What Is Anaphylaxis? Symptoms, Causes, Diagnosis, Treatment, and Prevention

Anaphylaxis is a serious allergic reaction that typically affects more than one area of your body at once.

It can be triggered by exposure to any allergen — a substance that you’ve become allergic to. The most common triggers are foods, medications, insect venom, and latex.

In an anaphylactic reaction, your immune system releases a number of chemicals to fight off what it perceives as a dangerous substance invading your body. These immune system chemicals can have different effects on different areas of your body.

If it’s severe enough and left untreated, anaphylaxis can cause you to go into shock. Signs of shock often include rapid breathing, pale or clammy skin, dizziness, and a rapid, weak pulse.

Anaphylaxis requires immediate treatment with an epinephrine injection — this usually is administered before the person gets to a hospital emergency room — where they should go as soon as possible.

If you know that you’re prone to an anaphylactic reaction, you should carry an epinephrine self-injection device with you at all times. You should also do everything in your power to avoid known triggers of anaphylaxis.

Because anaphylaxis involves so many different chemicals released by your immune system, scientists are still working to understand how and why different symptoms may occur.

Anaphylaxis has become more common in recent years, especially in children and younger adults, for reasons that aren’t fully understood. (1,2,3,4)

What Happens in Anaphylaxis

Anaphylaxis refers to a number of allergic reactions happening in your body at almost the same time. It’s set off by numerous chemicals released by your immune system in various tissues throughout your body.

These chemical groups include arachidonic acid metabolites, chemokines, cytokines, proteases, and proteoglycans. Each of these groups contains more specific chemicals.

One chemical implicated in anaphylaxis is histamine, which is the target of antihistamine drugs. Histamine is one player in the process that relaxes your blood vessels, tightens your throat and airways, and increases mucus production.

But because histamine is only one of numerous chemicals involved in an anaphylactic reaction, taking an antihistamine by itself tends to have limited benefits.

Each chemical released in an anaphylactic reaction is responsible for a specific task, and some are more associated with certain symptoms than others. (4)

How Allergies Lead to Anaphylaxis

An allergic reaction happens when your immune system overreacts to a substance it believes to be a threat, known as an allergen.

In response to an allergen, your immune system releases chemicals that cause allergy symptoms. These symptoms typically occur in one location on your body for a given allergen.

But in some cases, your immune system releases a wide range of chemicals, and in greater quantity than usual, causing reactions in multiple areas of your body. Collectively, these reactions are known as anaphylaxis. (5)

Allergies happen when your immune system becomes sensitized to a particular substance. It’s not fully understood why this happens with certain substances in some people and not in other people. Genetics is likely involved in some of these differences.

Once your body is sensitized to an allergen, an allergic reaction of any magnitude can occur. (1)

Anaphylactic and Anaphylactoid Reactions

Sometimes, a substance can set off a reaction that’s similar to anaphylaxis the very first time you’re exposed to it. This isn’t a true allergic reaction, since your body hasn’t become sensitized to the substance.

Called an anaphylactoid reaction, this can happen in response to drugs like morphine and aspirin, and contrast medium (a substance that’s introduced into the body for certain imaging tests). It can also be caused by exercise.

Even though the chemicals released in your body may be somewhat different in an anaphylactoid reaction compared with a true anaphylactic one, the symptoms, treatment, and risk of complications are the same for both. (1,3)

Any substance can cause an allergic reaction that leads to anaphylaxis, but some substances are more likely to trigger anaphylaxis than others.

Foods that are common triggers of anaphylaxis include the following:

• Peanuts and tree nuts
• Fish and shellfish
• Milk
• Eggs
• Soy
• Wheat
• Red meat

Drugs that are common causes of anaphylaxis include antibiotics, aspirin, blood and blood products, certain hormones, and opioids.

Bites or stings from fire ants, bees, wasps, and hornets are more likely than other types of insect bites or stings to cause anaphylaxis.

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Certain substances and activities can also trigger what’s known as an anaphylactoid reaction. The reaction isn’t allergic in the sense of your body being sensitized to an allergen, but the effects and treatment are virtually the same. Common triggers of an anaphylactoid reaction include contrast medium, aspirin and other NSAIDs, opioids, monoclonal antibodies, and exercise.

It’s possible for anyone to have an allergic reaction that causes anaphylaxis at any time, assuming he or she has previously been exposed to the allergen.

But certain groups are more likely to have anaphylaxis in response to particular allergens.

For example, children are more likely than adults to experience anaphylaxis in response to a food allergy. Adults are more likely to experience it because of an allergy to antibiotics, anesthetics, contrast medium, or insect venom.

Men are more likely than women to experience anaphylaxis from insect venom, while women are more likely to experience it from latex, aspirin, contrast medium, and muscle relaxants.

