The 4 Stages of Anaphylaxis – What Happens and What to Do

Anaphylaxis is one of the most frightening medical emergencies a person can experience or witness. It moves fast, it affects multiple body systems at once, and it can turn life-threatening in minutes.

Understanding how an anaphylactic reaction unfolds from the moment of exposure to the point of resolution or crisis – helps you recognize it earlier, respond more effectively, and know what to watch for even after a reaction appears to have passed.

Here's a clear, honest guide to how anaphylaxis progresses and what to do at each point along the way.

A Note Before We Begin

It's worth being upfront: the medical community describes anaphylaxis along a severity spectrum from mild to moderate to severe – rather than using a fixed set of numbered stages. According to the 2024 GA2LEN Anaphylaxis Consensus Report, anaphylaxis is dynamic, and reactions may initially appear mild before becoming severe.

The four stages described in this blog reflect how a reaction typically unfolds over time from exposure to early symptoms to full reaction to aftermath. This framework is educationally useful and widely used in patient and caregiver education, even if it doesn't map directly to a formal clinical staging system.

Stage 1 – Exposure to a Trigger

Every anaphylactic reaction begins with exposure to a trigger, something the immune system identifies, incorrectly, as a dangerous threat.

According to the ACAAI, the most common anaphylaxis causes include:

• Food – peanuts, tree nuts, shellfish, fish, milk, eggs, wheat, and sesame

• Insect stings – bees, wasps, hornets, yellow jackets, and fire ants

• Medications – antibiotics (particularly penicillin), aspirin, NSAIDs, and certain other drugs

• Latex – found in gloves, balloons, and some medical equipment

• Exercise – in rare cases, physical activity alone or in combination with food can trigger a reaction

• Unknown causes – sometimes called idiopathic anaphylaxis, where no trigger can be identified

Upon exposure, the immune system triggers a massive release of histamine and other chemical mediators from mast cells and basophils. These chemicals flood the body rapidly and what happens next depends on how severe the immune response is.

One important thing to understand: anaphylaxis is unlikely to occur on a person's very first exposure to an allergen. Sensitization, the immune system's initial recognition of the allergen  typically happens first. Subsequent exposures are when anaphylaxis can occur.

Stage 2 – Early Symptoms Appear

Within seconds to minutes of exposure, the first signs of an anaphylactic reaction begin to appear. This is the stage where early recognition matters most, because the earlier epinephrine is given, the better the outcome.

Early symptoms of an anaphylactic reaction often start in the skin or the respiratory system:

Skin symptoms:

• Hives, itching, or flushing across the body

• Swelling of the lips, face, or eyes

• Pale or flushed skin

Respiratory symptoms:

• Runny or congested nose

• Sneezing

• Mild shortness of breath or wheezing

• Anaphylaxis throat swelling – a sensation of throat tightening that can progress rapidly

Gastrointestinal symptoms:

• Nausea or stomach cramping

• Vomiting or diarrhea

Other early signs:

• Anxiety or a sense of something being wrong

• Tingling sensation in the mouth or lips (common with food-triggered reactions)

Mild anaphylaxis symptoms at this stage can be easy to dismiss or to mistake for a regular allergic reaction. This is one of the most dangerous traps in anaphylaxis management. According to ACAAI guidelines, epinephrine should be given at the first sign of suspected anaphylaxis, not after waiting to see whether symptoms worsen.

Anaphylaxis in infants can present differently – increased fussiness, crying, or pulling at the face and mouth may be the only early indicators, making recognition even more challenging in very young children.

Stage 3 – Full Anaphylactic Reaction

If epinephrine is not administered promptly, or if the reaction is particularly severe, the early symptoms progress into a full anaphylactic reaction. This is when the situation becomes life-threatening.

According to the 2024 GA2LEN Anaphylaxis Consensus Report, life-threatening anaphylaxis is characterized by airway, breathing, and cardiovascular compromise.

Severe anaphylaxis symptoms include:

• Airway – severe anaphylaxis throat swelling progressing to stridor, inability to speak or swallow, complete airway obstruction

• Breathing – severe wheezing, significant shortness of breath, respiratory failure

• Cardiovascular – rapid or weak pulse, sudden and severe drop in blood pressure, dizziness, fainting, collapse

• Neurological – confusion, loss of consciousness, seizures

At this stage, the body is in anaphylactic shock – a state of circulatory collapse where organs are not receiving adequate blood flow. Without immediate epinephrine and emergency medical care, this can be fatal.

