A type IV allergic reaction, also known as delayed hypersensitivity or cell-mediated hypersensitivity, is a specific immune response triggered by exposure to an allergen. Unlike immediate hypersensitivity reactions that occur within seconds or minutes, type IV reactions typically manifest after 24 to 48 hours following exposure to the allergen. These reactions are mediated by T-lymphocytes, a type of white blood cell involved in cell-mediated immunity.
During a type IV allergic reaction, allergen-specific T-cells recognize the allergenic substance and release various pro-inflammatory substances, such as cytokines, chemokines, and at times, toxic molecules. This immune response leads to an infiltration of immune cells, inflammation, and tissue damage in the affected area. Type IV reactions can occur on the skin or internally, depending on the route of allergen exposure.
Examples of type IV allergic reactions include allergic contact dermatitis (e.g., poison ivy rash) and certain drug allergies. Common symptoms associated with type IV reactions include localized redness, swelling, itching, and in some cases, the formation of blisters or pustules. Unlike type I allergic reactions, type IV reactions do not involve the production of antibodies or the release of histamine.
To diagnose a type IV allergic reaction, medical professionals typically perform a patch test, intradermal test, or a lymphocyte transformation test. Treatment typically involves avoiding the allergen, applying topical corticosteroids to affected skin, and using antihistamines to alleviate associated symptoms. In severe cases, oral corticosteroids or immunosuppressive medications may be prescribed.
