Contact Hypersensitivity

**Semantic type:** Disease or Syndrome

**Definition:** Contact hypersensitivity is an inflammatory response induced by antigen-reactive T cells that are exposed to antigen on the skin surface (epidermis). Poison ivy is a frequent cause of such reactivity, due to the urushiol from the plant, as are a variety of antigens derived from industrial exposure (e.g. rubber, chromate, nickel). These antigens induce the appearance of Th1-like T cells, which presumably produce IFN to activate macrophages.