Understanding Allergies -- the Basics

What Are Allergies?

Allergies are a reaction by your immune system to something that is usually not harmful. Allergies come in a variety of forms and can be anywhere from mildly bothersome to life-threatening. More than 50 million Americans have experienced various types of allergies each year.

Right now, researchers aren’t sure why people get allergies, but genes appear to play a role. Allergies may flare up and subside throughout your life.

Your immune system protects your body from foreign substances, known as antigens, by making antibodies and other things to fight them. Usually it ignores harmless substances, like food, and fights the dangerous ones, like bacteria.

An allergic reaction happens when your immune system attacks something harmless as if it were a threat. These harmless things are called allergens. Your body creates a chemical called histamine, which causes many of the symptoms that go with allergies. Things that can trigger an allergy attack range from pollen to pet dander to penicillin.

Most reactions aren’t serious, but some, like anaphylaxis, can be fatal. It can make you stop breathing or cause your blood pressure to drop too low. Allergies can't be cured, but there are lots of treatments to relieve the symptoms. If you have a severe allergy, see a doctor and get it treated.

Types of Allergies

Allergic Skin Conditions

Hives, or urticaria, is a rash with itchy, swollen bumps that can last for minutes or days. Sometimes they come with angioedema, a deeper swelling under your skin. It most often shows up around your eyes and lips, but sometimes it can affect your hands, feet, or other body parts.

Causes include foods, pollen, animal dander, drugs, insect stings, cold, heat, light, or even emotional stress. Often, you don’t know what caused them. If you have hives or angioedema that won’t go away, tell your doctor.

Eczema, also known as atopic dermatitis, is a skin condition sometimes triggered by allergies. Avoiding your allergy triggers can help you avoid flare-ups of eczema.

Respiratory Allergies

As many as 50 million Americans have hay fever. Your doctor may call it allergic rhinitis. It can cause inflammation in your sinuses, or sinusitis. Typical symptoms include itchy eyes, nose, roof of mouth, or throat, along with nasal congestion, coughing, and sneezing. If you (or members of your family) have other allergic conditions like eczema or asthma, you’re more likely to get hay fever.


Hay isn’t the only plant to blame. Ragweed, grasses, and other plants can also be triggers when their pollen takes a ride on the wind. Molds, dust, and animal dander (dead skin scales and saliva) are also on the list.

Spores in the air are to blame for mold allergies. Outdoor molds thrive in warm seasons or climates, while indoor molds grow year round in damp locations like basements and bathrooms.

Dust causes allergies because it’s home to pollen, mold spores, and tiny dust mites. It may also contain irritating fibers from fabrics, upholstery, and carpets.

Asthma is a lung disease causes your air passages to become inflamed and narrow. If it isn’t controlled, it can occasionally be fatal. There are many causes, including viruses, stuff in the air around you, and allergies to pollen, mold spores, animal dander, and dust mites.

Food Allergies

Food allergies are less common in adults than in infants and young children. It can be hard to figure out if a food is really to blame, because reactions may be delayed or could result from food additives or your eating habits.


Most food allergies in children are caused by:

  • Cow's milk
  • Egg whites
  • Peanuts
  • Tree nuts
  • Wheat
  • Soybeans

Other foods that often cause allergies include:

  • Berries
  • Shellfish
  • Corn
  • Beans
  • Yellow food dye No. 5
  • Gum arabic (an additive in processed foods)

Classic food allergy symptoms include stomach cramps, diarrhea, and nausea. In more severe cases, you might vomit, notice that your face and tongue are swollen, have chest congestion, or feel dizzy, sweaty, or faint.

Drug Allergies

The antibiotic penicillin is the most common cause of drug allergies. Other antibiotics, especially sulfa drugs, can also cause problems. Sulfa is also found in medications like the arthritis drug celecoxib (Celebrex). Nearly 1 million Americans (and a small number of people with asthma) have reactions to aspirin, but most are not true allergies. They’re sensitivities or intolerances.

Insect Sting Allergies

If you have a food allergy, you might be more likely to react to an insect sting. Venoms from bees, wasps, hornets, yellow jackets, and fire ants are common allergens. Some people have severe reactions to stings and can go into anaphylaxis.

What Triggers Anaphylactic Shock?

Any allergen can trigger it, but the most common are insect stings, certain foods (like shellfish and nuts), and some drugs. It begins within minutes after exposure and can get bad fast. An injection of epinephrine can slow your symptoms. Because anaphylaxis can stop your breathing or your heart, you may need CPR.

If you have severe allergies, you should keep two auto-injectors of epinephrine with you at all times. If you feel any sign of anaphylaxis, don’t wait to use your epinephrine auto-injector, even if you’re not sure the symptoms are allergy related. It won’t hurt you to take the shot just to be safe.

After you take the shot, call 911 immediately.

WebMD Medical Reference Reviewed by Nayana Ambardekar, MD on April 28, 2019



Asthma and Allergy Foundation of America: "Allergy Facts and Figures."

American Academy of Allergy, Asthma and Immunology: "Allergies."

The Food Allergy and Anaphylaxis Network: "About Food Allergy." 

MedlinePlus Medical Encyclopedia: "Allergy."

American College of Allergy, Asthma and Immunology: “Allergy Facts.”

National Heart, Lung and Blood Institute: “What Is Asthma?”

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