In contrast, delayed-type hypersensitivity reactions are mediated by specific T cells that produce TH1-type cytokines upon exposure to antigen. An example of this type of reaction is the PPD reaction, or tuberculin test. When the antigen is injected under the skin of an individual who was previously infected with Mycobacterioum tuberculosis, a reaction in the skin evolves over 48 to 72 hours in which there is local swelling and induration>10mm. If the site is biopsied, one finds a T-cells and macrophage infiltration. Injection of the same material in a noninfected individual produces little or no induration, and the histology is essentially negative. Whereas the cells in this case do not kill the organism, most individuals infected surround the organism in caseous inflammatory lesion, which does not allow the organism to spread.