An allergic reaction happens when the immune system overreacts to things like food, pollen, medications, bee venom, and other substances. Coming into contact with these allergens can cause swelling, hives, and other symptoms.
An allergen is anything that triggers an allergic reaction. Most allergens are harmless to the average person, but if the immune system sees them as a threat, even everyday things can become allergens.
The immune system’s job is to protect the body from harmful invaders. But when someone has an allergy, it mistakenly treats the allergen as dangerous and attacks it.
Every year, more than 50 million people in the U.S. have an allergic reaction. These reactions can lead to swelling, and if the airways are affected, it can become life-threatening.
In this article, we’ll look at allergy risk factors, symptoms, and treatments.
What is an allergy?
Allergies happen when the immune system overreacts to normally harmless substances.
The first time someone encounters an allergen, they usually don’t have a reaction. It takes time for the immune system to become sensitive to it.
Eventually, the immune system learns to recognize the allergen and produces antibodies to fight it. This process is called sensitization.
Some allergies are seasonal. For example, hay fever tends to flare up between April and May when pollen counts are high. Symptoms can get worse as pollen levels rise.
Symptoms
Allergic reactions cause inflammation and irritation, but symptoms vary depending on the allergen. Reactions can affect the gut, skin, sinuses, airways, eyes, or nose.
Here are some common triggers and their symptoms:
Dust & Pollen
- Stuffy or runny nose
- Itchy eyes and nose
- Watery, swollen eyes
- Coughing
Food
- Vomiting
- Swollen tongue
- Tingling in the mouth
- Swelling of lips, face, or throat
- Stomach cramps
- Shortness of breath
- Rectal bleeding (mostly in kids)
- Mouth itchiness
- Diarrhea
Insect Stings
- Wheezing
- Severe swelling at the sting site
- Sudden drop in blood pressure
- Itchy skin
- Trouble breathing
- Restlessness
- Hives (red, itchy rash spreading across the body)
- Dizziness
- Coughing
- Chest tightness
Medication
- Wheezing
- Swelling of the tongue, lips, or face
- Rash
- Itchiness
If symptoms get worse, they can lead to anaphylaxis.
Anaphylaxis Symptoms
Anaphylaxis is the most severe allergic reaction and can be life-threatening. It can happen within minutes or hours of exposure.
Research shows it usually affects the skin and breathing. Symptoms include:
- Hives, redness, and itching
- Trouble breathing
- Wheezing
- Swelling
- Low blood pressure
- Irregular heart rate
- Dizziness or fainting
- Loss of consciousness
Recognizing these signs quickly is crucial for getting help.
Causes
When an allergic reaction occurs, allergens bind to antibodies called IgE. These antibodies fight off harmful substances in the body.
Once the allergen attaches to IgE, certain cells (like mast cells) release chemicals that cause allergy symptoms.
One of these chemicals, histamine, tightens muscles in the airways and blood vessels. It also makes the nose produce more mucus.
Risk Factors
People are more likely to develop allergies if they:
- Are under 18
- Have a personal or family history of asthma or allergies
- Were born via C-section (some research suggests they miss exposure to the mother’s microbiome during birth)
Common Allergens
Allergens are everywhere. While any food can cause an allergy, these eight are the most common triggers:
- Eggs (especially whites)
- Fish
- Milk
- Peanuts
- Tree nuts
- Shellfish
- Wheat
- Soy
More on food allergies here.
Other frequent allergens include:
- Pet dander, fur, or saliva
- Mold & mildew
- Medications (like penicillin)
- Insect bites & stings
- Cockroaches, moths, midges
- Plant pollen
- Household chemicals
- Metals (nickel, cobalt, chromium, zinc)
- Latex
Diagnosis
If you suspect an allergy, a doctor can help identify the cause. Be ready to explain:
- Your symptoms
- When and how often they happen
- Possible triggers
- Family allergy history
- Whether others in your home have similar reactions
The doctor may run tests or refer you to a specialist.
Tests
Common allergy tests include:
- Blood tests – Measure IgE antibody levels.
- Skin prick tests – A tiny amount of allergen is applied to the skin. If it turns red or swells, it may indicate an allergy.
- Patch tests – Used for contact eczema. A small amount of allergen is taped to the skin and checked after 48 hours.
The American College of Allergy, Asthma & Immunology can help you find a certified allergist.
Treatment
Avoiding the allergen is best, but when that’s not possible, medications can help manage symptoms.
Medications (available over-the-counter or by prescription):
- Antihistamines – Block histamine, which causes reactions.
- Decongestants – Relieve nasal congestion.
- Corticosteroids – Reduce inflammation (available as pills, creams, nasal sprays, or inhalers).
- Immunotherapy – Gradually increases allergen exposure to build tolerance (given as shots or tablets).
- Antileukotrienes – Block chemicals that cause swelling (used if other treatments fail).