Activation revisited. Since the time of Metchinkoff (1) and his contemporaries it has been clear that tissue macrophages exist in various states of activity. One reads about the resting-wandering cells of the connective tissue, arising from the mesenchyme and stimulated by the products of local injury. Such observations performed with the light microscope, vital dyes, and keen insight have in the past two decades been expanded with our increasing knowledge of the mononuclear phagocyte system (2).

In the recent past the term “activated macrophage” has largely been relegated to the bactericidal process and to the role of these cells in cell-mediated immunity. Unraveled largely through the imaginative in vivo studies of Mackaness and North and their colleagues (3, 4), the two cell nature of the process, the effector role of the macrophage, and the unique sensitizing qualities of viable organisms were elucidated. Yet, it is apparent when one scans the varied roles of tissue macrophages in the physiology and pathology of mammals that this is a much restricted definition.

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