What Is Anaphylaxis?

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A sudden, severe immune response to an allergen trigger is medically known as anaphylaxis. While common allergies can cause itching, a runny or stuffy nose, or a rash, anaphylaxis causes an inappropriate whole-body reaction.


Anaphylaxis was originally identified in the early 1900s while conducting research to immunize dogs to sea anemone venom. Instead of developing an immunity to sea anemone venom, the dogs had worsened with each subsequent exposure.

While immunization was meant to be a prophylactic measure to help the dogs, the opposite effect was observed, so they termed the response the opposite of prophylaxis: anaphylaxis.

anaphylaxis causes
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Types of Anaphylaxis

Anaphylaxis reactions follow three specific patterns. Understanding what pattern your anaphylaxis follows may help both you and medical professionals to understand how best to treat your emergencies.

Uniphasic (1 phase) reactions are the most common anaphylaxis pattern. Approximately 70 to 90 percent of cases follow this pattern. Uniphasic reaction are the worst within 30 to 60 minutes and typically resolve within the next hour.

Biphasic (2 phase) reactions are five times more common in children than in adults and account for fewer than 23 out of 100 cases of anaphylaxis. Biphasic reactions are characterized by a recurrence of anaphylactic symptoms several hours after the resolution of symptoms.

Protracted reactions are the most severe form and the rarest pattern of anaphylaxis. Protracted reactions are persistent and can last from several days to several weeks.

Your Immune System

To help understand what causes anaphylaxis, it's helpful to know that your immune system is responsible for protecting your body from harmful substances like viruses or bacteria. It is one of the most complex systems of your body and it is made up of lymphatic organs (bone marrow and thymus), a variety of cell types, and proteins.

There are two different types of immunity: innate (defenses that you are born with) and adaptive (learned or acquired).

Innate Immune System

Your innate immune system is a natural defense that you are born with that helps prevent you from acquiring an infection or exposure to harmful agents.

Your skin is your body's first defensive barrier.

Proteins that exist in your saliva or other body fluids play an important role in your immune system. Your saliva contains an important protein called lysozyme that allows bacterial walls to be more easily destroyed. Special immune cells called phagocytes (including neutrophils, monocytes, or macrophages) are also important and function by surrounding and consuming bacteria or other potentially harmful substances.

Adaptive Immune System

Your adaptive immune system is the part of your defensive mechanism that learns throughout the course of your life.

When you are born, you have T and B cells that have receptors on them. As your body is exposed to different antigens (toxins), your T and B cells clone themselves to specifically fight against the exposed antigen. This is why once you have been exposed to some illnesses, subsequent illnesses are either shorter in duration, or you may not even know you were exposed.

Unlike innate immunity, the adaptive immune system cannot be passed on to your children.

Symptoms of Anaphylaxis

Anaphylaxis can affect multiple body parts and processes, including:

  •  Skin: rashes, itching, or swelling
  • Breathing: coughing, wheezing, or difficulty breathing
  • Heart: irregular heartbeat, low blood pressure, or chest pain
  • Abdomen: nausea, vomiting, or cramping
  • Neurological: headaches, confusion, or dizziness

The most common symptoms are swelling (particularly facial or angioedema), breathing problems, and low blood pressure.

If you are experiencing symptoms of anaphylaxis, emergency care, including an epinephrine shot, is needed immediately. Knowing what triggered the reaction can help avoid a future episode, as can other methods of prevention.


When you are first exposed to an allergen, your body may develop immune cells that are specific to the allergen. You will then have symptoms of allergies with subsequent exposures.

It is not well understood why some people develop allergies and others do not.

If you do develop an allergy, then your body will develop antibodies known as immunoglobulin E (IgE), which will respond every time your body is exposed to the allergen.

Immunoglobulin E reacts to exposure by activating basophils and mast cells, which are part of the white blood cells of your body.

Basophils and mast cells release mediators which cause changes within the body that directly relate to the symptoms associated with an allergic reaction. Mediators involved in anaphylaxis include:

  • Histamine: causes itching, flushing, low blood pressure, headache, runny nose, and bronchospasm
  • Tryptase: higher levels with more severe cases of anaphylaxis, except in food allergies
  • Platelet-activating factor: higher levels with more severe cases of anaphylaxis
  • Nitric oxide: causes hypotension related to dilation of blood vessels
  • Arachidonic acid metabolites: causes bronchospasm, hypotension, and erythema

Not all allergies will cause anaphylaxis. If you experience having worsening symptoms with repeated exposure to a food, medication, or insect sting, anaphylaxis is possible and you should avoid the allergen.

Anaphylaxis can also involve a non-IgE related reaction with your immune systems T-cells. Common agents that can cause non-IgE anaphylaxis includes:

  • Aspirin
  • NSAIDs: such as Ibuprofen, Aleve, or Celebrex
  • Contrast for radiographic images (x-ray, CT scans, MRI, or ultrasound)
  • IV immunoglobulin therapy
  • Heparin
  • Allergy to dialysis membranes

Activation of T-cells causes the same type of activation of basophils and mast cells described above.


Anaphylaxis is a medical emergency. It develops and progresses quickly and can be fatal. Hives, swelling, and wheezing are the symptoms that most often tip people off to a case of anaphylaxis.

Immediate administration of epinephrine (adrenaline), the only treatment for anaphylaxis, is necessary. Steroids and antihistamines may be used as well.

Those who have a known risk are prescribed an EpiPen, Auvi-Q, or other autoinjector, so they have this potentially life-saving medication on-hand at all times. It should be administered in the thigh, directly through the skin, though you can inject it through clothing if necessary.

Whether you have epinephrine available or not, it's imperative to call 911 immediately. While you wait for help, the person should lie down and elevate the legs, and the offending allergen (if known) should be removed. Pulse and breathing should be monitored.


Preventing anaphylaxis, logically, involves knowing what can cause this reaction in the first place. If you or your child have experienced this emergency before, you may already know that. If not, or your doctor has flagged this as a potential concern, it's worth having a detailed conversation about allergy testing and avoidance strategies. These may include diet changes, medication changes, wearing protective clothing when outdoors, and others.

You should also work with your doctor to manage any underlying conditions that could worsen the effects of anaphylaxis. Discuss whether immunotherapy might be appropriate, too.

Above all, be prepared: If you know that you are at high risk for anaphylaxis, wear a medical alert bracelet (and never take it off). Talk to family members, co-workers, and/or school officials about the possibility of such an emergency and what they should do to help, if needed. And be sure to always have an epinephrine auto-injector handy, if one has been prescribed.

A Word From Verywell

Experiencing anaphylaxis is very scary. Learning what causes anaphylaxis, along with treatments and ways to prevent episodes, will play an important part in dealing with severe allergic reactions. If you are unsure of whether or not you are experiencing a severe allergic reaction, you should always seek emergency help.

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Article Sources
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Additional Reading
  • An overview of the innate immune system. UpToDate website. 2017.
  • Loverde, D, Iweala, O, Eginli, A & Krishnaswamy, G. (2018). Anaphylaxis. CHEST, 153(2): 528-543. DOI: 10.1016/j.chest.2017.07.033
  • Pathophysiology of anaphylaxis. UpToDate website. 2018.