What are allergies? An easy-to-understand guide from experts

An allergy is when your immune system overreacts to protect you from something that isn’t actually a threat.¹ It could be harmless pollen, dust mites, mould, pets, food, insect stings or a particular drug and your symptoms will depend partly on how you came into contact with the trigger.^1,2 

Over 44% of British adults and 50% of kids have at least one allergic condition, according to Allergy UK.³ It can start at any age. Some people develop allergies as a baby, toddler or older child, while others first have symptoms as adults.⁴

This guide starts with the allergy basics. Find out about allergy causes, typical allergy symptoms depending on what you react to, and how to treat allergies. You may also find the answer you need in our FAQ section at the end.

What's causing my allergies?

The root cause of your allergies is the fact that your immune system responds in an exaggerated way to the natural world. ¹ It makes antibodies to watch for pollen, or whatever your allergen is, then triggers the body’s defences to fight as if against some kind of dangerous invasive germs. ¹

A substance that can trigger allergy is called an allergen.¹ Here’s how it works: ¹

1. First contact: Your immune system spots what it thinks is a threat and makes Immunoglobulin E (IgE) antibodies specific to that substance. Now you’re sensitised.

2. Contact again: The antibodies detect the substance and you may have an allergic reaction (it doesn’t always happen). In which case, you have an allergy.

3. Allergic reaction: The antibodies tell other cells to release chemicals like histamine to flush out the allergen, for instance with extra mucus (think of a runny nose), itching or digestive allergy symptoms.

Some factors increase the risk of having allergies. Others govern when your symptoms are likely to be most bothersome; for instance, allergens can be seasonal and only around at certain times of year.

Who is at risk of getting allergies?

Several factors may increase the risk of developing an allergy:

• Being born into a family with allergies^2,4
• Exposure to cigarette smoke, including in the womb^2,4
• Early feeding with formula milk and solid food²
• Respiratory infections as a young baby^2,4
• Early exposure to allergens like dust mites or pollen²
• Traffic pollution^4,5
• Already having an allergic condition^2,4

The family connection is very important. Kids have a 30-50% chance of getting allergies if their mum or dad has one and a 60-80% chance when it affects both parents (according to the study, Allergies in Children²). Scientists call this inherited tendency atopy.⁴

When is the allergy season?

Allergy season for you could be during the spring, summer or autumn if you have hay fever.⁵ Allergens you find mostly at home like dust mites can cause symptoms at any time,⁴ although it may be worse when life moves back indoors in autumn and the heating goes on.⁶

• Trees release pollen first⁵ when temperatures reach 13C⁷, often before their leaves emerge⁸
• Grass follows from early summer ⁵
• Weeds are last to cause problems. ⁵ Shorter days in late summer signal ragweed⁹ and mugwort¹⁰ to start flowering

It isn’t as neat as that, of course, because the allergy season can shift from year to year depending on the weather. They also tend to overlap. 

Which substances cause allergies?

It’s actually proteins that cause allergies¹ and these are some of the substances where you’ll find them:^1,2

• Pollen from trees, grass or weeds
• Waste particles from house dust mites
• Pet dander
• Mould
• Insect stings
• Foods such as nuts, dairy and shellfish 

Hay fever or pollen allergy ⁵

Pollen is the fine powdery dust that plants release for a few weeks each year during flowering. Allergic reactions to it are commonly known as hay fever. Breathing in the tiny particles can cause allergic rhinitis, which is inflammation in the lining of your nose.

House dust mite allergy

Dust mites live invisibly in most homes and can make you cough and sneeze.⁵ The microscopic bugs feed on your dead skin so they love your bed especially.² Interestingly, dust mite allergy isn’t a reaction to the mites themselves but rather to their waste products.⁵

Pet allergy

Pets means furry or hairy domestic animals like cats, dogs, horses, rabbits and guinea pigs (scaly pets aren’t usually such a problem, although birds can be¹¹). It’s animal dander, which is flakes of dead skin, fur or hair, that triggers allergic reactions. ⁵ Urine and saliva can also contain allergens.⁵ 

Mould allergy ⁵

There are a few dozen species of mould and fungi that can trigger allergic reactions with their spores. Outdoor types tend to fill the air from July to early autumn, and hibernate over winter, while indoor colonies thrive in warm humid conditions causing mould allergy symptoms all year round.

Insect sting allergies¹²

Bee and wasp stings can be very painful and leave most people with a small itchy bump. A bigger or more widespread swelling may be a symptom of a potentially dangerous allergy. Insect venom can trigger a life-threatening allergic reaction called anaphylaxis, which affects the airways, breathing and circulation.

Food allergies

Food allergies can develop at any time in your life but they’re most common in babies and children.⁴ The symptoms are often uncomfortable, like digestive problems and skin rashes, but food can also cause anaphylaxis, which is a medical emergency.² 

It’s important to know the difference between food allergies and intolerances. Food intolerance doesn’t involve the immune system, so carries less risk and you manage it differently.²

Can I be allergic to more than one thing?

Yes, it’s actually quite common. In fact, over half of people who seek medical advice about respiratory allergies are sensitised to more than one substance.¹³ Your immune system is constantly changing as you go through life, which means you can develop new allergies even into old age.¹⁴

Unexpected symptoms could also be a cross-reaction rather than a new allergy. Different substances can look confusingly similar to your immune system and trigger a similar response.¹⁵ For instance, it’s common for pollen types to cross-react with each other and with some types of fresh food. ¹⁵

Typical symptoms for different allergies 

The allergies described in this article are the kind that cause an immediate reaction so you’re likely to get symptoms soon after contact with the trigger, usually within an hour.¹⁶ What that feels like will depend on what you’re allergic to.

