Allergic Reaction

Overview

An allergic reaction happens when your body's defense system (the immune system) overreacts to something that is usually harmless. This substance is called an allergen. Your immune system sees it as a threat and releases chemicals like histamine, which cause symptoms such as itching, swelling, or trouble breathing. Reactions can be mild, like a few hives, or very serious and life-threatening, known as anaphylaxis (pronounced an-a-fi-lak-sis). Common allergens include certain foods, insect stings, medications, pollen, and animal dander.

Allergic reactions are very common. Many people have mild allergies like hay fever or skin rashes. Severe allergic reactions (anaphylaxis) are less common, but still affect millions of people worldwide. The number of people with allergies has been rising over recent decades.

Anyone can develop an allergy at any age. You are more likely to have allergies if a close family member has them, if you have other allergic conditions like asthma or eczema, or if you have had a previous allergic reaction. Children often develop food allergies, while adults may develop allergies to medications or insect stings.

Symptoms

Common symptoms

• Itchy, red, or watery eyes
• Sneezing and a runny or stuffy nose
• Itchy skin, hives (raised red bumps that can appear anywhere, also called urticaria), or a rash
• Mild swelling of the lips, face, or eyelids
• Stomach pain, feeling sick, or diarrhoea
• Tingling or itching in the mouth

Symptoms in children

• Fussiness and crying that is hard to soothe
• Rubbing the nose and eyes frequently
• Stomach pain or vomiting after eating a certain food
• Hives or a rash that appears suddenly
• Wheezing or a cough

Symptoms in older adults

• Similar symptoms as other adults, but reactions may be mistaken for other health conditions
• Skin reactions or itching from medications are more common
• Swelling or breathing difficulties may be more serious because of other age-related changes

Causes

Main causes

• Foods: peanuts, tree nuts (like almonds and walnuts), shellfish, fish, eggs, milk, soy, and wheat are common triggers
• Insect stings: from bees, wasps, hornets, or fire ants
• Medications: certain antibiotics, like penicillin, or over-the-counter pain relievers
• Latex: found in some gloves, balloons, and medical devices
• Pollen from trees, grasses, and weeds (hay fever)
• Dust mites and mould spores
• Animal dander (tiny skin flakes) from pets like cats and dogs

Risk factors

• Having a family history of allergies, asthma, or eczema
• Already having asthma or eczema yourself
• Being a child (although some children outgrow food allergies)
• Having had a previous allergic reaction, which means you are more likely to react again
• Exposure to very high levels of an allergen, such as multiple bee stings at once

When to see a doctor

See a doctor urgently if:

• Any sign of a severe reaction – call your local emergency number straight away
• If you have used an emergency epinephrine auto-injector, even if you feel better, you must go to the emergency department
• If symptoms of a mild reaction suddenly get worse or affect your breathing
• If you feel faint, your heart is racing, or you have a sense of doom

Book a routine appointment if:

• If you think you have an allergy and want to confirm the trigger
• To develop a written allergy action plan with your doctor
• When mild allergy symptoms interfere with your daily life, sleep, or work
• To discuss what to do if you accidentally come into contact with an allergen

Diagnosis

To diagnose an allergy, your doctor will talk with you about your symptoms, what you were doing or eating before the reaction started, and how quickly it came on. They will also ask about your general health and family history. A physical exam can check for signs like skin rashes or nasal swelling. If needed, they may refer you to an allergy specialist (an allergist or immunologist) for more detailed tests.

Tests that may be done

• Skin prick test: Tiny drops of possible allergens are placed on your skin, and the skin is gently pricked. A small itchy bump appears within 15–20 minutes if you are allergic to that substance.
• Blood tests: A sample of your blood is checked for specific antibodies (called IgE) that your body makes against certain allergens.
• Oral food challenge: This is done in a clinic or hospital under close medical supervision. You eat small, gradually increasing amounts of the suspected food to see if a reaction occurs.
• Elimination diet: You stop eating a suspect food for a few weeks, then reintroduce it while watching for symptoms. This is done with a healthcare provider’s guidance.

What to expect at your appointment

Allergy testing is generally very safe, and medical staff are prepared to treat any reaction right away. Skin tests can feel a little uncomfortable, like a tiny scratch. Blood tests are like any routine blood draw. A food challenge takes a few hours, and you will be watched carefully the whole time. Your specialist will explain everything ahead of time so you know exactly what will happen.

Treatment

Treatment for an allergic reaction depends on how severe it is. For mild symptoms, simple self-care and over-the-counter remedies can help. For severe allergies, your doctor will prescribe an emergency medicine and work with you to create a clear plan for avoiding triggers and handling accidental exposure.

