A delayed hypersensitivity test is an immune function test measuring the presence of activated T cells that recognize a certain substance.

The immune system protects against infection by viruses, bacteria, fungi, and parasites. After initial exposure to a foreign substance, or antigen, the immune system creates both antibodies and sensitized T cells. Both these immune agents respond when the body is reexposed to the antigen. Antibodies, which are circulating proteins, respond within minutes, to give what is termed an immediate hypersensitivity reaction. T cells responses occur over several days, and are thus called delayed hypersensitivity reactions. The cascade of events initiated by the T cells leads to hardening (induration) and redness (erythema) at the injection site.

A delayed hypersensitivity test (DHT) is performed for one of three reasons:
• To test for exposure to specific diseases, such as tuberculosis (TB). Tuberculosis testing is...