Immune responses protect the body from damage by fighting off invasive substances and infections. Sometimes, however, the immune system produces undesirable responses by identifying harmless substances as harmful. This is called a hypersensitivity response.

An antigen or allergen can refer to a toxic or foreign substance that causes an immune reaction. After detecting an antigen and perceiving it as a potential threat, the immune system mounts an immune response to dispose of it.

The body can produce different types of hypersensitivity reactions, depending on the antigen a person has exposure to and how the body responds to it.

This article discusses the different types of hypersensitivity reactions.

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A hypersensitivity reaction is an extreme or unnecessary immune response that the body has to an antigen.

There are four different types of hypersensitivity reactions. Some evidence suggests a potential fifth type, but this may actually be a subset of type 2 hypersensitivity reactions.

Each type of hypersensitivity reaction is an extreme immune response to an antigen. Each type of reaction differs based on the type of antigen the body identifies, what type of immune response the body generates, and how quickly the body produces the response.

Some people may refer to hypersensitivity reactions as allergies, as these are a form of hypersensitivity. Although people use these terms interchangeably, an allergic reaction typically refers to the signs and symptoms a person may experience, while a hypersensitivity reaction describes the immunological process that occurs in the body.

Type 1 hypersensitivity causes an immediate response and occurs after a person has exposure to an antigen. With this type of reaction, the body responds to an antigen by producing a specific type of antibody called IgE.

There are different components that can trigger type 1 hypersensitivity responses, including antigens that come from:

  • food products, such as nuts, shellfish, and soy
  • animal sources, such as cats, rats, or bee stings
  • environmental sources, such as mold, latex, and dust
  • allergic conditions, such as allergic rhinitis, allergic asthma, and conjunctivitis

There are two stages to type 1 hypersensitivity: the sensitization stage and the effect stage.

During the sensitization stage, the person encounters the antigen but does not experience any symptoms. During the effect stage, the person has exposure to the antigen again. As the body now recognizes the antigen, it is able to produce a response that results in the symptoms that people typically experience with an allergic reaction.

Some physical symptoms of type 1 hypersensitivity can include:

Responses can also cause:

The first step a doctor may take to diagnose type 1 hypersensitivity is assessing the person’s history, including taking information on signs and symptoms and reviewing their medical records. After this, they will conduct a physical examination in addition to blood and allergy tests to help identify which antigen caused the reaction.

There are different treatments for type 1 hypersensitivity, depending on the cause of the reaction and how the body responds. Some people may require emergency medical treatment with an immediate effect, whereas people with mild symptoms may require other medications. Also, people should try to avoid the allergen in the future.

Some treatment options may include:

Similar to type 1, type 2 hypersensitivity reactions also involve antibodies. In fact, type 2 and type 3 hypersensitivity both result from the same class of antibody, called IgG. The difference between them lies in the form of antigens that generate a response. Additionally, type 2 can also involve IgM antibodies.

Type 2 hypersensitivity causes cytotoxic reactions, meaning that healthy cells die as they respond to the antigens. This can cause long-term damage to cells and tissues, resulting in conditions such as:

Common causes of type 2 hypersensitivity reactions include drugs such as:

There are different subsets of type 2 hypersensitivity, depending on the trigger and the response. Methods of diagnosis vary according to these subsets, as a doctor must be cautious to avoid provoking further damage. Diagnosis may involve direct immunofluorescence to help identify causative antibodies.

Treatment for type 2 hypersensitivity typically involves immunosuppressants to prevent the action of unusual antibodies. Treatment options may include:

  • systemic glucocorticoids
  • cyclophosphamide and cyclosporin agents
  • intravenous immunoglobulin infusions
  • plasmapheresis

Type 5 hypersensitivity reaction

Type 5 hypersensitivity reactions are responses that occur when an antibody targets receptors present on a cell, which a hormone would normally activate.

Although more research is necessary, some experts suggest that this reaction may actually be a subset of type 2 rather than a type on its own.

In type 3 hypersensitivity, antigens and antibodies form complexes in the skin, blood vessels, joints, and kidney tissues. These complexes cause a series of reactions that lead to tissue damage.

Causes of a type 3 hypersensitivity reaction can include:

Type 3 hypersensitivity reactions can lead to:

When diagnosing type 3 hypersensitivity, a doctor may look at a person’s clinical history, perform a physical exam, and conduct various assessments, including tests of blood and urine samples, biopsies, and imaging scans.

There are many treatment options available, depending on the severity and presentation of the hypersensitivity response. Typically, treatment involves controlling the underlying condition. This may involve immunosuppression with systemic glucocorticoids and disease-modifying drugs.

Unlike the other types, type 4 hypersensitivity reactions are cell-mediated.

Instead of antibodies, white blood cells called T cells control type 4 hypersensitivity reactions. Experts can further subdivide these reactions into type 4a, type 4b, type 4c, and type 4d based on the type of T cell involved and the reaction it produces.

This type also differs from the other three in that it causes a delayed reaction.

The three subsets of type 4 hypersensitivity are as follows:

Some common causes of type 4 hypersensitivity reactions include exposure to poison ivy, certain metals, and drugs such as antibiotics or anticonvulsants.

Diagnosing contact dermatitis is often achievable using a skin biopsy and skin patch tests. A doctor is likely to use a chest X-ray when diagnosing tuberculin-type hypersensitivity. Granulomatous-type hypersensitivity is more challenging to diagnose, and a doctor may consider using any of the following to make an assessment:

  • X-ray
  • lymph node biopsy
  • enzyme analysis
  • salivary gland analysis

Treatment varies from case to case. With contact dermatitis, for example, a doctor may prescribe topical steroids. However, with tuberculin-type hypersensitivity, the doctor will use a normal procedure for tuberculosis. Common treatments for tuberculin-type hypersensitivity include:

  • rifampin
  • isoniazid
  • pyrazinamide
  • ethambutol

Treatment for granulomatous-type hypersensitivity can also include steroid therapy, but treatments vary depending on the condition that comes from the reaction. For example, a doctor may prescribe methotrexate as steroid treatment if a person presents with pulmonary sarcoidosis.

Although hypersensitivity reactions are allergic reactions, some people may experience a non-immune anaphylactic reaction to certain drugs or foods. Some people may call this a nonallergic drug hypersensitivity reaction or refer to it as a pseudoallergic, idiosyncratic, or anaphylactoid reaction.

These reactions are often hard to distinguish from allergic reactions, but they noticeably cause symptoms directly, without the release of antibodies or T cells.

A hypersensitivity reaction occurs when the body produces an unnecessary and undesirable immune response following exposure to a certain antigen.

Many people refer to hypersensitivity as a form of allergy. Most research suggests that there are four types of hypersensitivity reactions, but some evidence suggests that there may also be a fifth type.

Different classes of antibodies moderate types 1, 2, and 3. Experts consider these types immediate hypersensitivity reactions because they occur within 24 hours of exposure to the antigen.

T cells moderate type 4 reactions, and specialists consider this type a delayed hypersensitivity reaction because it usually occurs 48–72 hours after exposure.

Less is known about type 5, but it may be similar to — or a subset of — type 2.

Diagnosis and treatment vary according to the type of hypersensitivity reaction a person experiences and the symptoms and conditions that result from the reaction. With each situation, it is important to identify and remove the cause of the hypersensitivity reaction to minimize further harm.