For reasons that aren’t completely clear, people with a higher socioeconomic status — those with a higher income, education, and other measures of social status — are at greater risk for anaphylaxis.

Also for reasons that aren’t fully understood, prescription rates for self-injectable epinephrine — which is prescribed to treat anaphylaxis — are higher in northern than in southern areas of the United States.

You’re more likely to experience anaphylaxis if you have certain other medical conditions, including:

• Asthma
• Cardiovascular disease
• Mastocytosis (abnormal accumulation of mast cells in the skin, bone marrow, and internal organs)
• Substance use disorders

You’re at greater risk for anaphylaxis if you’re genetically predisposed to allergic diseases — a condition known as atopy. But this increased risk may not apply to certain allergens, such as penicillin, insulin, or some insect venoms.

Generally speaking, your risk for anaphylaxis goes down the longer you go without being exposed to an allergen that may cause it. (4)

Learn More About Causes of Anaphylaxis: Common Risk Factors, Genetics, and More

Surprising Triggers for Anaphylaxis

Most people have heard that peanuts can cause anaphylaxis if you’re allergic to them, but did you know that some women experience anaphylaxis during a certain phase of their menstrual cycle? Or that a bite from a lone star tick can lead to an allergy to red meat?

Lesser-Known Anaphylaxis Triggers

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Many seemingly weird things can bring on anaphylaxis in people who are susceptible to it, and in some cases it can be difficult to figure out what’s causing it.

If you’re experiencing symptoms of anaphylaxis but aren’t sure what’s causing it, see an allergist for help.

Learn More About Surprising Triggers for Anaphylaxis

Symptoms of anaphylaxis often develop within a few seconds or minutes after exposure to an allergen, but can take an hour or longer to develop in rare cases. (1,3)

An anaphylactic reaction can get worse quickly once it starts, to the point where you collapse, stop breathing, lose consciousness, or have seizures within one to two minutes of its onset — although it may take longer to fully worsen. If treatment isn’t given rapidly, such a reaction can be fatal.

Sometimes symptoms of anaphylaxis return four to eight hours after the initial exposure to an allergen, or even later (known as biphasic anaphylaxis). This second reaction tends to be milder than the first one, but in some cases it may be more severe or even fatal. (3,6)

Some people have what’s known as protracted anaphylaxis, in which symptoms last for several hours or longer. This possibility is one reason it’s important to be monitored in an emergency medical setting, even if your symptoms are initially resolved by an epinephrine injection. (6)

If you don’t receive prompt treatment, anaphylaxis can lead to a number of potentially life-threatening complications, including the following:

• Blocked or closed airway
• Cardiac arrest (no heartbeat)
• Respiratory arrest (no breathing)
• Shock (sudden drop in blood pressure)

Depending on how quickly you receive emergency medical attention, it may be possible to be revived if you experience these complications. (1,2)

There’s some evidence that Black and Hispanic children are at higher risk for food allergies that may lead to anaphylaxis than white children, as well as anaphylaxis itself.

In one study of children ages 0 to 17, when compared with non-Hispanic white children, African American children had significantly higher rates of allergies to wheat, soy, corn, fish, and shellfish. Hispanic children also had higher rates of allergies to corn, fish, and shellfish.

The same study found that African American and Hispanic children were at higher risk for adverse outcomes related to food allergies, including higher rates of anaphylaxis. This may be due, in part, to a higher rate of unrecognized food allergies. (8)

A separate analysis of emergency department visits in Florida from 2005 to 2006 for anaphylaxis found that Black people were more likely than other groups to have a food trigger for anaphylaxis, while white people were more likely to have it triggered by insect venom. (9)

The following organizations and websites offer more information on anaphylaxis and related conditions:

American Academy of Allergy, Asthma, and Immunology (AAAAI)

This advocacy and research organization has information on allergy-related conditions and treatments on its website, along with resources to help you find an allergist or ask a health expert a question.

Asthma and Allergy Foundation of America (AAFA)

This nonprofit group is the leading patient-centered organization for people with asthma and allergies. Its website contains useful health information, as well as an overview of allergy-related research, news, and advocacy opportunities.

Seattle Allergy & Asthma Research Institute

This clinical research organization provides information on its ongoing and currently recruiting studies, including how to find out if you’re eligible to participate in one.

Benaroya Research Institute at Virginia Mason

This research institute focuses on disorders of the immune system, including autoimmune disorders and allergies. Its website has information on a variety of research projects, including how to participate.

Food Allergy Research and Education (FARE)

This patient advocacy and research organization has provided over $100 million in funding for food allergy research. Its website provides detailed information on living with food allergies, along with its research-based projects and priorities.

Kids With Food Allergies

This website from the Allergy and Asthma Foundation of America (AAFA) contains practical guidance on living with food allergies, including allergy alerts regarding specific products, how to find a local or online allergy support group, and allergy-friendly recipes.