Allergic Reaction

Medically Reviewed By William C. Lloyd III, MD, FACS

What is an allergic reaction?

An allergic reaction is an inflammatory process that is triggered by a foreign substance known as an allergen. Allergens are usually proteins and individual susceptibility varies markedly from person to person. Common allergens include animal dander from cats or dogs, poison ivy (rhus), bee sting venom, airborne molecules such as ragweed pollen, and various types of drugs such as penicillin and sulfa antibiotics. Allergic reactions typically develop after repeated exposure to an allergen.

Symptoms of allergic reactions include skin lesions known as hives or urticaria, tongue or facial swelling, sneezing, itchy eyes, nausea, vomiting, and a variety of rashes. Allergic reactions may range from mild, self-limited sniffles to life-threatening conditions such as anaphylaxis and angioedema, in which the mouth and tongue may swell to a point that makes it difficult to breathe.

In most cases, physicians diagnose allergic reactions based on clinical signs and symptoms. This clinical diagnosis may be followed up with a series of blood tests or skin tests to establish the identity of the particular allergen.

Allergic reactions are treated with medications. Antihistamines may be used to treat mild allergic reactions. As allergic reactions become more severe, doctors typically prescribe steroids in addition to antihistamines. For life-threatening allergic reactions, injected epinephrine may be added. The definitive treatment for allergic reactions is to avoid the offending allergen entirely or to undergo desensitization therapy under the care of an allergist.

Seek immediate medical care (call 911) for serious symptoms of an allergic reaction, such as chest pain, chest tightness, difficulty breathing, nausea, rapid heartbeat, swelling in the throat or mouth, dizziness or unconsciousness, flushing with hives, or wheezing.

Seek prompt medical care if you have any but the mildest symptoms of an allergic reaction, or if you are being treated for an allergic reaction and you fail to improve or have concerns about your condition.

What are the symptoms of an allergic reaction?

Symptoms of an allergic reaction include runny nose, watery and itchy eyes, and itchy skin or hives. Symptoms of a more severe allergic reaction include swelling of the mouth or throat, difficulty breathing, tightness in the chest, dizziness or unconsciousness, nausea, and diarrhea.

Symptoms of a mild allergic reaction

You may experience allergy symptoms daily or just once in a while. At times any of these allergy symptoms can be severe:

Symptoms of a serious allergic reaction

You may experience symptoms of a serious allergic reaction even if your previous reactions have been mild. Symptoms of a severe allergic reaction include:

  • Difficulty breathing
  • Dizziness, lightheadedness or unconsciousness
  • Nausea, vomiting or diarrhea
  • Swelling in the mouth or throat
  • Tightness in the chest or chest pain
  • Wheezing

Serious symptoms that might indicate a life-threatening condition

In some cases, an allergic reaction can be life threatening. Seek immediate medical care (call 911) if you, or someone you are with, have any of these life-threatening symptoms:

  • Difficulty breathing
  • Exposure to an allergen known to have caused anaphylaxis in that person before
  • Swelling of the mouth, throat, or nose, especially if it limits breathing

What causes an allergic reaction?

Your body’s immune system identifies substances that are harmful, such as bacteria, and then seeks to destroy the harmful substance. Commonly, your immune system will interpret some normal and typically not harmful substances, like peanuts, as foreign and dangerous. In these cases, the immune system can be activated on exposure to that substance, and the immune system’s response can be so strong that it can cause life-threatening side effects, such as difficulty breathing.

What are the risk factors for an allergic reaction?

A number of factors increase the risk of developing an allergic reaction. Not all people with risk factors will get an allergic reaction. Risk factors for an allergic reaction include:

  • Family history of allergies
  • Family history of asthma
  • History of allergies
  • History of asthma

Reducing your risk of an allergic reaction

The best way to reduce your risk of an allergic reaction is to avoid the substance that triggers your allergy. If you do not know what causes your allergies, a visit to a health care provider can help identify the cause. You can also keep a journal of all the food you eat and look for patterns in which foods you eat and then develop allergic symptoms.

You may be able to lower your risk of an allergic reaction by:

  • Asking about the ingredients of foods at restaurants
  • Avoiding exposure to the allergen
  • Reading the ingredients on foods you buy

How is an allergic reaction treated?

There are a variety of treatments for allergic reactions, though none can cure the condition. Treatments include avoiding the triggers of an allergic reaction, drugs that reduce the severity of allergic symptoms, and drugs that change the way the immune system interprets certain triggers. Additionally, there are treatments, such as epinephrine, for severe allergic reactions.

Allergic reactions can range from mild to severe and can change in severity from one exposure to the next. Because allergic reactions can be so severe, it is important to decide on a treatment strategy with your health care provider and follow it to avoid complications.

Treatment of minor allergic reactions

Mild allergic reactions are very common and can often be treated with medications that relieve the associated symptoms including:

  • Allergen avoidance is the best treatment for allergies that can easily be avoided, such as shellfish or peanuts. Some environmental allergens, such as pollen, can be almost impossible to avoid.

  • Cromolyn sodium and other mast cell stabilizers can relieve inflammation in your lungs associated with allergies

  • Inhaled, nasal, oral, or topical corticosteroids (Pulmicort, Nasonex, prednisone, Alrex) can be used to decrease inflammation associated with allergies. Corticosteroids are most effective if used on a daily basis.

  • Leukotriene modifiers (Singulair) can help relieve nasal symptoms of allergies

  • Over-the-counter antihistamines (Benadryl) or prescription antihistamines (Clarinex) can counter some symptoms of allergies, including red eyes, itchiness, and runny nose

  • Over-the-counter decongestants (Sudafed) or prescription decongestants (Claritin-D) can relieve congestion associated with allergies

Treatment for moderate to severe allergic reactions

While some of the medications already listed can be used in cases of moderate and severe allergic reactions, other medications, ones that alter the immune system’s response, can be effective. Medications for moderate to severe allergic reactions include:

  • Epinephrine injection (EpiPen) can treat anaphylaxis, a life-threatening allergic reaction. This treatment is so effective that patients with known severe allergies often carry a mobile epinephrine injection kit with them at all times.A lower dose injector for children is now available (EpiPen Jr.)

  • Immunotherapy consists of injections of an allergen given in a controlled setting over a long period of time. It may help reduce the severity of subsequent allergic reactions. Immunotherapy is not used to reverse an acute severe allergic reaction.

What are the potential complications of an allergic reaction?

Complications of allergic reactions are usually limited when treated properly, as allergic reactions are often short-lived. However, some complications do exist.

Complications of untreated and severe allergic reactions can be serious, even life threatening in some cases. You can help minimize your risk of serious complications by following the treatment plan you and your health care professional design specifically for you. Complications of allergic reactions include:

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  1. Allergic reactions. Medline Plus, a service of the National Library of Medicine National Institutes of Health.
  2. Tips to remember: Allergic reactions. American Academy of Allergy, Asthma & Immunology.
Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2021 Jan 16
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