Many substances, both natural and manufactured, cause inflammation when introduced into living tissue. Some compounds cause a local inflammatory reaction in all exposed individuals and often are called toxic or irritant reactions. Skin testing with other compounds causes an inflammatory response in a limited number of subjects. Those reactions that occur on the basis of specific immune responses are termed hypersensitivity reactions, and the responding individuals are said to be hypersensitive to that antigen.

Intradermal skin testing may induce four or more types of cutaneous hypersensitivity reactions in humans<sup>35 Smith D.L., deShazo R.D. Delayed hypersensitivity skin testing Manual of Clinical Laboratory Immunology Washington, DC: American Society for Microbiology (1992).  202

Cliccare qui per andare alla sezione Riferimenti</sup>: (1) an immediate wheal and flare reaction related to the presence of antigen-specific immunoglobulin E (IgE) in the skin; (2) an IgE-mediated late-phase reaction that follows the wheal and flare, begins several hours after injection, and peaks at 12 to 24 hours; (3) a local vasculitic reaction peaking at 12 to 24 hours that results from the interaction of complement-fixing antibody and antigen at the injection site (Arthus reaction); and (4) a lymphocyte and macrophage-dependent delayed hypersensitivity reaction (DHR). Erythema and induration at the injection site peaks at 48 hours after the injection. The latter reaction is the subject of this article.