The macrophage: past, present and future.

As we approach the centenary of Elie Metchnikoff's Nobel Prize (1908), it is opportune to reflect upon the history of macrophage immunobiology, take stock of current knowledge and anticipate questions for the future. Starting from his appreciation of phagocytosis as an important determinant of host defence against infection and injury, we have learned a great deal about the distribution of macrophages throughout the body, their heterogeneous phenotype and complex functions in tissue homeostasis as well as in innate and acquired immunity. Recent discoveries of Toll-like and other plasma membrane, vacuolar and cytosolic recognition molecules have brought the macrophage and closely related dendritic cells to the centre of immunologic attention, but many earlier discoveries of their cellular and molecular properties have laid a broader foundation to the appreciation of their remarkable plasticity and adaptability to local and systemic cues. Discoveries of pro-inflammatory mediators such as TNF and other secretory products have provided valuable insights into the role of macrophages in many acute and chronic disease processes, and led to the development of effective therapeutics. Much remains to be discovered regarding both their specific functions and by study of their general cellular properties, in vitro and in vivo.