Resistance to the facultative intracellular bacteria, Brucella abortus and Listeria monocytogenes, is principally the result of acquisition of enhanced antibacterial activity by host macrophages, probably in response to lymphokines released by T lymphocytes. However, the present paper describes a surprisingly high resistance on the part of both congenitally athymic “nude” mice and of lethally irradiated mice compared with normal controls. This enhanced bactericidal activity was evident 24 hr after infection, and could also be demonstrated in macrophages from nude mice cultured in vitro. It was concluded that the macrophages of these animals had been activated before infection.

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This work was supported by the Australian National Health and Medical Research Council.

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