Anaphylaxis first aid at this stage:

Step 1 – Call 911 immediately. This is always the first action, before anything else.
Step 2 –  Administer epinephrine right away – auto-injector (EpiPen®, Auvi-Q®) or nasal spray (neffy®). Do not delay to see if symptoms improve.
Step 3 –  Lay the person flat and raise their legs if they are not having breathing difficulties, this helps maintain blood pressure.
Step 4 – Give a second dose of epinephrine if symptoms do not improve after 5 minutes and a second device is available.
Step 5 – Stay with the person until emergency services arrive.

Do not give antihistamines as a substitute for epinephrine. According to the ACAAI, antihistamines are not adequate first-line treatment for anaphylaxis – only epinephrine reverses the life-threatening aspects of the reaction.

Stage 4 – Resolution or Biphasic Reaction

With prompt epinephrine treatment, most anaphylactic reactions resolve – breathing improves, blood pressure stabilizes, and the person begins to recover. This is Stage 4, but it is not the end of the emergency.

Two important things happen at this stage:

1. The person still needs emergency room evaluation.
Even after a seemingly complete recovery, going to the emergency room is non-negotiable. According to the ACAAI's 2023 Anaphylaxis Practice Parameter update, patients should be observed in a medical setting for at least a period following resolution – the appropriate observation time depends on reaction severity and individual risk factors.

2. Biphasic reactions can occur.
According to a systematic review published in the National Institutes of Health's PMC database, the overall frequency of biphasic reactions was approximately 7.36% of anaphylactic reactions, with the mean time to the second phase being approximately 8 hours though the range extends from 1 to 72 hours.

A biphasic reaction is when anaphylaxis symptoms return after appearing to fully resolve  without any additional exposure to the trigger. According to the 2023 ACAAI practice parameter update, biphasic anaphylaxis is highly likely when a patient develops anaphylaxis after initial signs and symptoms have completely resolved for at least one hour before the onset of repeated anaphylaxis within 48 hours.

This is why staying in the hospital for observation matters even when someone feels completely fine after treatment.

Side effects after anaphylaxis that are common and expected include:

• Fatigue and weakness

• Headache

• Muscle aches

• Residual hives or flushing

• Anxiety

These generally resolve within hours to a day. If symptoms worsen or new symptoms develop, treat it as a potential biphasic reaction and seek emergency care immediately.

What Anaphylaxis in Infants Looks Like

Because anaphylaxis in infants deserves special attention – early signs in very young children differ from those in older children and adults. In infants and toddlers, age-specific symptoms may differ from older children and adults.

Infants may show:

• Sudden inconsolable crying or extreme fussiness

• Drooling or difficulty swallowing

• Color changes – pale, flushed, or bluish

• Vomiting

• Limpness or sudden loss of muscle tone

Any of these following a known allergen exposure or an unknown exposure should be treated as a potential anaphylactic reaction immediately.

The Most Important Thing to Take Away

Anaphylaxis is unpredictable. A mild first reaction does not predict a mild second reaction. A reaction that begins with only skin symptoms can progress to cardiovascular collapse within minutes.

The only reliable protection is being prepared – knowing the signs, having epinephrine immediately accessible, and acting without hesitation when a reaction begins.

Having epinephrine on your keys, in your pocket, or on your child's backpack is not overcaution. It is the difference between being able to help and watching helplessly while waiting for it to arrive.

Be Ready at Every Stage

Anaphylaxis moves fast. nCase Lite is designed to help keep your epinephrine  including neffy® –  within reach at all times, so you're ready from the very first sign.

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Quick Reference – The 4 Stages of Anaphylaxis

nCase Lite is a carrying case only –  medication not included. Always follow the manufacturer's instructions for storing and using your medication. nCase Lite is not a medical device. In an emergency, call 911. neffy® is a registered trademark of ARS Pharmaceuticals Operations, Inc. nCase Technologies is not affiliated with ARS Pharmaceuticals.