Allergic reactions

Allergic reactions are sensitivities to substances called allergens that come into contact with the skin, nose, eyes, respiratory tract, and gastrointestinal tract. Allergens 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, 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 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.

Common allergens include:

• Animal dander
• Bee stings or stings from other insects such as fire ants
• Dust
• Foods, especially milk, eggs, peanuts, tree nuts, fish, and shellfish
• Medicines
• Mold
• Pollens

Common symptoms of a mild allergic reaction include:

• A few hives (especially over the neck and face)
• Itching
• Nasal congestion
• Rashes
• Watery, red eyes

Symptoms of a moderate or severe reaction include:

• Abdominal pain
• Abnormal (high-pitched) breathing sounds
• Anxiety
• Chest discomfort or tightness
• Cough
• Diarrhea
• Difficulty breathing (wheezing)
• Difficulty swallowing
• Dizziness or lightheadedness
• Flushing or redness of the face
• Hives all over
• Nausea or vomiting
• Palpitations
• Swelling of the face, eyes, or tongue
• Unconsciousness

Contact 911 or the local emergency number for medical assistance 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).

To prevent allergic reactions:

• Avoid triggers such as foods and medicines 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.
• People who have had serious allergic reactions should wear a medical ID tag and carry emergency medicines such as a two-pack of injectable or intranasal epinephrine, according to their provider's instructions.
• There is no contraindication to using epinephrine in a severe allergic reaction.

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.

Nadeau KC. Approach to the patient with allergic or immunologic disease. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 230.

Wang J, Lieberman JA, Wallace DV, Waserman S, Golden DBK. Anaphylaxis in practice: a guide to the 2023 practice parameter update. J Allergy Clin Immunol Pract. 2024;12(9):2325-2336. PMID: 38944199 pubmed.ncbi.nlm.nih.gov/38944199/.

Last reviewed on: 3/31/2024

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.