What are allergies?

What are allergies

Millions of people around the world are affected by allergies. Are you one of them? The term allergy covers a range of conditions such as hay fever, eczema and food allergies.

But what are allergies? Why do some people get allergies? And what can cause allergic reactions?

In this article we’re going to be taking a close look at what allergies are. You’ll learn how allergies develop, typical symptoms to look for, common causes and what you should do if you think you have allergies.

Allergies are an overreaction of your immune system

If you have a pollen allergy, it may feel like the allergy starts in your nose. Or in your stomach if you have an allergic reaction to certain foods. But allergies start much deeper than that. An allergic reaction starts with your immune system.

Our immune system is there to defend our bodies against infectious bacteria, viruses, fungi and parasites. There are many types of cells involved in our immune system and each type plays a unique role, such as recognizing problems, communicating with other cells or performing the task of defending the body.

Allergies mean that your immune system is a bit too eager. When you have an allergic reaction, your immune system has mistaken a usually harmless substance as a threatening invader. This sets off a chain of reactions in defense. Substances which can cause an allergic reaction in some people are called allergens.

Allergies or a cold?
How to tell the difference

A man sneezing who is confused about just cold or allergy in pink

How do allergies develop?

There are several steps involved in developing an allergy. It’s unlikely you’d have an allergic reaction to a substance the first time you come across it. It’s usually necessary to come into contact with an allergen at least once before you develop an allergy to it. This process is called sensitization.

For instance, you might eat a certain food and your immune system is put on high alert. Your immune system will produce antibodies in defense against the supposed invader. These antibodies are called Immunoglobulin E (IgE). IgE antibodies fit like puzzle pieces to the particular structure of the allergen. It’s like a wanted sign for that substance.

It is possible to be sensitized and have IgE antibodies in your blood but not experience allergic reactions. Even though the immune system is primed for attack, it doesn’t act. We don’t know why this happens in some people. Having an allergy means you are both sensitized and experience an allergic reaction.

What happens when you have an allergic reaction?

When you next come into contact with the substance your body perceives as a threat, the custom-built IgE antibodies will recognize the allergen. And tell other cells to start the allergic reaction. This involves releasing different chemicals such as histamine.

The idea is to get rid of the intruder as fast as possible. Histamine is part of an inflammatory response to the intrusion. Various fluids and white blood cells travel to the site of the attack to help. This causes swelling and other typical symptoms of an allergic reaction. Histamine along with other chemicals gets the body to produce more mucus to flush out anything that doesn’t belong. It also causes itching to make you scratch the perceived harmful material off your skin.

An allergic reaction typically causes symptoms in your nose, lungs, throat, sinuses, ears, lining of the stomach or on your skin.

Substances which can cause allergies

The most common substances which can cause allergic reactions include:

  • Pollens from trees, grasses or weeds
  • Waste particles from house dust mites
  • Molds
  • Certain foods such as nuts, dairy, shellfish
  • Pet dander
  • Insect stings
A girl sneezing due to hay fever in yellow

Hay fever explained

Hay fever or pollen allergy

A common allergen is pollen, the fine powdery dust from trees, grasses and weeds. Pollen allergy is probably best known as hay fever. Hay fever is a form of seasonal allergy as it generally occurs at particular times of the year. This is usually during the flowering periods of specific plants and trees.

House dust mite allergy

Dust mite allergy is another common form of allergy. Dust mites are found all over the world, living in our homes and causing some of us to cough and sneeze. Dust mite allergy isn’t a reaction to the mites themselves, but to their waste and shed skin.

Pet allergy

Furry or hairy domestic animals produce allergens that can cause allergies. We’re talking cats, dogs, horses, rabbits and guinea-pigs and suchlike. It’s their dander that triggers the allergic reaction. Dander is flakes of their dead skin, fur or hair. Their urine and saliva also contain allergens that may cause a reaction.

Mold allergy

You might experience an allergic reaction caused by spores from mold and fungi. Spores are widespread in the air we breathe. Spores come from rotting organic matter and their development increases in warm and humid conditions.

Insect venom allergy

Some people are allergic to insect stings. For a few people, insect stings can be life-threatening.

Food allergies

Food allergy symptoms can appear at any age but they’re most common in babies and children. Food allergies can cause a variety of unpleasant symptoms, from digestive problems to skin reactions. In some people, a food allergy can cause severe symptoms and even be life-threatening. Food allergy is different to food intolerance though. Food intolerance is a less serious condition and doesn’t involve the immune system.

Typical allergy symptoms

The symptoms of allergies can vary from person to person. They can also vary in severity, from mild discomfort to the potentially life-threatening reaction called anaphylaxis.

