A life-threatening allergic reaction called anaphylaxis can cause shock, a sudden drop in blood pressure and trouble breathing. In people who have an allergy, anaphylaxis can happen minutes after exposure to a specific allergy-causing substance, called an allergen. Sometimes, there may be a delayed reaction, or anaphylaxis may happen without an obvious trigger.

Some common anaphylaxis triggers include:

• Medicines.
• Latex.
• Foods such as peanuts, tree nuts, fish and shellfish.
• Insect stings from bees, yellow jackets, wasps, hornets and fire ants.

If you're with someone having an allergic reaction with signs of anaphylaxis, call 911 or your local medical emergency number right away.

Don't wait to see whether symptoms get better. Seek emergency treatment right away. Severe untreated anaphylaxis can lead to death within half an hour.

Get emergency treatment even if symptoms start to improve. After anaphylaxis, it's possible for symptoms to start again. Being watched in a hospital for several hours most often is needed.

Symptoms of anaphylaxis include:

• Skin reactions, including hives, itching, and skin that becomes flushed or changes color.
• Swelling of the face, eyes, lips or throat.
• Narrowing of the airways, leading to wheezing and trouble breathing or swallowing.
• A weak and rapid pulse.
• Nausea, vomiting or diarrhea.
• Dizziness, fainting or unconsciousness.

After you call 911 or your local medical emergency number, do the following:

• Ask if the person is carrying an epinephrine autoinjector (EpiPen, Auvi-Q, others) to treat an allergic attack.
• If the person needs to use an autoinjector, ask whether you should help inject the medicine. This most often is done by pressing the autoinjector against the person's thigh.
• Have the person lie face up and be still.
• Loosen tight clothing and cover the person with a blanket.
• If there's vomiting or bleeding from the mouth, turn the person to the side to prevent choking.
• If there are no signs of breathing, coughing or movement, begin CPR. Keep doing about 100 chest presses every minute until paramedics arrive.

An antihistamine pill, such as diphenhydramine (Benadryl), isn't enough to treat anaphylaxis. These medicines can help relieve allergy symptoms, but they work too slowly in a severe reaction.

Don't give a person who has anaphylaxis anything to drink.

If you've had any kind of severe allergic reaction in the past, ask your healthcare professional if you should be prescribed an epinephrine autoinjector to carry with you.