Type IV Hypersensitivity (Delayed or Cellular Hypersensitivity)

Type IV hypersensitivity, also known as delayed-type hypersensitivity, is a reaction of the immune system that typically takes hours to days to develop after exposure to a substance, rather than immediately. It involves T-cells, a type of white blood cell, that recognize and attack the foreign substance (like certain allergens or infections). Common examples include reactions to poison ivy or the skin test for tuberculosis. Unlike other types of allergic reactions, which involve antibodies, Type IV is driven by the body's immune cells, leading to inflammation and tissue damage if the exposure persists.