Activation of the immune system requires that the presence of an incipient infection first be detected. This essential step is carried out by tissue macrophages, which alert innate immunity, and by dendritic cells whih alert the adaptive immune system. Both of these sentinel cell types are derived from circulating precursors known as blood monocytes. Here we briefly review current work which has recently expanded our understanding of the origin of blood monocytes and of the factors governing their further differentiation into tissue sentinels.
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