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This type of hypersensitivity reaction is mediated by the action of lymphocytes rather than by antibody.
The process develops over more than 12 hours and may take 2-3 days to evolve.
The reaction involves sensitised T cells, which in response to antigen release lymphokines and interleukins and may also directly kill target cells. There is a mononuclear cell infiltration into the site of antigen deposition.
An example of a type IV hypersensitivity reaction is the response to tuberculin.