Typical symptoms of an allergic reaction

Pollen allergy

Stuffy nose | Runny nose | Sneezing | Itchy nose | Itchy, red watery eyes | Cough

Dust mite allergy

Stuffy nose | Runny nose | Sneezing | Itchy nose | Itchy, red watery eyes | Cough | Skin reactions, rashes, eczema

Pet allergy

Stuffy nose | Runny nose | Sneezing | Itchy nose | Itchy, red watery eyes

Insect sting allergy

If symptoms go beyond the normal sensation of pain, local redness and wheals, seek medical attention or call an ambulance as soon as possible

Mold allergy

Stuffy nose | Runny nose | Sneezing | Itchy nose | Itchy, red watery eyes | Cough | Tight chest or wheezing

Food allergy

Bloating | Diarrhea | Nausea | Skin reactions | Itching | In severe cases, anaphylaxis

When allergies are life-threatening: anaphylaxis

Anaphylaxis (pronounced an–a–fi–LAK–sis) is a severe and often sudden allergic reaction. It can happen within minutes of being exposed to the allergen or up to a few hours later. Any one or more of the following symptoms may occur:

  • Skin reactions, including hives and itching and flushed or pale skin
  • Low blood pressure (hypotension)
  • Constriction of your airways and a swollen tongue or throat, which can cause wheezing and trouble breathing
  • A weak and rapid pulse
  • Nausea, vomiting or diarrhea
  • Dizziness or fainting

Anaphylaxis always requires an emergency medical response. It can be very serious if not treated immediately. Call 911 if you or somebody you’re with experiences any of these symptoms.

Can I be allergic to more than one thing?

If you have allergic reactions it’s possible you’re triggered by more than one substance. In fact, more than 50% of people with respiratory allergies are sensitized to multiple potential allergens. Having multiple allergies is also known as polysensitization. Throughout your life your immune system continues to develop as you meet each new threat. That means you can get new allergies at any time.


Children have a 30-50% chance of getting an allergy if their mom or dad has one. If both parents have allergies the risk increases to 60-80%.

Allergy symptoms can progress over time too. Symptoms which affect your nose and upper airways can progress to symptoms affecting your lower airways, such as coughing and wheezing.

Who gets allergies?

You can develop allergies at any stage of your life. Some people develop allergies as a child. While others might first experience allergy symptoms as an adult.

Allergies tend to run in families. If others in your family have allergies it’s statistically more likely that you’ll develop them as well. But this isn’t a blanket rule. Children have a 30-50% chance of getting an allergy if their mom or dad has one. If both parents have allergies the risk increases to 60-80%. This inherited tendency is called atopy, which means you might be more prone to allergy.

Why is the number of people with allergies increasing?

The number of people around the world affected by allergies increases each year. Particularly in developed countries. It’s not entirely understood why.

One of the main theories is it’s because we now live in a cleaner, more disinfected environment than ever before. This has reduced the quantity of germs our immune system has to tackle. It’s thought this might be the reason it overreacts when it makes contact with harmless substances.

What should I do if I think I have allergies?

Talk to your health care provider if you think you might have allergies. It can be helpful to keep a diary of your symptoms to take with you. If you think pollen is causing your symptoms our app can help you keep track over a period of time. If you’re concerned you’re having a reaction to foods you eat, keep a food diary. And note what the symptoms are in relation to what you ate.

A couple embracing

Allergy immunotherapy

Allergy testing and diagnosis

Detecting sensitization with allergy testing can help identify potential allergy triggers. It’s a useful means of finding out what might be causing your symptoms. Once you know that you can take steps to reduce your contact with your specific triggers.

Allergy testing alone won’t give you a diagnosis of allergy though. For that, your allergy test results, your symptoms and your unique medical history will all be taken into consideration. When you have a diagnosis of allergy, you’ll be able to discuss treatment options with your health care provider.

Allergy medication

When you have allergies the first step is usually trying to avoid the allergen triggering your symptoms. When you can’t, allergy medicines may help. Antihistamines, corticosteroids and decongestants are all common treatments to ease symptoms of allergy. If you have eczema there are also skin care regimes designed to soothe and moisturize.

Always follow your health care provider’s or pharmacist’s advice as well as the instructions in the patient information leaflet when taking allergy medication.

Allergy immunotherapy treatment

For some people, symptoms of allergy are difficult to keep in check. Temporary allergy symptom relief might not be enough. If that’s you, your health care provider may discuss allergy immunotherapy with you.

Allergy immunotherapy treats the cause of your allergy. The idea is to stimulate your immune system and get it used to your triggers through repeated tiny doses of the allergen over a period of time. This can reduce your symptoms and the need for allergy medicines.

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