Type IV hypersensitivity

Type IV Hypersensitivity

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Type IV Hypersensitivity, also known as delayed-type hypersensitivity (DTH), is an immune response that involves the activation of T-cells and macrophages, leading to tissue damage. Unlike the immediate reactions seen in other types of hypersensitivity, Type IV reactions are characteristically delayed, occurring several hours to days after exposure to the antigen.

Overview

Type IV Hypersensitivity is mediated by T lymphocytes rather than by antibodies. It plays a crucial role in the immune system's defense against pathogens, but can also contribute to the pathology of certain autoimmune and infectious diseases, as well as allergic reactions to drugs, chemicals, and environmental agents.

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Illustration of a delayed-type hypersensitivity reaction. (Placeholder image)

Mechanisms

The mechanism of Type IV Hypersensitivity involves:

• Sensitization phase: Antigen-specific T-cells are activated by antigen-presenting cells (APCs).
• Effector phase: Upon re-exposure to the antigen, activated T-cells release cytokines that recruit and activate macrophages, leading to inflammation and tissue damage.

Subtypes

Type IV Hypersensitivity can be further divided into four subtypes, based on the mechanisms and effector cells involved:

• Type IVa: Th1 cells activate macrophages.
• Type IVb: Th2 cells activate eosinophils.
• Type IVc: Cytotoxic T lymphocytes (CTLs) directly damage target cells.
• Type IVd: T cells recruit neutrophils.

Examples of Type IV Hypersensitivity Reactions

• Contact dermatitis, such as poison ivy rash.
• Tuberculin skin test reaction.
• Celiac disease, where gluten induces T-cell mediated injury to the intestinal lining.
• Rejection of transplanted organs.
• Certain drug hypersensitivities.

Diagnosis

Diagnosis of Type IV Hypersensitivity involves clinical evaluation and may include skin testing, such as the patch test for contact dermatitis or the tuberculin skin test for tuberculosis exposure. In some cases, biopsy of affected tissues may be necessary to identify the presence of T-cells and macrophages.

Treatment and Management

Treatment focuses on reducing inflammation and managing symptoms. This may include:

• Avoidance of known allergens or triggers.
• Topical or systemic corticosteroids to reduce inflammation.
• Immunosuppressive drugs in severe cases, such as organ transplant rejection.

External Links

• British Society for Immunology
• American Academy of Allergy, Asthma & Immunology

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