A localized skin response mediated by T cells, which occurs 24 to 72 hr after injection of a specific antigen to which the person has been previously sensitized. It is used routinely to screen for tuberculosis infection through injection of purified protein derivative of Mycobacterium tuberculosis. In patients with immunodeficiency, common microbial antigens to which most people have been exposed, such as diphtheria, tetanus, measles, or Candida, are used to determine the presence of defects in T-cell mediated immunity (CMI). If patients do not develop induration at the site, indicating a positive response to the antigen, a CMI defect is present. Delayed hypersensitivity is a type IV hypersensitivity reaction mediated by cytokines released by macrophages and helper T cells.