<n0:ComplexDefinition xmlns:n0="http://ncicb.nci.nih.gov/xml/owl/EVS/ComplexProperties.xsd#"><n0:def-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.</n0:def-definition><n0:def-source>NCI</n0:def-source></n0:ComplexDefinition>