Allergic reactions are sensitivities to substances called allergens that come into contact with the skin, nose, eyes, respiratory tract, and gastrointestinal tract. They can be breathed into the lungs, swallowed, or injected.
Allergic reactions are common. The immune response that causes an allergic reaction is similar to the response that causes hay fever. Most reactions happen soon after contact with an allergen.
Many allergic reactions are mild, while others can be severe and life-threatening. They can be confined to a small area of the body, or they may affect the entire body. The most severe form is called anaphylaxis or anaphylactic shock. Allergic reactions occur more often in people who have a family history of allergies.
Substances that don't bother most people (such as venom from bee stings and certain foods, medications, and pollens) can trigger allergic reactions in certain people.
First-time exposure may produce only a mild reaction. Repeated exposures may lead to more serious reactions. Once a person has had an exposure or an allergic reaction (is sensitized), even a very limited exposure to a very small amount of allergen can trigger a severe reaction.
Most severe allergic reactions occur within seconds or minutes after exposure to the allergen. Some reactions can occur after several hours, particularly if the allergen causes a reaction after it has been eaten. In very rare cases, reactions develop after 24 hours.
Anaphylaxis is a sudden and severe allergic reaction that occurs within minutes of exposure. Immediate medical attention is needed for this condition. Without treatment, anaphylaxis can get worse very quickly and lead to death within 15 minutes.
Common allergens include:
Bee stings or stings from other insects
Foods, especially nuts, fish, and shellfish
Common symptoms of a mild allergic reaction include:
Calm and reassure the person having the reaction. Anxiety can make symptoms worse.
Try to identify the allergen and have the person avoid further contact with it. If the allergic reaction is from a bee sting, scrape the stinger off the skin with something firm, such as a fingernail or plastic credit card. Do not use tweezers. Squeezing the stinger will release more venom.
If the person develops an itchy rash, apply cool compresses and an over-the-counter hydrocortisone cream.
Watch the person for signs of increasing distress.
Get medical help. For a mild reaction, a health care provider may recommend over-the-counter medications, such as antihistamines.
For a severe allergic reaction (anaphylaxis), check the person's airway, breathing, and circulation (the ABC's of Basic Life Support). A warning sign of dangerous throat swelling is a very hoarse or whispered voice, or coarse sounds when the person is breathing in air. If necessary, begin rescue breathing and CPR.
Calm and reassure the person.
If the allergic reaction is from a bee sting, scrape the stinger off the skin with something firm (such as a fingernail or plastic credit card). Do not use tweezers -- squeezing the stinger will release more venom.
If the person has emergency allergy medication on hand, help the person take or inject the medication. Avoid oral medication if the person is having difficulty breathing.
Take steps to prevent shock. Have the person lie flat, raise the person's feet about 12 inches, and cover him or her with a coat or blanket. Do not place the person in this position if a head, neck, back, or leg injury is suspected or if it causes discomfort.
Do not assume that any allergy shots the person has already received will provide complete protection.
Do not place a pillow under the person's head if he or she is having trouble breathing. This can block the airways.
Do not give the person anything by mouth if the person is having trouble breathing.
When to Contact a Medical Professional
Call for medical assistance (911) right away if:
The person is having a severe allergic reaction. Do not wait to see if the reaction is getting worse.
The person has a history of severe allergic reactions (check for a medical ID tag).
Avoid triggers such as foods and medications that have caused an allergic reaction in the past. Ask detailed questions about ingredients when you are eating away from home. Carefully check ingredient labels.
If you have a child who is allergic to certain foods, introduce one new food at a time in small amounts so you can recognize an allergic reaction.
Persons who have had serious allergic reactions should wear a medical ID tag and carry emergency medications (such as a chewable form of Chlor-Trimeton and injectable epinephrine or a bee sting kit) according to your health care provider's instructions.
Do not use your injectable epinephrine on anyone else. They may have a condition (such as a heart problem) that could be made worse by this drug.
Brown SGA, Kemp SF, Lieberman P. Anaphylaxis. In: Adkinson NF Jr., Bochner BS, Burks AW, et al., eds. Middleton's Allergy Principles and Practice. 8th ed. Philadelphia, PA: Elsevier Mosby; 2013:chap 77.
Wasserman SI. Approach to the person with allergic or immunologic disease. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 257.
Stuart I. Henochowicz, MD, FACP, Associate Clinical Professor of Medicine, Division of Allergy, Immunology, and Rheumatology, Georgetown University Medical School, Washington, DC. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.