We previously found an antigen-specific factor capable of augmenting delayed-type hypersensitivity (DTH) in the culture supernatant of the mixture of immune spleen cells and erythrocyte antigen, or in the serum of mice immunized with heterologous erythrocytes and exhibiting delayed-type footpad reaction. To elucidate whether this kind of factor (DTH-augmentation factor; DAF) participates in the establishment of DTH to various kinds of antigen besides erythrocyte antigen, we chose a bacterial antigen, Listeria monocytogenes, which is a facultative intracellular bacterium. In the present study, we demonstrated that the immune serum from mice immunized with viable Listeria augmented the delayed-type footpad reaction to Listeria. Furthermore, acquired resistance against Listeria was also augmented by the transfer of such immune serum. Such augmentation of acquired resistance was observed in sites infected locally and in the spleen of mice infected systemically. This effect was also seen in sera from mice immunized with heat-killed Listeria emulsified with complete Freund's adjuvant.
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