Type IV Hypersensitivity

Definition

Type IV hypersensitivity, also known as delayed-type hypersensitivity, is a cell-mediated immune response that occurs when T lymphocytes react to an antigen, leading to inflammation and tissue damage. This reaction typically takes 24 to 72 hours to develop after exposure to the antigen, distinguishing it from other hypersensitivity types that involve antibody-mediated responses. It plays a significant role in autoimmune disorders and transplant rejections, demonstrating how the immune system can sometimes turn against the body or its own tissues.

5 Must Know Facts For Your Next Test

1. Type IV hypersensitivity is primarily mediated by T cells, particularly CD4+ helper T cells and CD8+ cytotoxic T cells, rather than antibodies.
2. Common examples of Type IV hypersensitivity reactions include contact dermatitis (e.g., poison ivy), tuberculin skin tests (e.g., Mantoux test), and graft-versus-host disease.
3. The delayed nature of this hypersensitivity means that symptoms may not appear until days after exposure to the allergen, making it less immediate than other hypersensitivity types.
4. Chronic Type IV hypersensitivity can lead to tissue damage and conditions such as rheumatoid arthritis or sarcoidosis, where ongoing inflammation causes significant health issues.
5. The process involves antigen presentation by macrophages or dendritic cells, leading to T cell activation and subsequent release of cytokines that orchestrate the inflammatory response.

Review Questions

• How does Type IV hypersensitivity differ from other types of hypersensitivity reactions in terms of its immune mechanism?
  • Type IV hypersensitivity is unique because it is mediated by T lymphocytes rather than antibodies. While other types involve immediate allergic reactions driven by immunoglobulins, Type IV relies on the activation of T cells that respond to antigens presented by antigen-presenting cells. This leads to a delayed response characterized by inflammation and tissue damage, distinguishing it from the immediate reactions seen in Types I, II, and III.
• Discuss the role of cytokines in the pathophysiology of Type IV hypersensitivity reactions.
  • Cytokines play a critical role in Type IV hypersensitivity by facilitating communication between immune cells during the inflammatory response. After T lymphocytes recognize an antigen, they release various cytokines that promote further activation of other immune cells like macrophages. This recruitment leads to a sustained inflammatory response that contributes to tissue damage over time, highlighting the importance of cytokine signaling in both the development and persistence of these hypersensitivity reactions.
• Evaluate the implications of Type IV hypersensitivity in autoimmune disorders and transplant rejections.
  • Type IV hypersensitivity has significant implications for autoimmune disorders and transplant rejections due to its mechanism of self-reactivity. In autoimmune conditions like rheumatoid arthritis or multiple sclerosis, T cells mistakenly target self-antigens, leading to chronic inflammation and tissue damage. Similarly, during transplant rejections, the recipient's T cells recognize donor tissues as foreign, resulting in a vigorous immune attack. Understanding this hypersensitivity type is crucial for developing treatments that manage these conditions and improve transplant outcomes.

Related terms

T Lymphocytes: A type of white blood cell that is crucial for the immune response, particularly in recognizing and responding to specific antigens.

Cytokines: Signaling proteins released by immune cells that mediate and regulate immunity, inflammation, and hematopoiesis.

Granulomas: Small clusters of immune cells that form in response to chronic inflammation, often associated with Type IV hypersensitivity reactions.