IT has only been a decade since the "mononuclear phagocyte system" was defined, linking monoblasts, promonocytes, monocytes, and the structurally heterogeneous tissue macrophages. Cells of this lineage develop over several days in the marrow, emigrate from the blood within hours to days, and survive for months in the tissues, where they are involved in important homeostatic mechanisms as well as diverse pathologic events. For 70 years, investigators followed the lead of Metchnikoff, focusing on phagocytosis and the intracellular digestion of particles by macrophages. In the past few years, it has emerged that the macrophage is also a secretory cell. Through . . .