Overview
Allergies happen when the immune system overreacts to a normally harmless substance such as pollen, pet dander, food, or insect stings. The immune system treats these substances as threats and triggers a response that causes symptoms ranging from a runny nose to a life-threatening reaction called anaphylaxis.
How common is it?
Allergic conditions affect roughly 1 in 4 people in the UK at some point in their lives. Hay fever alone affects about 1 in 5 adults.
Causes and risk factors
When a susceptible person encounters an allergen for the first time, the immune system produces IgE antibodies against it. On future exposures, these antibodies trigger mast cells to release histamine and other chemicals that cause allergy symptoms.
Common risk factors
- Family history of allergies or asthma
- Having another allergic condition (eczema, asthma)
- Childhood exposure to certain environments
- Westernised diet and lifestyle (hygiene hypothesis)
- Air pollution
- Early or late introduction of allergenic foods
Symptoms
- Sneezing and runny or blocked nose
- Itchy, watery red eyes
- Skin rash, hives, or eczema flare
- Coughing or wheezing
- Stomach pain, diarrhoea, or vomiting (food allergies)
- Swelling of the face, lips, or throat
- Severe reaction with drop in blood pressure (anaphylaxis)
When to see a doctor
Go to emergency immediately for throat swelling, difficulty breathing, or feeling faint after an allergen exposure. See your GP if allergy symptoms are affecting daily life or if you want to identify your specific triggers.
Diagnosis
Skin prick tests expose a small area of skin to common allergens to look for a reaction. Blood tests measure specific IgE antibodies. Oral food challenges are the gold standard for food allergies.
Treatments
Antihistamines
Block histamine receptors to reduce sneezing, itching, and runny nose. Modern non-drowsy types (cetirizine, loratadine) are effective for most mild to moderate allergies.
Adrenaline auto-injector (EpiPen)
For people at risk of anaphylaxis. Injected into the thigh during a severe reaction while waiting for emergency services.
Allergen immunotherapy
Gradual exposure to increasing amounts of an allergen over months to years to retrain the immune system to tolerate it. Available for pollen, insect venom, and some food allergies.
Self-care and lifestyle
- Identify and avoid known triggers where practical
- Keep windows closed during high pollen season
- Wash hands and change clothes after outdoor exposure
- Read food labels carefully and inform restaurants of food allergies
Prevention
Introduce allergenic foods like peanuts early in infancy (around 4 to 6 months) to reduce the risk of food allergy. There is no reliable way to prevent environmental allergies, but reducing exposure to triggers helps manage severity.