Resistance to infection with Listeria monocytogenes involves a series of cellular interactions, many of which are carried out by cytokines. Macrophages, NK cells and neutrophils participate in early stages of Listeria resistance. The neutrophil is specially important for clearance of the liver phase of listeriosis. Macrophages and NK cells interact by way of IL-12 and TNF, which induce the NK cell to produce IFN-gamma. IFN-gamma is the major macrophage-activating cytokine. The CB-17 SCID mouse shows these cellular interactions restricting the growth of Listeria, without its elimination. CD4 and/or CD8 T cells bring about sterilizing immunity. Macrophages influence the lymphocyte response by way of antigen presentation and also by promoting Th 1 differentiation. Thus, elimination of Listeria requires a symbiosis between innate immunity and the T-cell system.