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Allergic reactions

Anaphylaxis; Anaphylaxis - first aid

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.

Images

Allergic reactions
Dermatographism - close-up
Dermatographism on the arm
Hives (urticaria) on the arm
Hives (urticaria) on the chest
Hives (urticaria) - close-up
Hives (urticaria) on the trunk
Dermatographism on the back
Dermatographism - arm
Allergic reactions

Considerations

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, medicines, 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 (and thus is sensitized to an allergen), 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. In very rare cases, reactions develop after 24 hours.

Anaphylaxis is a sudden and severe allergic reaction that occurs within minutes or hours 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.

Causes

Common allergens include:

Symptoms

Common symptoms of a mild allergic reaction include:

Symptoms of a moderate or severe reaction include:

First Aid

For a mild to moderate reaction:

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.

  1. If the person develops an itchy rash, apply cool compresses and an over-the-counter hydrocortisone cream.
  2. Watch the person for signs of increasing distress.
  3. Get medical help. For a mild reaction, a health care provider may recommend over-the-counter medicines, 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.

  1. Call 911 or the local emergency number.
  2. Calm and reassure the person.
  3. 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.
  4. If the person has injectable emergency allergy medicine (Epinephrine), administer it at the beginning of a reaction. Do not wait to see if the reaction gets worse. Avoid oral medicine if the person is having difficulty breathing.
  5. Take steps to prevent shock. Have the person lie flat, raise the person's feet about 12 inches (30 centimeters), and cover them 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

If a person is having an allergic reaction:

When to Contact a Medical Professional

Contact for medical assistance (911 or the local emergency number) right away if:

Prevention

To prevent allergic reactions:

Related Information

Allergen
Allergies
Insect bites and stings
Anaphylaxis

References

Barksdale AN, Ross W. Allergy, anaphylaxis, and angioedema. In: Walls RM, ed. Rosen's Emergency Medicine: Concepts and Clinical Practice. 10th ed. Philadelphia, PA: Elsevier; 2023:chap 106.

Custovic A, Tovey E. Allergen control for prevention and management of allergic diseases. In: Burks AW, Holgate ST, O'Hehir RE, et al, eds. Middleton's Allergy: Principles and Practice. 9th ed. Philadelphia, PA: Elsevier; 2020:chap 84.

Higgins T. Allergic reaction. In: Higgins T, ed. Medicine for the Outdoors. 7th ed. Philadelphia, PA: Elsevier; 2024:78-79.

Shaker MS, Wallace DV, Golden DBK, et al. Anaphylaxis-a 2020 practice parameter update, systematic review, and grading of recommendations, assessment, development and evaluation (GRADE) analysis. J Allergy Clin Immunol. 2020;145(4):1082-1123. PMID: 32001253 pubmed.ncbi.nlm.nih.gov/32001253/.

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Review Date: 3/31/2024  

Reviewed By: Deborah Pedersen, MD, MS, Allergy & Asthma Care, PC, Taunton, MA. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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