy, red and swollen skin – it may mean there is an allergy


  • Patch test – for patients with contact dermatitis (eczema). Special metal discs with trace amounts of a suspect allergen are taped onto the back. The doctor checks for a skin reaction 48 hours later, and then again after a couple of days.

The National Health Service says that commercial allergy-testing kits are not recommended and that patients should have these tests done by specialized health care professionals.

The National Institute for Health and Clinical Excellence (NICE), in the UK, found that the IgE antibody test and the skin prick test were cost effective compared with no test.

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Recent developments on allergy treatment from MNT news

Avoiding peanut after building tolerance is safe for children

Consuming peanut products from a young age can protect at-risk infants from allergy, and they continue to be protected a year after ceasing consumption, reports a study published in the New England Journal of Medicine.

Treatment options for allergies

The most effective treatment and management of an allergy is to avoid exposure to the allergen.

However, sometimes it is not possible to completely avoid an allergy. A person with hay fever cannot avoid exposure to pollens, unless he/she closes all the windows in the house and never goes out. Even then, there is a risk of other people bringing pollen into the house.

It is also important to educate patients so that they know how to identify their allergenic foods properly. Many people with peanut or tree nut allergy could not correctly identify which items they were allergic to in a study carried out at Ohio State University.

Study leader, Professor Todd Hostetler, says:

“When we ask patients to avoid peanuts and tree nuts, we shouldn’t assume patients know what they’re looking for, because they may not. It’s worthwhile to do some education about what a tree nut is, what a peanut is, and what they all look like.”

Medications for allergies

Drugs can help treat the symptoms of allergy, but do not cure it. The majority of allergy medications are OTC (over-the-counter, no prescription required). Before considering a medication, speak to a pharmacist or your doctor.

    • Antihistamines (histamine antagonists): they block the action of histamine, a chemical released in the body as part of an allergic reaction. Some antihistamines are not suitable for children


    • Decongestants: some patients say they help with a blocked nose in cases of hay fever, pet allergy or dust allergy. Decongestants are short-term medications


    • Leukotriene receptor antagonists (anti-leukotrienes): for asthma when other treatments have not worked. Anti-leukotrienes block the effects of leukotrienes, chemicals that cause swelling. Leukotrienes are released in the body when there is an allergic reaction


  • Steroid sprays: applied to the inside lining of the nose. Corticosteroid sprays help reduce nasal congestion.


Also known as hyponsesitization. This type of therapy rehabilitates the immune system. The doctor administers gradually increasing doses of allergens over a period of years. The aim is to induce long-term tolerance by reducing the allergen’s tendency to trigger IgE production.

Immunotherapy is only used in cases of severe allergies.

Allergist Bobby Lanier, MD, executive medical director of the American College of Allergy, Asthma and Immunology, says “It (immunotherapy) is the only allergy treatment that has been shown to prevent the progression of allergic disease, reduce the risk of developing asthma, and provide long term remission after therapy ends. Studies demonstrate it reduces the need for medication, as well as use and cost of health care services. And unlike medications, allergen immunotherapy has a long track record of safety in children, the elderly and pregnant women.”

A study published in Annals of Allergy, Asthma and Immunology found that childhood allergies are more effectively reduced by a shortened length of immunotherapy. Lead author Dr. Iwona Stelmach says that “the recommended duration of immunotherapy for long-term effectiveness has been three to five years. Our research shows that three years is an adequate duration for the treatment of childhood asthma associated with house dust mites. An additional two years adds no clinical benefit.”

Treatment for anaphylaxis

Epi-Pen (1)
Epinephrine auto-injector for adults

Anaphylaxis is a medical emergency. The patient may require resuscitation, including airway management, supplemental oxygen, intravenous fluids and close monitoring.

The patient, who often has to be hospitalized, will need an injection of adrenaline (epinephrine) into the muscle. Antihistamines and steroids are often used as adjuncts.