Next we’ll explore how to distinguish allergies from colds, recognise anaphylaxis (a severe systemic allergic reaction) and understand the link between allergy and asthma.

How can you tell the difference between a cold and an allergy?

It can be difficult, especially in kids who tend to have many more colds than grown-ups.¹⁸ But here are some of the main differences:

More likely to be an allergy if… ⁵

• Starts suddenly
• Thin, clear runny nose
• Itchy, red and watery eyes
• Happens at the same time each year (pollen) or never really goes away (dust mites, pet dander)
• You may need regular symptom medication for weeks or months at a time

More likely to be a cold if…¹⁹

• Builds over 2-3 days
• Thick, yellowy-green runny nose⁸
• Sore throat and hoarse voice
• Possibly a fever
• Aches and pains
• Happens at any time of year but particularly in autumn and winter
• Usually goes within 14 days without treatment

Can allergy lead to asthma?

Yes, it can. Up to 40% of people with hay fever go on to develop allergic asthma and up to 80% of those with allergic asthma have hay fever (according to the study, Allergic rhinitis: the Ghost Diagnosis in patients with asthma²⁰). The common factor is inflammation, making you sneeze first and later wheeze.²⁰

The journey from respiratory allergy to allergic asthma is part of a recognised pattern. It’s known as the allergic march. ⁴ It tends to start with eczema in babies, then food allergy and finally affecting the airways.⁴

When allergies are life-threatening: Anaphylaxis

Anaphylaxis (pronounced ana-fil-ax-is) is a severe allergic reaction affecting your whole body. ²¹ It can happen within minutes or up to a few hours after contact with the trigger, commonly food, insect stings and medication.²¹

You may have one of them or more of these symptoms:²¹

• Flushed or pale skin, itching or hives
• Swelling of the tongue or throat
• A weak or rapid pulse
• Diarrhea, feeling or being sick
• Low blood pressure (hypotension)
• Narrowing of the airways, which may cause wheezing and trouble breathing 
• Dizziness or fainting

Anaphylaxis is an emergency as it can be life-threatening. Call an ambulance immediately and use your adrenaline nasal spray or auto-injector if you’ve been prescribed one.²¹ People known to be at risk of severe allergic reactions usually carry two devices at all times.²¹ But you should still go to hospital even if you feel better in case of a delayed secondary reaction.²¹

What should I do if I think I have allergies?

Talk to your GP if you think you might have allergies, letting them know what your symptoms are and when you get them, as well as any ideas you have about the cause. They may arrange an allergy test to help make the diagnosis.

You might find you’re allergic to pollen, dust mites, cats or certain foods. Your doctor can then tell you about ways to treat allergy symptoms, which can include avoiding your trigger, as well as short-term or long-term allergy relief. ^1,2,5

Allergy testing and diagnosis

The most common types of allergy testing are skin prick tests and blood tests. ^1,2 A positive usually means you’re sensitised to a particular substance but not necessarily allergic to it. ^1,3 That’s why your doctor needs details of your symptoms and medical history to help interpret your test results. ^1,3

Where can I get an allergy test done?

It happens at the GP’s surgery or your allergist’s office. A skin prick test means having drops with different allergens put on your forearm and the skin gently scratched. If you get an itchy swelling, that’s a positive test.¹ If it’s an allergy blood test, the sample goes off to the lab to check for specific antibodies that reveal sensitisation.¹

Short-term allergy relief

It’s no fun dealing with allergy symptoms but you may find that antihistamines, corticosteroids and decongestants can help.^5,16 These come in topical forms, like nasal sprays, eye drops, creams and inhalers, and oral forms, depending on the medication. A saline (salt water) nasal spray is a simple drug-free treatment available from the pharmacy. Lubricating artificial tears can help sore itchy eyes. ²² If you have eczema there are also creams that can soothe your skin.²³

Your GP can tell you what allergy medicine is right for you, antihistamines or corticosteroids. Make sure to follow their advice as well as the instructions in the patient information leaflet.

Long-term allergy relief ^{1, 2, 5, 6}

Long-term allergy relief is based on retraining your immune system. The treatment is called allergy immunotherapy^{1, 2, 5}  and involves repeated controlled doses of the substance you’re allergic to.¹ The goal is for your immune system to stop seeing it as a threat, which can greatly reduce allergy symptoms during and after treatment.^2,5 Allergy immunotherapy may also lower the risk of allergies progressing over time, for instance to allergic asthma.^2,5,6

Treatment comes as tablets, drops and injections] depending on your age and trigger. Ask your doctor if you might be eligible. ^{1, 2, 5, 6}

The short version

Allergies are the result of having an immune system that overreacts to harmless substances like pollen, dust mites, mould, certain foods and insect bites and stings.¹ Respiratory allergy causes cold-like allergic rhinitis and itchy eyes.⁵ With food you may get digestive problems but also skin reactions and anaphylaxis, a severe systemic allergic reaction. ² Insect sting, latex and drug allergies can also trigger anaphylaxis, which affects the whole body and can be life-threatening.²¹

Allergies tend to run in families and there’s a pattern to how they may develop called the allergic or atopic march.⁴ It often starts with eczema in babies, then food allergies, respiratory allergies and finally allergic asthma.⁴ 

Your doctor may suggest allergy testing to help make a diagnosis and inform the treatment plan. That’s likely to involve avoiding the trigger, plus short-term symptom relief like antihistamines and corticosteroids, and maybe retraining the immune system with allergy immunotherapy. Getting to the root of the problem early could help stop symptoms progressing.