Allergies: Basic Info You Need to Know

Written by Shawna Seed
Medically Reviewed by Melinda Ratini, MS, DO on April 02, 2025
5 min read

An allergy is when your immune system over reacts to a harmless substance and lets out a chemical in your body called histamine. Histamine can make your nose run, eyes water, and more. 

You get allergies when your immune system responds to substances such as pollen, pet dander, or certain foods. Your antibodies identify these allergens as bad for you, even though they're not.

How common are they?

Nearly 18 million adults in the United States have hay fever, or allergic rhinitis. That's about one-third of adults and about one-quarter of children in the U.S.

Doctors don’t know exactly how many adults are diagnosed with allergies for the first time. But nasal allergies affect more Americans every year, according to the American College of Allergy, Asthma and Immunology.

Whatever the case, allergies are all over, and they’re big business. They’re the sixth-leading cause of chronic illness in the U.S., according to the CDC. And they cost Americans more than $18 billion a year.

Allergic reactions happen when you come into contact with an allergen. Your immune system reacts by releasing histamines and other chemicals into your blood, causing symptoms that can irritate your skin, sinuses, or digestive system.

It starts when you come into contact with common allergens such as:

  • Airborne allergens: Pollen, pet dander, dust mites, mold
  • Certain foods: Peanuts, tree nuts, shellfish, eggs, and dairy
  • Insect stings: Bees and wasps
  • Medications
  • Latex

Your allergy symptoms will vary, depending on what you're allergic to and how you’re exposed. Allergens can enter your body in several ways:

  • Through your nasal passages and into your lungs
  • Through your mouth
  • Through your skin
  • Through absorption from an insect sting

If you have a mild allergic reaction, common symptoms might be:

  • Itchy, watery eyes
  • Sneezing
  • Itchy, runny nose
  • Feeling tired or ill
  • Rashes and hives

Symptoms of a food allergy could include:

  • Tingling in your mouth
  • Swelling of your lips, tongue, face, or throat
  • Hives
  • Stomach cramps, vomiting, or diarrhea
  • Anaphylaxis, which is a serious whole-body allergic reaction

An allergic reaction to an insect sting might cause any of these:

  • Swelling, redness, and pain at the site of the sting
  • Itching or hives
  • Chest discomfort or tightness
  • Coughing
  • Anaphylaxis

Serious allergy symptoms

A serious allergic reaction can lead to problems such as:

  • Belly pain
  • Anxiety
  • Chest tightness
  • Coughing
  • Diarrhea
  • A hard time breathing or swallowing
  • Dizziness
  • Swelling of the face, eyes, or tongue
  • Anaphylaxis

If you have any of these serious symptoms, seek medical help right away.

Some allergic reactions are mild, but others can produce life-threatening problems, including anaphylaxis. You have to treat anaphylaxis with epinephrine (adrenaline) within minutes.

If you have an epinephrine auto-injector or nasal spray, use it and repeat after 5 to 10 minutes if your symptoms haven’t improved. If your symptoms don't ease quickly and completely, if they're serious, or if you don't have a second dose of epinephrine on hand, call 911. 

Signs of anaphylaxis include:

  • Hives and itching all over
  • Wheezing or shortness of breath
  • Hoarseness or tightness in your throat
  • Swelling of your face, eyelids, lips, tongue, or throat
  • Tingling in your hands, feet, lips, or scalp

 

An allergy test measures how your body responds to certain triggers. If your immune system overreacts, you have an allergy. Your body will produce antibodies called immunoglobulin E (IgE).

There are different types of allergy tests. Which one your doctor chooses depends on your symptoms and the suspected causes. 

Skin-prick or scratch test

Your health care provider will use a thin needle that contains an allergen to prick your skin. Or they might put droplets of allergens on your skin and then scratch your skin so the allergens can get in. The test could use your forearm or back. If you're allergic, within 15 minutes, a rash or raised spots – called wheals – will appear. You might be tested for many allergies at once. 

Intradermal skin test

If the skin-prick test doesn't yield enough information, your doctor may try this test next. A small amount of an allergen is injected into the outer layer of your skin. The doctor is looking for a rash or other reaction. 

Patch test

This checks for contact dermatitis, a reaction that happens when your skin comes in contact with an allergen. Your health care provider puts drops of an allergen on the surface of your skin and covers the spot with a bandage. Or the allergen may be on the bandage itself. You leave the bandage in place for at least 2 days. Then you go back to your doctor, who removes the bandage and checks for a reaction. 

Blood test

Your health care provider draws a blood sample and sends it to a lab. At the lab, allergens are added to your blood, and then it's tested for levels of IgE antibodies. This test has a higher rate of false positives, indicating you have an allergy when you really don't.

Challenge test

Typically, an allergy specialist, called an allergist, does this test. You'll swallow a small amount of a suspected allergen – a food or drug – while a health care provider closely watches you in case you get anaphylaxis. If you do, you'll get an epinephrine shot right away to stop the reaction. 

To prepare for an allergy test, you'll need to stop your allergy medication 3 to 7 days ahead of time. Those drugs can interfere with the test. 

There isn't a diet for allergies, but your diet might help with how bad your symptoms get. Of course if you're allergic to any food, you'll want to avoid it to prevent a reaction.

Here are some of the foods that researchers are studying to see if they might help ease allergies.

Good fats

Researchers are studying whether polyunsaturated fatty acids such as omega-3s can help prevent allergies in children. Foods such as salmon, mackerel, tuna, flaxseeds, chia seeds, and walnuts are high in omega-3s, which help ease inflammation. Researchers think eating these may lower the risk of childhood asthma and allergy.

In one Swedish study, children who had higher levels of these fatty acids in their blood at age 8 were less likely to have nasal allergies by age 16. But more research is needed to confirm if that is because of the polyunsaturated fatty acids in their diets or something else.

Mediterranean diet

A large study of children in Crete (part of Greece) found that children who stuck to a Mediterranean diet full of fruits, vegetables, whole-grain breads and cereals, and legumes and nuts were less likely to have hay fever.

Steps you can take to help control your allergies include: 

What is immunotherapy?

Certain medicines can reduce your immune system's response to the substances that you're allergic to. To get the most benefit, you'll need to take it for 3 to 5 years. This can be an expensive option, but it offers the best hope of lasting results. 

There are two types of immunotherapy, allergy shots and sublingual immunotherapy or “SLIT” that dissolves under your tongue. The effects can last even after you stop taking the shots or SLIT. If you're looking for long-term allergy relief, talk to your doctor to see if immunotherapy is right for you.