After the patient has been stabilized, doctors may recommend they remain in hospital under observation for up to 24 hours in case of biphasic anaphylaxis. Biphasic anaphylaxis is the recurrence of anaphylaxis within 72 hours with no further exposure to the allergen.

Patients who have had severe allergic reactions should carry an epinephrine autoinjector with them, which may include the EpiPen, EpiPen Jr, Twinject, or Anapen.

Many doctors and health authorities advise patients to wear a medical information bracelet or necklace with information about their condition.


How to prevent allergies

Even though treatments can help alleviate allergy symptoms, patients will need to try to avoid exposure to specific allergens. In some cases this is not easy. Avoiding pollen in late spring and summer is virtually impossible, even the cleanest houses have fungal spores or dust mites.

If you have friends or family with pets, avoiding them might be difficult. Food allergies can be challenging to manage, because traces of allergens can appear in the most unlikely meals.

Reducing your exposure to dust mites

  • Go for hard floor surfaces rather than carpets
  • Replace your window curtains with roller blinds
  • Regularly vacuum cushions, chairs, and soft toys. Where possible, wash them at a high temperature setting
  • Do not use woolen blankets or feather pillows
  • Instead of dry dusting, which can scatter allergens into the air, wipe surfaces with a damp cloth.

Preventing allergy to cats or dogs

[Owner squeezing dog's head]
If you are visiting a pet owner’s house, take an antihistamine before arriving.

It is not the pet itself but proteins found in its urine, saliva, flakes of dead skin or hair that can cause allergic reactions.

If you cannot avoid being in contact with a pet you are allergic to, see if you can come to an arrangement where it is not allowed into certain parts of the house, for example upstairs.

Do not allow the pet into your bedroom. Grooming a dog or cat outside regularly can help (try to get somebody to do this for you).

The pet’s bedding and soft toys should be washed at a high temperature setting regularly.

If you have to go into a pet owner’s house, taking an antihistamine medication beforehand may help.

Preventing mold spore allergy

  • Test your house for mold
  • Check the plumbing in your house. Leaks create damp areas which are ideal environments for molds
  • You can probably clean small moldy areas yourself. An environmental service can help clear mold from difficult-to-get-to areas
  • If mold is detected inside drywall, it must be cut out and replaced
  • Make sure all hard surfaces are mold free
  • Avoid having carpets in damp areas of your house
  • Replace moldy tiles or carpets
  • Make sure your bathrooms are well ventilated
  • Dehumidifiers and air conditioners help keep the house dry. Make sure filters are changed regularly.
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Preventing food allergies

Preventing allergic reactions affects children psychologically – scientists from the University of Padua, Italy, reported that children with food allergies suffer from anxiety and loneliness. Approximately 17% of kids with food allergies never attend peers’ parties, while 24% always take “safe” foods along with them.

Before considering buying and eating a particular food, read the list of ingredients on the label. A considerable number of prepared foods contains allergens, such as milk, eggs or peanuts.

US Allergen Information Example
Clear allergen food labeling is legally required in many countries

According to the National Institute of Allergy and Infectious Diseases, US food manufacturers have been required by law since 2006 to list the ingredients in prepared foods.

They are also required to disclose whether their foods contain – fish, shellfish, wheat, soy, tree nuts, peanuts, milk or eggs – known as the top allergenic foods. They also have to list the food preservatives, sulphur dioxide and sulphites .

In August 2013, the U.S. FDA introduced a new rule defining the term “gluten free”. The FDA (Food and Drug Administration) said that the new regulation will help people with celiac disease avoid gluten more effectively.

Simple hygiene – straightforward cleanliness measures can help reduce your risk of coming into contact with a food allergen. If you are allergic to peanuts, for example, washing your hands with soap and water will remove all or most traces of the allergen. Keeping working surfaces clean with a good household cleaner also helps.

