Type IV hypersensitivity, also known as delayed hypersensitivity or cell-mediated hypersensitivity, refers to an immune response that develops within 24 to 72 hours after exposure to an allergen or antigen. It is considered the delayed type of hypersensitivity reaction, as it requires sensitized T cells to trigger the response.
In Type IV hypersensitivity reactions, the immune system identifies harmless substances as harmful and launches an inflammatory response. This reaction occurs when T lymphocytes, particularly T helper cells (Th1), recognize specific antigens presented by antigen-presenting cells (APCs). These APCs process and present antigens to the T cells, leading to the release of cytokines and activation of macrophages.
The activated macrophages secrete various inflammatory mediators, including pro-inflammatory cytokines, which recruit other immune cells such as neutrophils and monocytes to the site of inflammation. This influx of immune cells causes tissue damage and the characteristic delayed inflammatory response observed in Type IV hypersensitivity reactions.
Examples of Type IV hypersensitivity reactions include contact dermatitis from exposure to plants like poison ivy, latex allergies, and tuberculin reactions seen in individuals with a previous exposure to Mycobacterium tuberculosis. These reactions typically manifest as localized inflammation, redness, swelling, and sometimes the formation of blisters.
Type IV hypersensitivity reactions are distinct from other types of hypersensitivity reactions, such as immediate hypersensitivity (Type I), antibody-mediated hypersensitivity (Type II), and immune complex-mediated hypersensitivity (Type III), as they have a delayed onset and primarily involve the activation of T cells rather than antibodies.
