|Found this medical article:
"Delayed hypersensitivity does not start to be noticeable until several hours to a full day after exposure to the antigen. It may last for over a week. T lymphocytes recognize the foreign substances, usually after the antigen is eaten, degraded, and presented (in pieces) by so-called antigen-presenting cells such as Langerhans cells in the skin, or macrophages. Urushiol metabolites are presented by this and other mechanisms. The T lymphocytes pour out inflammatory signal substances called cytokines. These call in armies of white blood cells called monocytes, which become macrophages. The macrophages become activated by the cytokines and attack everything in the vicinity, and can cause severe tissue damage. "