Which type of hypersensitivity reaction is known for its delayed response?

Type IV hypersensitivity reactions are characterized by their delayed response, typically occurring 24 to 72 hours after exposure to the antigen. This type of reaction is mediated by T cells (specifically, CD4+ Th1 cells and CD8+ cytotoxic T cells) and does not involve antibodies like the other hypersensitivity types.

In type IV reactions, the T cells recognize and respond to specific antigens, leading to recruitment of other immune cells and a localized inflammatory response. Common examples include contact dermatitis (such as reactions to poison ivy), tuberculin reactions (like the tuberculin skin test for tuberculosis), and some autoimmune conditions. The delay is due to the time required for T cell proliferation and the subsequent immune response to take effect, distinguishing type IV from the other hypersensitivity types, which are more immediate or involve antibodies.

This differentiation is crucial for understanding which types of immune responses are at play in various allergic and autoimmune diseases, and it highlights the importance of timing in diagnosing and treating such conditions effectively.