Allergic reactions are sensitivities to substances, called allergens, that are contacted through the skin, inhaled 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 occur more often in people with 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.

While first-time exposure may only produce a mild reaction, repeated exposures may lead to more serious reactions. Once a person has had an allergic reaction (is sensitized), even a very limited exposure to a very small amount of allergen can trigger a severe reaction.

Allergic reactions can be mild or serious. They can be confined to a small area of the body or may affect the entire body.

Most severe allergic reactions occur within seconds or minutes after exposure to the allergen. However, some reactions can occur after several hours, particularly if the allergen causes a reaction after it has been ingested. 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. It can get worse very fast and lead to death within 15 minutes if treatment is not received.

Common allergens include:

• Animal dander
• Bee stings or stings from other insects
• Foods, especially nuts, fish, and shellfish
• Insect bites
• Medications
• Plants
• Pollens

Common symptoms of a mild allergic reaction include:

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

Symptoms of a moderate or severe reaction include:

• Cramps or pain in the abdomen
• Chest discomfort or tightness
• Diarrhea
• Difficulty breathing
• Difficulty swallowing
• Dizziness or lightheadedness
• Fear or feeling of apprehension or anxiety
• Flushing or redness of the face
• Nausea and vomiting
• Swelling of the face, eyes, or tongue
• Weakness
• Wheezing
• Unconsciousness

For a mild to moderate reaction:

1. Calm and reassure the person having the reaction, as anxiety can worsen symptoms.
2. 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.
3. If the person develops an itchy rash, apply calamine lotion and cool compresses. You may also apply over-the-counter cortisone cream.
4. Watch the person for signs of increasing distress.
5. Get medical help. For a mild reaction, a physician may recommend over-the-counter medications (such as antihistamines).

For a severe allergic reaction (anaphylaxis):

1. 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 air in. If necessary, begin rescue breathing and CPR.
2. Call 911.
3. Calm and reassure the person.
4. 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.
5. 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.
6. 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.

Call for immediate medical emergency assistance if:

• The person is having a severe allergic reaction, always call 911. 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, even a mild one, in the past. This includes asking detailed questions about ingredients when you are eating away from home. Also carefully examine 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 know that they have had serious allergic reactions should wear a medical ID tag.
• If you have a history of serious allergic reactions, carry emergency medications (such as diphenhydramine 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 negatively affected by this drug.

Simons FE. Anaphylaxis. J Allergy Clin Immunol. 2008;121:S402-S407.