What is a defining characteristic of type IV hypersensitivity?

Type IV hypersensitivity, also known as delayed-type hypersensitivity, is characterized by a response that takes time to develop, typically several hours to days after exposure to the antigen. Unlike other hypersensitivity reactions, such as types I, II, and III, which involve antibodies and occur rapidly, type IV is primarily mediated by T cells, particularly CD4+ T helper 1 (Th1) cells and CD8+ cytotoxic T cells.

This delayed response is essential for the body's ability to react to intracellular pathogens, as well as for the rejection of transplanted tissues and the allergic response to certain chemicals and drugs. The activation of T cells leads to the recruitment of other immune cells, such as macrophages, which further contribute to inflammation and tissue damage. This time-dependent mechanism distinguishes type IV hypersensitivity from immediate hypersensitivity reactions that occur within minutes.

Understanding this characteristic is crucial for diagnosing and managing conditions that involve type IV hypersensitivity, such as contact dermatitis, graft-versus-host disease, and certain autoimmune disorders.