Understanding Hypersensitivity Type 4

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Understanding Hypersensitivity type 4, also known as delayed-type hypersensitivity, is an allergic reaction that unfolds over a longer timeframe compared to other hypersensitivity reactions. Unlike the immediate response seen in type 1 allergies, type 4 reactions take hours or even days to develop. This T cells causes delayed response. And a specific type of immune cell, leading to inflammation and tissue damage.

Understanding Hypersensitivity Type 4

Causes of Hypersensitivity Type 4:

  • Contact Dermatitis: This common skin reaction arises from direct contact with allergens like poison ivy, nickel, or certain fragrances. The allergens trigger a T cell response, leading to redness, itching, and blistering at the contact site.
  • Allergic Patch Testing: This diagnostic test involves applying small amounts of potential allergens to the skin to identify triggers for allergies like contact dermatitis. A positive patch test results in a delayed-type hypersensitivity reaction at the test site.
  • Tuberculin Skin Test: This test uses a weakened form of the tuberculosis bacteria to assess past or present exposure to tuberculosis.
  • A positive test indicates a T cell response to the bacteria, suggesting past infection or vaccination.
  • Hypersensitivity Pneumonitis: This lung condition can develop from repeated inhalation of certain antigens, such as mold spores or bird feathers.
  • T cells orchestrate an inflammatory response in the lungs, leading to coughing, shortness of breath, and fatigue.

The T Cell Orchestration in Hypersensitivity Type 4:

Initial Sensitization: During the first encounter with an allergen, specialized antigen-presenting cells (APCs) take up the allergen and present fragments to T cells.

  • T Cell Activation: These T cells recognize the allergen fragments and become activated.
  • Memory T Cell Formation: Some activated T cells become memory T cells, primed to respond swiftly upon future encounters with the same allergen.
  • Delayed Response: Upon re-exposure to the allergen, memory T cells are rapidly activated and release inflammatory chemicals.
  • Inflammation and Tissue Damage: These chemicals attract other immune cells to the site, leading to inflammation and tissue damage.

Symptoms of Hypersensitivity Type 4:

Symptoms vary depending on the affected tissue and the specific allergen. Here are some common examples:

  • Contact Dermatitis: Redness, itching, blistering at the contact site.
  • Allergic Patch Test: Red, itchy, or bumpy patch at the test site.
  • Tuberculin Skin Test: Red, indurated (hardened) bump at the injection site.
  • Hypersensitivity Pneumonitis: Coughing, shortness of breath, fever, fatigue.

Diagnosis and Management of Hypersensitivity Type 4:

  • Skin Patch Testing: This test helps identify allergens responsible for contact dermatitis.
  • Tuberculin Skin Test: This test aids in diagnosing tuberculosis infection.
  • Imaging Tests (in some cases): X-rays or CT scans may be used to assess lung involvement in hypersensitivity pneumonitis.
  • Identifying and Avoiding Triggers: Avoiding contact with known allergens is crucial for managing type 4 hypersensitivity.
  • Corticosteroid Creams: Topical corticosteroids may help reduce inflammation and itching in contact dermatitis.
  • Oral Corticosteroids (in severe cases): For severe cases of contact dermatitis or hypersensitivity pneumonitis, oral corticosteroids may be prescribed to manage inflammation.


Understanding Hypersensitivity type 4 reactions may not be immediate, but they can still cause significant discomfort and inflammation. Understanding the causes, symptoms, and management strategies empowers individuals to work with healthcare professionals to identify triggers and control their condition.

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