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Symptoms and Signs of Allergic Reaction

Doctor's Notes on Allergic Reaction

An allergic reaction is an exaggerated response by the body's immune system to a substance in the environment. Allergies are caused by an inappropriate or misguided reaction to foreign substances. The substances that trigger allergic reactions are called allergens. There are many different kinds of allergens, including pollens, plants, eggs, nuts, foods, and animal dander.

Signs and symptoms of an allergic reaction include itching and watering of the eyes, nasal congestion, sneezing, runny nose, scratchy throat, and cough due to postnasal drip. Other associated symptoms can include fatigue, lethargy, hives, rash, and itching of the skin. Anaphylactic reactions are serious allergic reactions that can be life-threatening. In an anaphylactic reaction, swelling of the throat leads to difficulty breathing.

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Medically Reviewed on 4/8/2019

Allergic Reaction Symptoms

The look and feel of an allergic reaction depends on the body part involved and the severity of the reaction. Some reactions may be localized and limited, while others could involve multiple body systems. Reactions to the same allergen vary among individuals.

  • Anaphylaxis is the term for any combination of allergic symptoms that is rapid, or sudden, and potentially life-threatening. Call 9-1-1 or activate emergency medical services immediately for suspected anaphylaxis.
    • One sign of anaphylaxis is shock. Shock has a very specific meaning in medicine. Shock may lead rapidly to death. The organs of the body are not getting enough blood because of dangerously low blood pressure. The person in shock may be pale or red, sweaty or dry, confused, anxious, or unconscious.
    • Breathing may be difficult or noisy, or the person may be unable to breathe.
  • Shock is caused by sudden dilation of the blood vessels. This is brought on by the action of the mediators. If the drop in blood pressure is sudden and drastic, it can lead to unconsciousness, even cardiac arrest and death.
  • Symptoms and signs of an allergic reaction include any, some, or many of the following:
    • Skin: irritation, redness, itching, swelling, blistering, weeping, crusting, rash, eruptions, or hives (itchy bumps or welts)
    • Lungs: wheezing, tightness, cough, or shortness of breath
    • Head: swelling or bumps on the face and neck, eyelids, lips, tongue, or throat, hoarseness of voice, headaches
    • Nose: stuffy nose, runny nose (clear, thin discharge), sneezing, postnasal drip
    • Eyes: red (bloodshot), itchy, swollen, or watery or swelling of the area around the face and eyes
    • Stomach: pain, nausea, vomiting, diarrhea, or bloody diarrhea
    • Other: fatigue or feeling tired, sore throat, dizziness, or lightheadedness

Allergy shots: These are given to some people who have persistent and disruptive allergy symptoms.

  • The shots do not treat symptoms, but by altering the immune response, they prevent future reactions. This is referred to as immunotherapy.
  • Treatment involves a series of shots, each containing a slightly greater amount of the antigen(s) that cause the reaction.
  • The shots are administered every two to four weeks for two to five years.
  • Ideally, the person will become "desensitized" to the antigen(s) over time.
  • The effectiveness of shots varies between individuals.

Probiotics: There have been studies that have looked into the use of probiotics (live microorganisms thought to be beneficial to the body) to treat allergies, particularly atopic dermatitis in infants. Results of a recent meta-analysis were mixed, and their effectiveness has not yet been proven. More study is needed about the role of probiotics in the management of allergic reactions.

Acupuncture: Studies have shown mixed results in the effectiveness of acupuncture to treat allergies. It may be helpful for year-round types of allergies but not seasonal allergies such as hay fever.

Allergic Reaction Causes

Almost anything can trigger an allergic reaction.

  • The body's immune system involves the white blood cells, which produce antibodies.
    • When the body is exposed to an antigen (a foreign body such as pollen that can trigger an immune response), a complex set of reactions begins.
    • The white blood cells produce an antibody specific to that antigen. This is called "sensitization."
    • The job of the antibodies is to help white blood cells detect and destroy substances that cause disease and sickness. In allergic reactions, the antibody belongs to the class of immunoglobulins known as immunoglobulin E or IgE.
  • This antibody type promotes production and release of chemicals and hormones called "mediators."
    • Mediators have effects on local tissue and organs in addition to activating more white blood cell defenders. It is these effects that cause the symptoms of the reaction.
    • Histamine is one of the better-known allergy mediators produced by the body.
    • If the release of the mediators is sudden or extensive, the allergic reaction may also be sudden and severe, and anaphylaxis may occur.
  • Allergic reactions are unique for each person. Reaction time to allergens can vary widely. Some people will have an allergic reaction immediately; for others, it may take hours to days to develop.
  • Most people are aware of their particular allergy triggers and reactions.
    • There are more than 160 allergenic foods. Certain foods are common allergens, including peanuts, strawberries, shellfish, shrimp, dairy, and wheat.
    • Babies can also have food allergies. Common foods that can cause allergic reactions in babies include milk, eggs, nuts, and soy. People should talk to their child's pediatrician if they are concerned about food allergies in their baby.
    • Food intolerance is not the same as food allergies. Allergies are an immune system response, while food intolerance is a digestive system response in which a person is unable to properly digest or break down a particular food.
    • People can be allergic to wheat but not gluten. According to the American College of Allergy, Asthma & Immunology, there is no such thing as a gluten allergy; but one can have a sensitivity to this protein that results in gastrointestinal symptoms.
    • Certain fruits or vegetables may cause an itchy mouth or scratchy throat after eating in people who have oral allergy syndrome.
  • Seasonal allergic rhinitis (also called hay fever) is an allergy that occurs in the spring, summer, or early fall caused by allergies to pollens from trees, grasses or weeds, or to mold spores.
  • Vaccines and medications (antibiotics such as penicillin and amoxicillin, aspirin, ibuprofen, iodine), general anesthesia and local anesthetics, latex rubber (such as in gloves or condoms), dust, mold or other fungi, animal dander from pets and other animals, and poison ivy are well-known allergens. Other known allergens can include detergents, hair dyes, cosmetics, and the ink in tattoos.
  • Bee stings, fire ant stings, penicillin, and peanuts are known for causing dramatic reactions that can be serious and involve the whole body.
  • Minor injuries, hot or cold temperatures, exercise, stress, or emotions may trigger allergic reactions.
  • Sun exposure may cause allergic reactions in some people, often referred to as "sun poisoning."
  • Often, the specific allergen cannot be identified unless someone has had a similar reaction in the past.
  • Allergies, and the tendency to have allergic reactions, is hereditary -- that is, it runs in some families.
  • Many people who have one trigger tend to have other triggers, as well.
  • Risk factors for allergic reactions include certain medical conditions that can make a person more likely to have allergic reactions:

Allergy Life-Threatening Allergy Triggers Slideshow

Allergy Life-Threatening Allergy Triggers Slideshow

Peanuts are one of the most common cause of food-related allergy death. They can trigger anaphylaxis -- a reaction that may be fatal if not treated right away. Symptoms usually start within minutes of exposure. But they can also start within seconds or take hours to develop. Call 911 at the first sign of swelling, hives, trouble breathing, a rapid pulse, or dizziness.


Kasper, D.L., et al., eds. Harrison's Principles of Internal Medicine, 19th Ed. United States: McGraw-Hill Education, 2015.