Final answer:
Type IV hypersensitivity reactions, also known as delayed-type hypersensitivities, are coordinated by TH1 cells, which secrete cytokines that activate macrophages and lead to tissue damage.
Step-by-step explanation:
Type IV Hypersensitivity Reactions and T cells
Type IV hypersensitivity reactions are coordinated by the cytokines produced by TH1 cells. These reactions, also known as delayed-type hypersensitivities (DTH), involve the sensitization of the immune system to an antigen presented by antigen presenting cells (APCs). Upon a subsequent encounter with the same antigen, the sensitized memory TH1 cells release cytokines that activate macrophages, leading to tissue damage. This mechanism underlies conditions such as contact dermatitis and the tuberculin skin test. To clarify, the T cells that secrete cytokines helpful for antibody responses are TH2 cells.
In contrast, TH17 cells are associated with defense against chronic mucocutaneous infections and are not typically involved in type IV hypersensitivity reactions. The specific subtype involved in a given immune response is dependent on the types of cytokines secreted, which varies with the nature of the pathogen.
Type IV hypersensitivity is unique among hypersensitivity reactions in that it is not mediated by antibodies but rather by T cell responses, including CD4 TH1 and CD8 cytotoxic T lymphocytes (CTLs). It's important to recognize that different T cell subtypes such as TH1, TH2, and TH17 have distinct roles within the immune system.