It can be especially difficult to avoid food allergens when eating out. Explain clearly to the waiter (and the chef if you can) that you have a food allergy and how it important it is for you to avoid certain food(s).

Many waiters and chefs believe a small amount of allergen will not harm the allergic customer. A survey of 90 table-service restaurants in Brighton, England, found that 1 in 3 kitchens were not separating common food allergens (fish, nuts, milk, wheat, peanuts and eggs) from other foods.

Be careful when selecting items in bakeries or buffets, because it is likely that various kinds of foods have come into contact with one another.

A report issued by the American Academy of Pediatrics in 2008 suggested that food allergies may be delayed or prevented in high-risk infants if they are breastfed for four months or more.

Mother’s diet during pregnancy may affect baby’s allergy risk – Dr Gaëlle Boudry, of the INRA research institute in Rennes, France, and team identified a possible link between a mother’s diet during pregnancy and the risk of allergies in her offspring.

If the mother’s diet contained a group of PUFAs (polyunsaturated fatty acids), such as those found in flaxseed, walnut oil or fish, the baby’s intestines developed differently, making him/her less likely to suffer from allergies.

Dr. Boudry says:

“There is intense research interest in maternal diet during pregnancy. In the western diet, the group of polyunsaturated fatty acids that we have shown to help gut function are actually disappearing our dietary intake of fish and nut oils is being replaced by corn oils which contain a different kind of fatty acid.

Our study identifies that a certain group of polyunsaturated fatty acids known as n-3PUFAs causes a change in how a baby’s gut develops, which in turn might change how the gut immune system develops. These changes are likely to reduce the risk of developing allergies in later life.”

Preventing pollen allergies (Hay fever)

[Woman smelling flowers]
There are a number of ways to minimize the impact of hay fever on your summer.

If you suffer from hay fever, there are some measures you can take to make your spring and/or summer months more tolerable.

With a certain amount of planning and effort, you can significantly limit your suffering, and maybe even avoid it completely.

Mark Dykewicz, M.D., professor of internal medicine and chief of allergy and clinical immunology at the Saint Louis University School of Medicine, says:

“There are a number of simple steps you can take to help relieve symptoms and minimize your suffering when allergy season kicks into high gear. That’s good news for many of the millions of Americans who traditionally suffer every year from seasonal allergies.”

Dr. Dykewicz offers the following five tips to prevent or relive symptoms of pollen allergy:

    1. Use OTC antihistamines. For many patients they are very effective at reducing the classic symptoms of hay fever. Go for the more recent products which are less likely to cause drowsiness


    1. Keep all the doors and windows in your house closed. This helps prevent pollens and outdoor molds from entering


    1. Go out as little as possible in the morning or when pollen counts are high. On windy days it is better to stay in. Pollen counts tend to be higher between 5am and 10am


    1. Keep the windows of your car closed when you are traveling. Make sure the air filter is regularly serviced


  1. If you have been out, change clothes and have a shower when you are back home. Pollen can gather on clothes, skin and hair.

The following measures can also help reduce the severity and frequency of hay fever symptoms:

    • If you have a lawn, try to get somebody else to mow it


    • Stay away from very grassy areas, such as fields and parks


    • When you are outside, wear wraparound sunglasses


    • Avoid drying your clothes and sheets outside when pollen counts are high


  • Always be aware of the pollen count in your area.

If you are also allergic to cats and dogs, stay away from pets when pollen levels are high. People who are allergic to pets may find their ragweed allergy symptoms get worse when exposed to dogs or cats.

Preventing anaphylaxis

If you are vulnerable to anaphylaxis, a severe allergic reaction, make sure you have an “allergy action plan”.

Parents should inform their school, day care center, etc., regarding their child’s allergy and what to do in an anaphylactic emergency.

Tell your work colleagues and friends so that they can help you in an emergency.

You should always carry an epinephrine autoinjector, e.g. an EpiPen, and wear a medical alert bracelet. It is advisable to receive professional counseling on how to avoid triggers.

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