Hypersensitivity Reactions

Overview

A hypersensitivity reaction happens when your body's defence system overreacts to something that is usually harmless. This can be an allergy, such as to food, pollen, or medicine, or a non-allergic reaction, such as to sunlight, cold, or pressure on the skin. Reactions can be mild, like itching or a rash, or severe and need emergency help.

Yes. Allergies and other hypersensitivity reactions are very common. Many people have mild reactions at some point, and some people have reactions that need ongoing care.

Symptoms

Common symptoms

• Sneezing, runny or blocked nose, and itchy, watery eyes
• Hives: raised, itchy welts on the skin
• A red, itchy rash or flare-up of eczema
• Swelling of the lips, eyes, face, or tongue
• Wheezing, cough, or shortness of breath
• Stomach pain, nausea, vomiting, or diarrhoea
• Itchy mouth or throat after eating certain foods

Symptoms in children

• Eczema flare-ups or dry, itchy skin
• Hives or a spreading rash
• Vomiting, stomach pain, or diarrhoea after eating
• Irritability, poor feeding, or being more sleepy than usual
• Noisy breathing, coughing, or trouble breathing

Symptoms in older adults

• A rash that may be less obvious or easier to miss
• Sudden dizziness, weakness, or falls caused by low blood pressure
• Confusion or feeling faint during a reaction
• Breathing problems that can become serious more quickly
• Reactions to new medicines or foods that were previously fine

Causes

Main causes

• Foods such as nuts, milk, egg, shellfish, fish, wheat, soya, or sesame
• Insect stings from bees or wasps
• Medicines, especially when started for the first time
• Latex and some medical products
• Pollen, dust mites, animal dander, or mould
• Contact with metals, cosmetics, fragrances, or plants
• Physical triggers such as cold, heat, sunlight, or pressure on the skin
• Infections that can trigger hives or rashes

Risk factors

• Having had a previous allergic or hypersensitivity reaction
• A family history of allergies, asthma, or eczema
• Having asthma, which can make breathing reactions more serious
• Having eczema or other allergic conditions
• Jobs or hobbies with repeated exposure to latex, chemicals, or certain foods
• Being a young child, when food allergies and eczema often first appear

When to see a doctor

See a doctor urgently if:

• You have a reaction that is getting worse
• You have facial swelling, a widespread rash, or vomiting after a suspected trigger
• A new medicine causes a rash or swelling
• Your asthma or breathing symptoms get worse during a reaction
• A child has a reaction and is sleepy, floppy, or not feeding

Book a routine appointment if:

• Mild symptoms keep coming back and you want to find the cause
• You want to know what triggers your symptoms
• You need an allergy action plan for home, school, or work
• You have had a reaction at school, work, or during a hobby
• You are unsure whether a symptom is an allergy or something else

Diagnosis

Your doctor will ask about your symptoms, when they happen, how quickly they start, and what you were exposed to before they began. They will also examine you. If an allergy is suspected, they may refer you to an allergy clinic for testing.

Tests that may be done

• Skin prick test: a small amount of a suspected allergen is placed on the skin with a tiny scratch
• Blood test for allergy-related antibodies, which are proteins your body makes in response to a trigger
• Patch test: patches are placed on the skin for a few days to check for contact reactions
• Supervised food challenge: a specialist gives small amounts of a food under close medical supervision
• Breathing tests if asthma or lung symptoms are part of the reaction

What to expect at your appointment

Testing is usually done in an allergy clinic. Your doctor will explain which tests are safe for you. Some tests need you to stop certain medicines beforehand, so always ask your healthcare team for instructions. Test results are used together with your history - they do not give all the answers on their own.

Treatment

Treatment depends on the type of reaction and how severe it is. The main steps are avoiding the trigger, calming symptoms, and having a clear plan for severe reactions. Mild reactions may need simple symptom relief. Severe reactions need emergency care.

Self-care at home

• Avoid the trigger once you know what it is
• Keep a symptom diary to help spot patterns
• Use a cool compress or cool bath for itchy skin
• Keep skin moist with gentle, fragrance-free products
• Do not scratch or rub a rash if you can avoid it
• Read food and product labels carefully if you have a food or contact allergy
• Keep your emergency action plan with you and share it with family, school, or work
• Tell healthcare staff about any previous reactions before tests or treatment

Medical treatments

Doctors may suggest medicines that calm the allergic response, reduce itching or swelling, or ease stomach symptoms. For breathing symptoms, they may suggest inhaled medicines or breathing treatments. People at risk of severe reactions may be prescribed an emergency auto-injector device and taught how and when to use it. In some cases, specialists offer treatments that slowly train the immune system to tolerate a trigger, such as allergy immunotherapy. Severe reactions are treated in hospital with close monitoring and emergency care.

When is surgery considered?

Surgery is not used to treat hypersensitivity reactions. In rare cases, a specialist may discuss surgery for a related problem, such as severe nasal blockage, but this is not a treatment for the reaction itself.

Living with this condition

Many people live full and active lives with hypersensitivity reactions. The key is knowing your triggers, recognising early symptoms, and having a plan you trust. Mild reactions can often be managed at home, while severe reactions need emergency help.

Lifestyle tips

• Learn to recognise your early warning signs
• Carry your emergency plan and any prescribed rescue device
• Tell friends, family, school, and work about your triggers and plan
• Check ingredients and ask about cross-contact when eating out
• Wear medical alert jewellery if you have a severe allergy
• Keep your home clear of known triggers where possible
• Plan ahead for travel, parties, and school trips

Diet and exercise

Eat a balanced diet. If you have a food allergy, avoid only the foods you have been advised to avoid, and ask for a dietitian's help if you need to cut out several foods. Exercise is usually safe and healthy. Some people get symptoms during or after exercise, especially if they have eaten a certain food or are hot, so discuss this with your doctor and follow your action plan.

Mental health and emotional wellbeing

Living with a severe allergy or unpredictable reactions can cause worry, stress, or anxiety. This is common and understandable. Talking to your healthcare team, a counsellor, or a support group can help. If you ever have thoughts of harming yourself or are in crisis, contact your local emergency number or a local crisis support service right away.

Prevention

You cannot always prevent a hypersensitivity reaction, but you can lower the risk by avoiding known triggers, recognising early symptoms, and having an emergency plan. Early treatment of mild reactions can sometimes stop them from becoming worse.

Vaccines

There is no vaccine that prevents hypersensitivity reactions. Keep up your routine vaccinations and ask your healthcare provider if you have questions about vaccines and allergies.

Screening programmes

There is no general screening test for everyone. Allergy testing is used only when symptoms suggest a possible trigger. If you have had a reaction, your doctor can advise whether testing is helpful.

Complications

If left untreated

• A severe allergic reaction, called anaphylaxis, can be life-threatening without emergency care
• Poorly controlled asthma or breathing problems
• Ongoing hives, eczema, or skin infections from scratching
• Sleep problems, tiredness, and difficulty concentrating
• Missing school, work, or social activities because of symptoms or worry
• Anxiety around food, medicines, or going out

Long-term outlook

Most people do very well with good trigger avoidance, an action plan, and regular advice from their healthcare team. Severe reactions are serious but uncommon when people know the warning signs and get help quickly. Many children outgrow some allergies, and adults can often manage reactions well with the right support.

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