Type IV delayed hypersensitivity depends on the action of sensitized T cells. The reaction peaks 2 to 3 days after exposure to antigen. Examples are contact dermatitis, damage in a variety of infectious diseases, rejection of tissue grafts, and some autoimmune diseases.
■ Explain the events that occur in the skin during a positive delayed hypersensitivity skin test.
■ Describe a patch test for contact hypersensitivity.
■ In the tuberculin skin test, why would there be no reaction if there is no infection?
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