Self-care at home

• If your symptoms are mild, you may try a cool, damp cloth on itchy skin or take a soothing oatmeal bath.
• Use over-the-counter antihistamine tablets or creams to calm itching, sneezing, and hives (ask a pharmacist for help choosing one).
• Keep your skin moisturized and avoid scratching, which can make itching worse.
• Stay well hydrated and rest if you feel unwell.
• Most importantly, if you have been prescribed an epinephrine auto-injector for severe allergies, always carry two with you and make sure you and those close to you know how to use it.

Medical treatments

For ongoing mild to moderate allergies, doctors may suggest antihistamines to block the chemical that causes symptoms. Some antihistamines can make you drowsy, so your doctor can help you find the right option. Nasal sprays and eye drops are also useful for hay fever symptoms. For severe allergies, the main treatment is an injection of epinephrine. This medicine works quickly to raise blood pressure, open the airways, and reduce swelling. After using epinephrine, you must go to the emergency department for observation. Some people with persistent allergies benefit from immunotherapy, where tiny, controlled amounts of the allergen are given over time (as injections or tablets placed under the tongue) to help the body become less sensitive. This is only done under specialist care.

When is surgery considered?

Surgery is not a treatment for allergic reactions.

Living with this condition

Living with allergies means learning what triggers your symptoms and planning ahead to stay safe. This might involve reading food labels carefully, asking about ingredients when eating out, or checking pollen counts. Many people wear medical alert jewelry (a bracelet or necklace) that tells others about their allergy in an emergency. If you have a severe allergy, always carry your prescribed emergency medicine. Having a written allergy action plan, created with your doctor, helps you, your family, and your school or workplace know exactly what to do if a reaction occurs.

Lifestyle tips

• Avoid your known triggers as much as possible. For food allergies, this means checking every label every time, even for products you have used before.
• For dust mite allergies, use allergen-proof covers on pillows and mattresses, wash bedding in hot water weekly, and vacuum with a HEPA filter.
• During high pollen seasons, keep windows closed, shower and change clothes after being outdoors, and consider wearing wraparound sunglasses.
• If you are allergic to insect stings, wear shoes outdoors, avoid bright clothing and strong perfumes, and cover food and drinks when outside.

Diet and exercise

If you have a food allergy, it is essential to strictly avoid that food. Read ingredient lists on packaged foods and always let restaurant staff know about your allergy. Exercise is safe and encouraged, but a small number of people experience symptoms triggered by exercise alone or in combination with certain foods. If you have ever had an allergic reaction during physical activity, talk to your specialist. For anyone with a severe allergy, always have your emergency medication with you when you exercise, and tell a coach or gym staff about your allergy.

Mental health and emotional wellbeing

Managing an allergy, particularly a severe one, can take a toll on your mental well-being. It is normal to feel anxious, overwhelmed, or isolated by the constant need to be vigilant. Fear of accidental exposure can affect social activities, travel, and relationships. It is important to acknowledge these feelings and seek support. Talking to a counsellor or psychologist, joining a support group, or simply opening up to trusted friends and family can help you feel less alone. If you are struggling to cope or feel hopeless, please reach out to a mental health crisis line or speak to your doctor immediately.

Prevention

You cannot always stop an allergy from developing in the first place. However, you can prevent most allergic reactions by avoiding your known triggers and following the plan created with your doctor. For parents of infants at high risk of food allergy, introducing common allergy-causing foods like peanut early in life, under medical advice, may help reduce the chance of developing an allergy. If you are at risk of anaphylaxis, carrying your emergency medication and knowing when to use it can prevent a life-threatening outcome.

Vaccines

Vaccines are not used to prevent allergic reactions.

Screening programmes

There is no routine population screening for allergies. If you have symptoms that suggest an allergy, your doctor may recommend testing. If you have a strong family history of allergies, you can discuss this with your healthcare provider to decide if any early steps might be right for your family.

Complications

If left untreated

• Mild allergic reactions can cause ongoing discomfort, poor sleep, and trouble concentrating at school or work.
• Chronic allergies, such as untreated hay fever, may lead to sinus infections, ear infections, or worsening asthma.
• A severe allergic reaction (anaphylaxis) can quickly become life-threatening if not treated with epinephrine. It can cause loss of consciousness, breathing to stop, or the heart to stop beating.

Long-term outlook

With proper management, the vast majority of people with allergies live full and active lives. Learning your triggers and having a clear action plan puts you in control. Science is constantly advancing, and new treatments like immunotherapy and alternative therapies are being studied. Many children outgrow certain allergies, and even adults may find their reactions become less severe over time. By working closely with your healthcare team, you can look forward to a positive future.

Find support

International organisations

• World Allergy Organization ↗
• Asthma and Allergy Foundation of America ↗

Local organisations

• Allergy UK ↗ · United Kingdom
• Allergy & Anaphylaxis Australia ↗ · Australia

Helplines

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