a doctor testing for an allergic reaction

What is an Allergic Reaction?

An allergic reaction is the immune system’s unnecessary attempt to protect the body from substances it perceives to be harmful. Substances percieved as threatening are called allergens or antigens. To most people, allergens are harmless substances in the environment.

Ragweed, peanuts, nickel, and penicillin are just a few common allergens. Allergies are a type of genetic hypersensitivity disorder of the immune system, which causes the body to react to allergens as if they were harmful bacteria, viruses, or other pathogens. Initially, a mild response may occur, causing symptoms such as watery eyes, sneezing, or hives, but with subsequent exposures, allergic reactions can become progressively exaggerated and can even be life-threatening. There is no treatment for allergies except avoidance of triggers and management of symptoms.

The American Academy of Allergy Asthma and Immunology (AAAI) estimates that one in five Americans suffer from all types of allergies and that allergies have been on the rise since the 1980s. Allergic reactions are defensive—but unwarranted—responses that occur when allergic individuals are exposed to something their body has become sensitized to. They can affect a localized area of the body or be systemic, affecting the whole body. Allergic reactions can occur suddenly within a matter of seconds and last as long as several days.

Initial Exposure

When your body is first exposed to an allergen, your immune system produces certain proteins in the blood called antibodies or immunoglobulins (IgE). Antibodies correspond to each substance you are allergic to. They act like specialized forces that identify the enemy, and never forgetting, they circulate in the blood and searching for the intruder. The more frequent exposure is, the more antibodies may be produced. For example, if you are allergic to ragweed, peanuts, and penicillin, then you will have a number of antibodies that target ragweed, some that search for traces of peanut, and also some that are penicillin-specific. This first stage of allergic reaction development is called sensitization.

Subsequent Exposures

When your body is reacquainted with an allergen after being sensitized, it will have a two-fold response. In the early onset phase, your antibodies detect and bind the allergen’s molecules to blood cells called mast cells, which function like microscopic bombs that destroy harmful invaders. When the mast cells detonate, they release a chemical called histamine into the bloodstream. Histamine kickstarts the inflammatory process, which includes tissue and blood vessel swelling, redness, mucous production, and itching. These symptoms are commonly associated with allergic reactions. In the delayed response phase, symptoms may become more intense and long-lasting.

Severity of Allergic Reactions

Sometimes allergic reactions are classified by the type of trigger. Below are a few of the most common groups of allergens with examples:

  • indoor and outdoor allergens such as dust mites, pet dander, mold, pollen, or poison plants
  • foods such as milk, eggs, soy, peanuts, shellfish, tree nuts, fish, and wheat
  • drugs such as penicillin, sulfur-based drugs, and some vaccines
  • latex
  • venom from honey bee or yellow jacket stings and bites from fire ants

The National Institutes of Health (NIH) lists the following symptoms and signs of allergic reactions that are considered mild:

  • hives (especially over the neck and face)
  • itching
  • nasal congestion
  • rashes
  • watery, red eyes

The following symptoms and signs of allergic reactions are considered moderate to severe:

  • abdominal pain
  • wheezing
  • anxiety
  • chest tightness
  • cough
  • diarrhea
  • difficulty breathing
  • difficulty swallowing
  • dizziness
  • flushing, redness in the face
  • nausea or vomiting
  • palpitations
  • swelling of the face, eyes, or tongue
  • loss of consciousness

Systemic allergic reactions, called anaphylaxis or anaphylactic shock, will rapidly affect the whole body and can be life-threatening. The following serious symptoms and signs of allergic reactions are accompanied by a risk of fatality:

  • loss in blood pressure
  • loss of consciousness
  • difficulty breathing

Anaphylaxis commonly follows an insect sting or ingesting medications or foods you are allergic to. Anaphylaxis is an allergic emergency and requires immediate injections of a drug called epinephrine. Immediate medical care must be given, as anaphylaxis can be fatal.

Last Updated: October 18, 2016