Type IV Hypersensitivity

Definition

Type IV hypersensitivity, also known as delayed-type hypersensitivity, is an immune response mediated by T cells that occurs 24 to 72 hours after exposure to an antigen. This form of hypersensitivity does not involve antibodies but rather relies on the activation of CD4+ T helper cells and CD8+ cytotoxic T cells that recognize specific antigens, leading to inflammation and tissue damage.

5 Must Know Facts For Your Next Test

1. Type IV hypersensitivity is primarily mediated by T lymphocytes rather than antibodies, distinguishing it from other types of hypersensitivity reactions.
2. Common examples include reactions to poison ivy, nickel allergies, and tuberculin skin tests, where the skin reacts to previously encountered antigens.
3. The process involves antigen recognition by T cells, leading to their activation and the release of cytokines that recruit other immune cells to the site of exposure.
4. This type of hypersensitivity can cause chronic inflammation and tissue damage if the immune response is prolonged or excessive.
5. Certain autoimmune diseases, such as type 1 diabetes and multiple sclerosis, can involve mechanisms related to type IV hypersensitivity.

Review Questions

• What are the key immune cells involved in type IV hypersensitivity, and how do they contribute to the response?
  • Type IV hypersensitivity involves primarily CD4+ T helper cells and CD8+ cytotoxic T cells. When these T cells encounter a specific antigen presented by antigen-presenting cells (APCs), they become activated. Activated CD4+ T cells release cytokines that help recruit and activate additional immune cells, while CD8+ T cells can directly kill infected or altered cells. This cell-mediated response leads to inflammation and can cause significant tissue damage over time.
• Compare and contrast type IV hypersensitivity with other types of hypersensitivity reactions in terms of mechanism and clinical manifestations.
  • Unlike type I, II, and III hypersensitivities that involve antibodies (IgE or IgG), type IV hypersensitivity is characterized by a T cell-mediated response without antibody involvement. While type I reactions are immediate and involve anaphylaxis or allergic responses, type IV reactions are delayed, manifesting as conditions like contact dermatitis or graft-versus-host disease. The inflammation in type IV hypersensitivity typically peaks 48-72 hours post-exposure due to the activation of memory T cells responding to previously encountered antigens.
• Evaluate the implications of type IV hypersensitivity in autoimmune diseases and potential therapeutic approaches.
  • Type IV hypersensitivity plays a significant role in the pathogenesis of several autoimmune diseases such as rheumatoid arthritis and multiple sclerosis. In these conditions, inappropriate activation of T cells against self-antigens leads to chronic inflammation and tissue damage. Understanding these mechanisms opens up potential therapeutic avenues, such as the use of immunomodulators or targeted biologics that inhibit specific components of T cell activation or cytokine signaling pathways. This approach could help manage symptoms and reduce the progression of autoimmune diseases related to type IV hypersensitivity.

Related terms

T Cells: A type of white blood cell that plays a central role in cell-mediated immunity, including helper T cells (CD4+) and cytotoxic T cells (CD8+).

Antigen-Presenting Cells (APCs):

Cells such as dendritic cells, macrophages, and B cells that process and present antigens to T cells, crucial for initiating immune responses.

Contact Dermatitis: An inflammatory skin condition resulting from exposure to allergens or irritants, often associated with type IV hypersensitivity reactions.