The site of a delayed hypersensitivity reaction to tuberculin or bovine serum ablumin was shown to contain mechanisms that expressed increased antibacterial activity, as evidenced by restricted growth of a local inoculum of Listeria monocytogenes. As was the case with a delayed hypersensitivity reaction, the local generation of antibacterial activity was antigen specific and T-cell dependent. Antibacterial resistance was always expressed at the site of injection of specific antigen in sensitized mice, even though under certain circumstances there was no measurable increase in footpad thickness at this site. It thus appears that nonspecific antibacterial resistance represents a sensitive and quantitative method for measuring delayed hypersensitivity. More importantly, this study serves to provide a functional meaning for the delayed-type hypersensitivity reaction in that it demonstrates that such a reaction causes the focusing of mechanisms that can restrict the growth of bacteria at a site which may represent a source of microbial invasion.
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