Infection in mice with Listeria monocytogenes results in a substantial accumulation of migrant macrophages in the liver. The immigrant cells populate both the infective foci and intervening sinusoids. They have the labeling characteristics of blood monocytes, and their appearance in infective foci in the liver corresponds to the expression of a high level of antimicrobial immunity in this organ. The infected liver acquires additional new macrophages by Kupffer-cell division. The proliferation of these cells, however, is not essential for the expression of immunity in the liver. The results indicate that the macrophages which express immunity to a primary infection with L. monocytogenes are those derived from circulating monocytes. Most of these cells are quickly lost once the parasite is eliminated from the tissues.

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