What Is Anaphylaxis? Symptoms, Causes, and What to Do

Anaphylaxis is a severe, life-threatening allergic reaction that usually happens quickly and can affect breathing and blood circulation. It’s a medical emergency and needs immediate action.

If you want the full service and treatment overview, start here: Anaphylaxis treatment & adrenaline auto-injectors.

 

Why anaphylaxis is different from a “normal” allergy

A mild allergy might cause itching, sneezing, or a local rash. Anaphylaxis is different because it can become systemic (whole body) and can quickly turn into airway or circulation problems.

If you want the full comparison logic, see: Anaphylaxis vs allergic reaction.

 

How fast does anaphylaxis happen?

Anaphylaxis often starts within minutes of contact with a trigger, which is why early recognition matters. Some reactions can happen later, but the key point is that anaphylaxis can progress rapidly and should never be “wait and see.” 

For the full timeline logic, see: How fast does anaphylaxis happen?.

 

Common causes (triggers) of anaphylaxis

The most common trigger groups are:

1. Foods (e.g., peanuts/tree nuts, milk, eggs, seafood)

2. Medicines (some antibiotics, pain medicines like NSAIDs)

3. Insect stings (bee/wasp stings)

If you need a complete trigger breakdown, go to: What causes anaphylaxis?.

 

Signs and symptoms of anaphylaxis (what to look for)

Anaphylaxis can involve skin symptoms, but the more important red flags are the Airway / Breathing / Circulation pattern.

Airway (throat/tongue)

• Swelling of the tongue or throat

• Tight throat, hoarse voice, trouble speaking 

Breathing (lungs)

• Trouble breathing, noisy breathing, wheeze

• Persistent cough or chest tightness 

Circulation (blood pressure / collapse)

• Dizziness, fainting, collapsing

• Pale, clammy skin (children may look “pale and floppy”) 

Skin + gut (can occur, but don’t rely on these alone)

• Hives (urticaria), flushing, itching

• Vomiting, stomach cramps, diarrhoea 

 

Can anaphylaxis happen without hives?

Yes. Some people focus on skin symptoms and miss the danger signs. Breathing difficulty, throat swelling, and collapse are more critical than whether hives are present. 

 

What to do immediately (minimum action logic)

If you think someone is having an anaphylactic reaction:

1. Use an adrenaline auto-injector immediately if the person has one and symptoms suggest anaphylaxis.

2. Call 999 and clearly say “anaphylaxis” (even if they start to feel better).

3. Do not delay emergency care  observation is often needed because symptoms can return. 

 

Why adrenaline is the key first treatment

Anaphylaxis can cause airway swelling and dangerous circulation changes. Adrenaline (epinephrine) is the treatment designed to rapidly counter these life-threatening effects. This is why guidance consistently treats adrenaline as the critical first response in suspected anaphylaxis. 

Why people are advised to carry two auto-injectors

In real life, a single dose may not be enough for everyone. UK safety advice commonly recommends that people at risk carry two adrenaline auto-injectors at all times, and still call emergency services after use.

 

What happens after an anaphylactic reaction?

Even after symptoms improve, emergency follow-up matters. Guidance highlights assessment and referral after emergency treatment, and clinical pathways often involve observation and later specialist review. 

 

When should you get an EpiPen in the UK?

If you’re at risk of anaphylaxis, the practical path is: risk assessment → prescription → training → carry two → renew before expiry.