Evaluation of delayed hypersensitivity: from PPD to poison ivy.

Intact cell mediated immunity (CMI) requires a complex interaction between lymphocytes, macrophages, and cytokines. The presence of functional CMI can be easily tested in vivo by delayed hypersensitivity (DTH) skin testing. Delayed hypersensitivity skin testing has been commonly used in three ways; anergy screening, testing for infection with intracellular pathogens, and testing for sensitivity to contact allergens. Accurate anergy screening requires intradermal testing with four or more common recall antigens such as Candida albicans, tetanus toxoid, mumps, and purified protein derivative. Problems of antigen potency and availability can be overcome by using a multiple time test-like device that contains seven different recall antigens (Multitest CMI). Although testing for infection was once common, it is now only recommended for tuberculosis screening because most other antigens are either not predictive of infection or not commercially available. Accurate testing for contact allergy requires careful attention to technique, and limitation of allergens to the 20 antigens that are approved by the American Academy of Dermatology.