Allergic Reactions: Symptoms, Triggers, and What to Do in an Emergency

Last reviewed: July 2026

What You Need to Know

• Epinephrine is the only treatment for anaphylaxis. Nothing else works. Delay is what kills — anaphylaxis can be fatal in under 15 minutes, and drug-triggered reactions have caused death in an average of five.
• There’s now a needle-free option. The FDA approved neffy, an epinephrine nasal spray, in August 2024 — the first major innovation in epinephrine delivery in over 35 years.
• Sesame is the ninth major food allergen, required on US labels since January 2023.
• Prevention advice reversed completely. Early introduction of peanut — not avoidance — cuts peanut allergy dramatically in at-risk infants. If you’re following advice from before 2017, you’re doing the opposite of what works.
• Reactions escalate unpredictably. A mild reaction last time does not guarantee a mild reaction next time.

What an Allergic Reaction Actually Is

Your immune system exists to fight genuine threats — viruses, bacteria. In an allergic reaction, it misidentifies something harmless as dangerous: pollen, dust mite protein, a peanut, a bee’s venom protein.

The immune system produces IgE antibodies against that substance. Those antibodies attach to mast cells throughout your body. The next time you encounter the allergen, it binds to those antibodies and the mast cells release a flood of chemicals, including histamine. That flood is the reaction: hives, swelling, itching, runny nose, wheezing, vomiting.

This is why first exposures are often uneventful. The first bee sting may just hurt. The second one can cause hives or breathing trouble — because by then your immune system has been sensitized, and the machinery is ready to fire. This is one of the more dangerous features of allergy: the reaction you had before does not predict the reaction you’ll have next.

Allergy vs. Intolerance vs. Autoimmune — They’re Not the Same

This distinction matters, and the internet blurs it constantly.

	What it is	Can it cause anaphylaxis?
Allergy (e.g. peanut, milk, wheat allergy)	Immune system attacks a harmless protein. IgE-mediated.	Yes
Intolerance (e.g. lactose intolerance)	A digestive enzyme problem. Unpleasant, not immune-mediated.	No
Celiac disease	Autoimmune. Gluten triggers the immune system to damage the small intestine.	No — but it causes serious long-term harm if untreated
Non-celiac gluten sensitivity	Poorly understood, contested.	No

A wheat allergy and celiac disease are entirely different conditions, requiring different testing and different management — and the previous version of this article treated “wheat, gluten” as one thing. If you react to bread, you need to know which of these you have. Don’t guess, and don’t go gluten-free before being tested for celiac — it makes the test unreliable.

Epinephrine: What You Need to Carry

Epinephrine has been used for allergic reactions for more than a century and is the only universally recommended first-line therapy. It works by tightening blood vessels and opening airways — reversing the two things that kill in anaphylaxis.

Auto-injectors

The traditional option (EpiPen and others), FDA-approved since 1987. Into the outer thigh, through clothing if necessary. Hold as instructed.

neffy — epinephrine nasal spray

Approved by the FDA in August 2024. Sprayed into one nostril; no needle.

• 2 mg — adults and children weighing at least 66 lbs (30 kg)
• 1 mg — children weighing 33–66 lbs (the FDA has since removed the earlier age restriction)

Why this matters more than it sounds: the FDA explicitly noted that people — particularly children — may delay or avoid treatment out of fear of needles. In anaphylaxis, delay is the mechanism of death. Removing the needle removes a real barrier.

Rules for carrying it

• Carry two. One dose isn’t always enough.
• Carry it with you. Epinephrine in a car, a locker, or the school office is epinephrine you don’t have.
• Everyone around you should know where it is and how to use it — partner, colleagues, teachers, coaches.
• Check the expiry date. Set a calendar reminder.
• Use it early. The most common fatal error is waiting.
• Medical ID jewellery genuinely helps. See peanut allergy bracelets.

Anaphylaxis can progress to cardiac arrest. Knowing CPR is worth your time — see CPR can make you a lifesaver.

What Triggers Allergic Reactions

Food — the top 9 major allergens

Under US law, these must be declared on packaged food labels:

Milk · Eggs · Fish · Crustacean shellfish · Tree nuts · Peanuts · Wheat · Soybeans · Sesame

Sesame was added on January 1, 2023 under the FASTER Act. A note that catches people out: some manufacturers responded to the new rule by adding sesame to products and declaring it, rather than keeping it out. Foods you’ve safely bought for years may now contain sesame. Read labels again.

Also worth knowing: highly refined oils derived from a major allergen are exempt from the labeling requirement.

“May contain” and shared-equipment warnings are voluntary and unregulated — the levels are not standardized. Talk to your allergist about how cautious to be. See understanding food labels and what “peanut-free” actually means.

More on the most common serious one: peanut allergy symptoms, peanut allergy testing, and peanut allergy treatment.

Medications

Antibiotics (especially penicillins), NSAIDs, and chemotherapy agents are common culprits. Drug reactions can be severe and fast.

But here’s something most people don’t know: the overwhelming majority of people labeled “penicillin-allergic” are not. Childhood rashes get recorded as allergies and follow people for life — and the label pushes them onto broader, more expensive, less effective antibiotics with worse side effects. Ask your doctor about penicillin allergy testing and “delabeling.” It’s a genuinely worthwhile conversation. See making the most of your medications.

