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allergic reactions

Alternative Names

acquired sensitivity reaction, induced sensitivity reaction, acquired hypersensitivity, hypersensitivity reaction


An allergic reaction is a sensitivity to a specific substance a person is exposed to.

What is going on in the body?

Allergic reactions are fairly common. Most reactions happen soon after contact with an allergen. An allergen is a trigger that causes the reaction after touching a certain part of the body. For example, the skin may be directly exposed, the lungs may be exposed from inhaling the allergen, the blood may be exposed from an injection, or the blood or gut may be exposed from swallowing an allergen. Usually these reactions are mild and can be treated at home with simple methods. Some people have a sudden, life-threatening allergic reaction that occurs within minutes, which is called anaphylaxis. Anaphylaxis can progress rapidly and result in shock and even death if medical help is not obtained quickly.

Common triggers include:
  • some foods such as peanuts or foods with sulphites, such as beer, wine, processed foods, and seafood
  • medications
  • pollens or plants
  • mould
  • pets with feathers or fur
  • metals
  • synthetic materials
  • tiny organisms such as bacteria
  • insect stings, such as bees and wasps
Usually, the first exposure produces only a very mild reaction or no reaction at all. For some people, repeated exposures may lead to more serious reactions. Even a small amount of a trigger can lead to a serious reaction in some people.

Allergic reactions can affect small areas or the entire body. Most reactions occur within seconds or minutes of exposure. Some reactions can occur days or weeks from the time of exposure.

What are the signs and symptoms of the condition?

Mild allergic reactions may cause:
  • redness of the skin or a rash
  • hives, or raised swellings on the skin that itch
  • itching
  • worsening of asthma or an asthma flare-up, which causes trouble breathing
  • swelling of the tongue, eyelids, or face
  • fever
  • joint or muscle pain
  • coughing, sneezing, and nasal congestion
Severe reactions may cause severe forms of the above changes as well as:
  • dizziness or light-headedness
  • difficulty swallowing
  • chest discomfort
  • abdominal pain or cramping
  • unconsciousness

What are the causes and risks of the condition?

There are few things that do not cause an allergic reaction in some people. Most people never have an allergic reaction. Those with a family history of allergies are more likely to develop them. People with asthma, hay fever, or a skin condition known as eczema are more likely to develop this condition.

What can be done to prevent the condition?

The first reaction cannot generally be avoided. Avoidance of the trigger is the first step in preventing a second attack. This can be difficult in some cases, especially with common substances. Breastfeeding instead of bottle-feeding is thought to reduce the chance of a child having certain food allergies.

How is the condition diagnosed?

The history of the person having a reaction shortly after exposure to a trigger is needed. In some cases, special skin or other testing may be needed to determine the trigger. In skin testing, small amounts of the suspected substance can be injected under the skin. If a person is allergic to the substance, a skin reaction usually occurs.

What are the long-term effects of the condition?

There are no long-term effects if the trigger is avoided. Many allergies are mild and pose little threat to the affected person. Some allergies are life threatening and may cause death.

What are the risks to others?

Allergic reactions are not contagious and pose no risk to others.

What are the treatments for the condition?

First aid for the treatment of mild to moderate reactions includes:

  • Reassure the person to calm them down. Anxiety can increase the severity of the reaction.
  • The trigger should be identified if possible and removed. If it is an insect stinger, for example, this can often be removed. The stinger should not be squeezed in this setting. This causes the release of more bee venom, which is probably the trigger.
  • If the person develops a rash, calamine lotion and cool compresses or ice can be applied. The person should be watched closely to see if he or she gets worse.
  • Medical help should be sought. For a mild reaction, over-the-counter antihistamines, such as diphenhydramine or chlorpheniramine, may be advised. Prescription medications may be needed in some cases.
A severe allergic reaction is treated as above with a few extra measures.

  • The person's airway, breathing and pulse should be checked. If necessary, cardiopulmonary resuscitation (CPR) should be started.
  • Many people who have severe allergic reactions carry medication, such as adrenaline, in case exposure to the trigger occurs. The person may need help injecting the medication. Oral medication is not advised if a person is having trouble breathing.
  • The person should be laid down. Their feet should be raised above the level of their head. However, if the person has or may have a head, neck, back or leg injury, the person should not be moved.
  • The person should be covered with a blanket or coat for warmth.

What are the side effects of the treatments?

All medications have possible side effects. For example, antihistamines may make someone sleepy or have a dry mouth. Specific side effects depend on the drugs used.

What happens after treatment for the condition?

Anyone with a known serious allergic reaction should wear a medical tag at all times. This tag identifies the allergy. The thing a person is allergic to should be avoided. Friends and relatives should be made aware of the allergy in case of an exposure or emergency. Those with a drug allergy should tell their doctor.

How is the condition monitored?

The affected person should avoid the substance they are allergic to. People with severe allergies may be given a drug for injection in case of an exposure.

Author: James Broomfield, MD
Reviewer: eknowhow Medical Review Panel
Editor: Dr John Hearne
Last Updated: 25/04/2005
Potential conflict of interest information for reviewers available on request  

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