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.

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.

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 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.

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.

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.

Diagnosis involves the steps that your healthcare professional takes to find out whether you have an allergy. Your healthcare professional will likely:

• Ask detailed questions about your symptoms.
• Do a physical exam.
• Have you keep a detailed diary of symptoms and possible triggers.

If you have a food allergy, your healthcare professional will likely:

• Ask you to keep a detailed diary of the foods you eat.
• Ask if you've stopped eating the suspected food during the allergy exam.

Your healthcare professional also may recommend one or both of the following tests. Be aware that these allergy tests can give results that might not be accurate.

• Skin test. A healthcare professional uses a tiny needle to prick or scratch your skin and expose you to small amounts of the proteins found in your potential allergens. If you're allergic, you'll likely develop a raised bump called a hive at the test area on your skin. If the test result suggests you don't have an allergy but your healthcare professional still thinks you might, you may need an exam called an intradermal skin test. Your healthcare professional injects a small amount of allergen into the outer layer of the skin.
• Blood test. This test measures the amount of antibodies that the blood makes to destroy allergens. These antibodies are called immunoglobulin E (IgE) antibodies. The test is known as specific IgE (sIgE) blood testing. It's also commonly called radioallergosorbent test (RAST) or ImmunoCAP testing. Your blood sample is sent to a lab to find out if you're sensitive to suspected allergens.

If your healthcare professional thinks that something other than an allergy is the cause of your symptoms, other tests might be needed to help find the cause.

Allergy treatments include:

• Staying away from allergy triggers, also known as avoidance. Your healthcare professional helps you take steps to spot and stay away from your allergy triggers. Often, this is the most important step in preventing allergic reactions and curbing symptoms.
• Medicines. Depending on your allergy, medicines can help turn down your immune system's reaction and ease symptoms. Your healthcare professional might suggest medicine in the form of pills or liquid, nasal sprays, or eye drops.

• Immunotherapy. This treatment can help treat severe allergies. It also can help for allergies that don't get better with other treatments. Immunotherapy involves getting a series of shots of purified allergen extracts. These extracts train the immune system not to overreact to your suspected allergen. Most often, the shots are given over a period of a few years.

  Another form of immunotherapy is a tablet that's placed under the tongue until it dissolves. This is known as sublingual immunotherapy. Sublingual medicines placed under the tongue are used to treat some pollen allergies.

• Emergency epinephrine. If you have a severe allergy, you might need to carry an emergency epinephrine shot at all times. An epinephrine shot (Auvi-Q, EpiPen, others) can ease symptoms of a severe allergic reaction until you get emergency treatment.

Some allergy symptoms get better with steps that you can take at home:

• Sinus congestion and hay fever symptoms. These often get better with saline nasal sprays. These sprays rinse out the sinuses with a salt and water solution. You can use a neti pot or a specially designed squeeze bottle to flush out thickened mucus and irritants from your nose. But be aware that using a neti pot or other device wrong can lead to infection.
• Household airborne allergy symptoms. Lessen your exposure to dust mites or pet dander by washing bedding and stuffed toys in hot water often. Also, keep the indoor humidity level low. And regularly using a vacuum with a fine filter such as a high-efficiency particulate air (HEPA) filter. Think about replace carpeting with hard flooring too.
• Mold allergy symptoms. Reduce moisture in damp areas, such as your bath and kitchen, by using ventilation fans and dehumidifiers. Fix leaks inside and outside your home. You also can improve airflow by leaving doors between rooms open and moving furniture away from walls.

Research suggests that a type of alternative medicine called acupuncture may help ease some hay fever symptoms. With acupuncture, a practitioner places very thin needles through the skin at specific points on the body.

For symptoms that could be caused by an allergy, see your main healthcare professional. You might be referred to a doctor called an allergist who treats allergies.

What you can do

Ask if you should stop taking allergy medicines before your appointment, and for how long. For example, antihistamines can affect the results of an allergy skin test.

Make a list of:

• Your symptoms, including any that don't seem related to allergies. Note when the symptoms began.
• Your family's history of allergies and asthma, including specific types of allergies, if you know them.
• All medicines, vitamins and other supplements you take, including doses.
• Questions to ask your healthcare professional.

Some basic questions to ask your healthcare professional include:

• What is the most likely cause of my symptoms?
• Are there other possible causes?
• Will I need allergy tests?
• Should I see an allergy specialist?
• What treatment do you recommend?
• I have these other health conditions. How can I best manage them together?
• What emergency symptoms should my friends and family be aware of?

Feel free to ask other questions.

What to expect from your doctor

Your healthcare professional is likely to ask you questions such as:

• Have you recently had a cold or other respiratory infection?
• Are your symptoms worse at certain times of the day?
• Does anything seem to improve or worsen your symptoms?
• Are your symptoms worse in certain areas of your home or at work?
• Do you have pets, and do they go into bedrooms?
• Is there dampness or water damage in your home or workplace?
• Do you smoke, or are you exposed to secondhand smoke or other pollutants?
• What treatments have you tried so far? Have they helped?