Insect stings

Bees, wasps, hornets, fire ants. Your immune system reacts to proteins in the venom — not to it being “toxic.” Local swelling and pain are normal. Hives away from the sting site, breathing difficulty, or dizziness are not: that’s anaphylaxis.

Venom immunotherapy is highly effective for people with a history of systemic sting reactions and can be close to protective. Worth asking about.

Animals

Dander (skin flakes), saliva, and urine — not the hair itself, which is why “hypoallergenic” breeds are largely marketing. Symptoms are usually respiratory.

Environmental and seasonal

Pollen (trees, grasses, weeds), dust mites, mold, cockroach. Hay fever is the familiar version.

Latex

Not in the original version of this page, and it should have been. Common in healthcare settings, and it cross-reacts with banana, avocado, kiwi, and chestnut — the “latex-fruit syndrome.”

Cosmetics and chemicals

Fragrances, preservatives, hair dye, henna tattoos, nickel, paint fumes, ammonia. Often contact dermatitis rather than IgE allergy, but it can be miserable.

Alpha-gal syndrome — the one nobody expects

A tick bite can trigger an allergy to red meat. Reactions are typically delayed by 3–6 hours after eating, which makes it easy to miss entirely. If you’re getting unexplained hives or anaphylaxis in the middle of the night, mention this to your doctor.

Prevention: The Advice That Completely Reversed

This is the single most important update on this page for parents.

For years, the advice was to delay introducing allergenic foods to at-risk babies. That advice was wrong, and it may have made things worse.

The landmark LEAP trial found an 81% relative reduction in peanut allergy when peanut was introduced early — between 4 and 11 months — and continued regularly. The guidelines changed accordingly.

Current guidance:

• Introduce peanut (in an infant-safe form — never whole nuts, which are a choking hazard) early in the first year of life.
• For most infants, this can be done at home.
• The one group that should be assessed by a doctor first is infants with severe eczema, egg allergy, or both.
• Once solids begin, a diverse diet appears protective.

If your parenting information predates 2017, throw it out. Talk to your pediatrician — see the ABCs of picking a pediatrician.

The Atopic March: Allergies Travel in Packs

Allergic conditions cluster, and they tend to appear in a sequence — eczema in infancy, then food allergy, then asthma, then allergic rhinitis. Which is why allergies so often show up alongside asthma, sinusitis, recurrent ear infections, and sleep problems.

Practical consequence: if your child has food allergy, take breathing symptoms seriously. Uncontrolled asthma is a major risk factor for fatal anaphylaxis — the two conditions compound each other. See asthma: help your child breathe easier and easing your child’s ear infection.

Diagnosis and Treatment

Testing: skin prick tests and specific IgE blood tests show sensitization — not necessarily clinical allergy. A positive test on its own does not mean you’re allergic. It must be interpreted alongside your actual history, which is why self-ordered allergy panels cause so much unnecessary food avoidance. The gold standard remains a supervised oral food challenge.

For mild reactions: antihistamines help with hives, itching, and runny nose. Nasal steroids help with allergic rhinitis. See safety check: over-the-counter medications.

Immunotherapy: allergy shots (and, for some allergens, under-the-tongue tablets) can genuinely reduce or eliminate allergy over time. Oral immunotherapy for peanut is now an option for some children. Discuss with an allergist.

Biologics: newer injected medications can reduce reaction severity in people with multiple food allergies. Ask.

When to Get Help

Call 911 and use epinephrine for any sign of anaphylaxis (top of this page).

See a doctor if you have:

• Any reaction involving breathing, swelling, or dizziness — even if it settled
• Reactions that are getting worse over time
• An unexplained reaction with no obvious trigger
• A “penicillin allergy” you’ve never had tested

Go to the ER for a severe reaction — see before you go to the ER, read this and determining if or when you should go.

An allergist is the right specialist. Your primary care doctor can refer you — getting the most from your doctor will help you make it count.

The Bottom Line

Most allergic reactions are mild and manageable. The ones that aren’t kill fast, and they kill mostly through delay — waiting to see if it gets worse, reaching for an antihistamine, standing up to get to the car.

If you or your child carries epinephrine: carry two, carry them on you, and use them early. That’s the whole game.

Related Reading

• Peanut Allergy Symptoms
• Peanut Allergy Test
• Peanut Allergy Treatment
• Peanut Allergy Bracelets
• What Does “Peanut-Free” Mean?
• Asthma: Help Your Child Breathe Easier
• Understanding Food Labels
• CPR Can Make You a Lifesaver

Sources: U.S. FDA — approval of neffy (epinephrine nasal spray), August 2024 · U.S. FDA — The FASTER Act: Sesame Is the Ninth Major Food Allergen · NIAID Addendum Guidelines for the Prevention of Peanut Allergy · LEAP trial (Learning Early About Peanut Allergy) · American Academy of Pediatrics guidance on early food introduction.

This article is for information only and is not a substitute for medical care. If you suspect anaphylaxis, use epinephrine and call 911.