What Are Hypersensitivity Reactions?

Medically Reviewed By Marc Meth, MD, FACAAI, FAAAI
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Hypersensitivity reactions, or allergies, occur as a response to a foreign body. There are four different types of hypersensitivity reactions. A hypersensitivity reaction is commonly referred to as an “allergy.” A hypersensitivity reaction involves an abnormal response to a foreign body, or an antigen. The body can mount one of four different immune responses to this antigen. The antigen can be anything from a food to a medication.

There are four different types of hypersensitivity reactions. Types I–III are immediate hypersensitivity reactions because they occur within 24 hours. Type IV is a delayed hypersensitivity reaction that occurs between 48–72 hours after exposure to the antigen.

This article explains each type of hypersensitivity reaction, as well as their symptoms and treatment options.

Type I

Peanuts on a pink background
Yaroslav Danylchenko/Stocksy United

Type I hypersensitivity is the most well-known type of reaction. In extreme cases, it can lead to anaphylaxis. Anaphylaxis symptoms usually involve several parts of the body, including the skin, airways, and cardiovascular system. In this type of reaction, IgE antibodies bind to an antigen, which activates a chain of reactions that results in the release of multiple chemicals, including histamine.

This type of reaction includes allergic (atopic) disorders, which affect the nose, eyes, skin, and lungs.


Symptoms of type I hypersensitivity reactions include:

In the case of an anaphylactic reaction, the airway swells, and the person can experience shock, diarrhea, and organ dysfunction. This is a life threatening medical emergency, and you should call 911 if this occurs.


Examples of type I hypersensitivity reactions include:

  • food allergies
  • venom allergies, such as bee stings
  • drug allergies, such as penicillin
  • latex allergies
  • environmental allergies, such as:
    • pet dander
    • pollen
    • mold spores
    • dust mites


For mild allergy symptoms, you can use antihistamine medications — such as Claritin, Zyrtec, or Allegra — to treat symptoms such as sneezing and hives.

You can use nasal sprays to control hay fever.

Some people who are at risk of anaphylaxis carry a medication pen called an EpiPen. This pen works by injecting adrenaline into the thigh muscle to stop airway swelling and cardiovascular symptoms.

If you have environmental allergy symptoms, an allergy specialist can work with you on desensitization treatment. This involves gradually exposing you to the allergen with the aim of creating a desensitized immune response. This is commonly referred to as “immunotherapy” or “allergy shots.”


Doctors diagnose type I hypersensitivity reactions based on your history of signs and symptoms following exposure to the allergen.

The most common type of test used to detect type I hypersensitivity reactions is a skin pricking test. In this test, a healthcare professional introduces a small number of allergens directly under the skin. The skin’s reaction is compared with that of a control to see if a positive allergic reaction occurs.

Type I hypersensitivity reactions can also be detected with blood work. For example, your doctor might use an Immunocap test.

Learn more about allergies here.

Type II

Type II hypersensitivity reactions result in cell and tissue damage. These reactions are also known as “autoimmune reactions.”

This type of reaction is due to the abnormal binding of antibodies IgG and IgM to normal host targets. The IgG and IgM antibodies activate an immune response that causes inflammation and damages cells.


Signs and symptoms of type II hypersensitivity reactions include:


Examples of type II hypersensitivity reactions include:

  • rejection of an organ transplant
  • Hashimoto thyroiditis
  • Goodpasture syndrome
  • hemolytic disease of the newborn
  • myasthenia gravis
  • pemphigus
  • immune thrombocytopenia
  • autoimmune hemolytic anemia
  • autoimmune neutropenia


Immunosuppression is the main type of treatment for type II hypersensitivity reactions. For this, doctors often use systemic glucocorticoids to suppress the illness when it first appears or flares up.

Intravenous immunoglobulin infusions are sometimes used to deliver human-derived antibodies to target the abnormal antibodies and neutralize them.

Plasmapheresis, which is a procedure used to filter out a person’s antibodies and return clean blood to their body, is another possible treatment option for this type of hypersensitivity reaction.


Doctors diagnose type II hypersensitivity reactions by confirming the presence of IgG and IgM antibodies through a blood test.

Diagnosis is also based on your history and physical symptoms.

Learn more about autoimmune diseases here.

Type III

Type III hypersensitivity reactions involve IgG antibodies binding to foreign antigens in the blood. This antibody-antigen complex gets stuck in certain locations — such as the blood vessels, skin, kidneys, or joints — and causes local damage.


Signs and symptoms of type III hypersensitivity reactions include:

  • purpuras, which are small, non-blanching hemorrhages on the skin
  • rheumatoid nodules
  • a flushed rash on the face
  • kidney damage


Examples of type III hypersensitivity reactions include:


Treatment for type III hypersensitivity reactions involves immunosuppression, often using glucocorticoids. Disease-modifying antirheumatic drugs — such as methotrexate, cyclosporine, and cyclophosphamide — and biologic agents — such as TNF-blockers — are also used.


Diagnosing type III hypersensitivity reactions involves a medical history and physical exam based on your symptoms.

There is no single test to identify the cause of this type of hypersensitivity reaction.

Punch biopsies of vasculitis rashes can show whether or not there is local inflammation and tissue breakdown.

Type IV

Type IV hypersensitivity reactions do not involve antibodies. These are T-cell-mediated reactions that are activated by an antigen. Helper T cells recognize the antigen and send messengers that activate the immune system with killer T cells to destroy the target host.

This type of reaction occurs 48–72 hours after exposure to an antigen.


Symptoms of type IV hypersensitivity reactions include:

  • an itchy rash
  • flushed skin
  • fever
  • weakness
  • weight loss
  • skin vesicles with crust formation
  • a decline in lung function

In some cases, such as with severe drug reactions, there is severe tissue damage and fluid loss that can result in hypovolemic shock. There may also be severe skin blistering that looks like a third-degree burn. In other cases, there is multiorgan involvement — particularly with the heart, lungs, kidneys, and liver.

Learn when to contact a doctor about a rash here.


The most common example of type IV hypersensitivity reaction is allergic contact dermatitis. The reaction that some people get to the Mantoux test, which is used to check for active tuberculosis, is also a form of type IV hypersensitivity.

There are also delayed drug reactions, including:

  • Steven-Johnson syndrome (SJS)
  • drug hypersensitivity syndrome
  • erythema multiforme
  • lichenoid drug eruptions
  • morbilliform drug reactions


Avoiding the trigger allergen is the best form of treatment and prevention for type IV hypersensitivity reactions.

For contact dermatitis, applying topical steroid creams can help manage skin symptoms. In the case of more severe allergic reactions, your doctor might prescribe oral corticosteroids and taper the dosage over a number of weeks.

If SJS is present, acute care in a hospital with life saving treatment — such as fluid therapy and antibiotics — is warranted.


Doctors can diagnose these types of reactions using patch tests. With these tests, they apply a small number of potential allergens to the skin and leave them there for 2 days. If a patch of eczema forms at the site of exposure, the patch test is positive.

A detailed medical history and physical exam are also required for an accurate diagnosis.


Hypersensitivity reactions are allergic reactions that occur when the body mounts an immune response to a foreign body.

Types I–III occur within 24 hours of the exposure, while type IV is a delayed response, occurring 48–72 hours after the exposure.

Diagnosis of hypersensitivity reactions involves a detailed medical history, a physical exam, and, sometimes, skin testing.

Treatment involves the use of immune-suppressing medications and, in some cases, life saving measures.

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Medical Reviewer: Marc Meth, MD, FACAAI, FAAAI
Last Review Date: 2022 Mar 10
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