Peritoneal exudates from rats which have survived an infection with L. monocytogenes can protect cyclophosphamide-treated recipients against a Listeria challenge. They are more effective in this respect than cells obtained from the spleen or thoracic duct lymph. Since exudate cells from normal rats and inocula prepared from the resident peritoneal cell populations of infected donors are unable to inhibit the challenge infection, the protective cells must belong to a class of lymphocytes that emerges from the blood in response to inflammation. It is significant therefore that thoracic duct lymphocytes formed during an acute Listeria infection can move into exudates induced by a variety of inflammatory stimuli. The affinity of newly formed lymphocytes for inflamed tissue points to a mechanism whereby the host marshalls its cellular defenses at sites of bacterial invasion. The tendency of short-lived lymphocytes to leave inflamed vessels might also explain their short-circulating life-span.

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