Overview

Allergies happen when the immune system reacts to a foreign substance that gets inside the body. These substances are known as allergens. They include pollen, bee venom and pet dander. Allergies also can happen due to certain foods and medicines that don't cause reactions in most people.

The immune system makes protective proteins called antibodies that attack invaders such as germs. But with allergies, the immune system makes antibodies that mark a specific allergen as harmful, even though it isn't. Coming into contact with the allergen causes an immune system reaction that can inflame the skin, sinuses, airways or digestive system.

Allergic reactions vary from person to person. They can range from minor irritation to a life-threatening emergency called anaphylaxis. While most allergies can't be cured, treatments can help relieve your allergy symptoms.

Symptoms

Allergy symptoms depend on what allergen is involved. The symptoms can affect the airways, sinuses and nasal passages, skin, and digestive system. Allergic reactions can range from mild to severe. Sometimes, allergies can trigger a life-threatening reaction known as anaphylaxis.

Hay fever, also called allergic rhinitis, can cause:

  • Sneezing.
  • Itching of the skin, nose, eyes or roof of the mouth.
  • Runny, stuffy nose.
  • Tiredness, also called fatigue.
  • Watery, red or swollen eyes, also called allergic conjunctivitis.

A food allergy can cause:

  • Tingling in the mouth.
  • Swelling of the lips, tongue, face or throat.
  • Itchy welts called hives.
  • Stuffy nose, sneezing or teary eyes that itch.
  • Stomach cramps, vomiting or diarrhea.
  • Anaphylaxis.

An insect sting allergy can cause:

  • Pain and a large area of swelling called edema at the sting site.
  • Itching or hives all over the body.
  • Skin warmth and a change in skin color, also called flushing.
  • Cough, chest tightness, wheezing or shortness of breath.
  • Anaphylaxis.

A medicine allergy can cause:

  • Hives.
  • Itchy skin or rash.
  • Facial swelling.
  • Wheezing.
  • Shortness of breath.
  • Vomiting or diarrhea.
  • Feeling dizzy.
  • Anaphylaxis.

Atopic dermatitis, an allergic skin condition also called eczema, can cause skin to:

  • Itch.
  • Form red or brown patches that may be harder to see on darker skin tones.
  • Flake, peel or crack.

Anaphylaxis

Some types of allergies can trigger a severe reaction known as anaphylaxis. Certain foods, insect stings and medicines are among the allergens that can set off this life-threatening emergency. Anaphylaxis can cause you to go into shock. Other symptoms include:

  • Passing out.
  • A drop in blood pressure.
  • Severe shortness of breath and throat tightness.
  • Skin rash with hives or welts.
  • Dizziness.
  • A rapid, weak pulse.
  • Upset stomach, vomiting or diarrhea.
  • A sense of doom.

When to see a doctor

You might see a healthcare professional if you have symptoms that you think are caused by an allergy, and over-the-counter allergy medicines sold without a prescription didn't give you enough relief. If you have symptoms after starting a new medicine for a health condition, call the healthcare professional who prescribed it right away.

For a severe allergic reaction, also called anaphylaxis, call 911 or your local emergency number. Or get emergency medical help. A shot of prescription medicine called epinephrine is needed to treat anaphylaxis. If you carry an epinephrine auto-injector (Auvi-Q, EpiPen, others), give yourself a shot right away.

Even if your symptoms get better after an epinephrine shot, go to the emergency room. Healthcare professionals need to make sure your symptoms don't come back when the effects of the shot wear off.

If you've had a severe allergy attack or any symptoms of anaphylaxis in the past, make an appointment to see your healthcare professional. Allergy testing and the creation of a long-term treatment plan to manage anaphylaxis can be a challenge. So you'll likely need to see a doctor called an allergist who finds and treats allergies and other immune system conditions.

Causes

Various substances can cause allergies. An allergy starts when the immune system mistakes a substance that's usually harmless for a dangerous invader. The immune system then makes antibodies that stay on the alert for that specific allergen. When you're exposed to the allergen again, these antibodies can release some immune system chemicals that cause allergy symptoms.

Common allergy triggers include:

  • Airborne allergens, such as pollen, animal dander, dust mites and mold.
  • Certain foods, especially peanuts, tree nuts, wheat, soy, fish, shellfish, eggs and milk.
  • Insect stings, such as from a bee or wasp.
  • Medicines, especially penicillin or penicillin-based antibiotics.
  • Latex or other substances you touch, which can cause allergic skin reactions.

Risk factors

Risk factors for allergies include:

  • Having a family history of asthma or allergies, such as hay fever, hives or eczema.
  • Being a child.
  • Having asthma or another allergic condition.

Complications

Having an allergy raises the risk of certain other health conditions called complications, including:

  • Anaphylaxis. If you have severe allergies, that raises your risk of this serious allergic reaction. Foods, medicines, latex and insect stings are the most common triggers of anaphylaxis.
  • Asthma. If you have an allergy, you're more likely to have asthma. Asthma is an immune system reaction that affects the airways and breathing. Often, asthma is triggered by exposure to an allergen in the environment. This is known as allergy-induced asthma.
  • Infections of the sinuses, ears or lungs. Your risk of getting these conditions is higher if you have hay fever or asthma.

Prevention

Preventing allergic reactions depends on the type of allergy you have. General measures include the following:

  • Stay away from known triggers. Do this even if you get treatment for your allergy symptoms. If, for instance, you're allergic to pollen, stay inside with windows and doors closed when pollen is high. If you're allergic to dust mites, dust and vacuum and wash bedding often. You also can use "mite-proof" covers for items such as pillows, comforters, mattresses and box springs.
  • Keep a diary. When trying to figure out what causes your allergic symptoms or makes them worse, track your activities and what you eat. Also note when symptoms happen and what seems to help. This may help you and your healthcare professional find your triggers.
  • Wear a medical alert bracelet. If you've had a severe allergic reaction, wear a medical alert bracelet or necklace. It lets others know that you have a serious allergy in case you have a reaction and you're not able to speak.
By Mayo Clinic Staff