THE MONONUCLEAR PHAGOCYTE-DENDRITIC CELL DICHOTOMY - MYTHS, FACTS, AND A REVISED CONCEPT

Since Aschoff's reticuloendothelial system was abandoned a few decades ago, classification and characterization of the mononuclear phagocyte and dendritic cell systems have evolved separately or even in competi tion with one another. New information has now become available indica ting that monocytes/macrophages and dendritic cells have a common orig in in the bone marrow, and may even transdifferentiate. Morphological and functional distinctions-although valid under certain conditions-ha ve been blurred by revelation of the versatility of monocytes/macropha ges and dendritic cells in response to different contextual needs in i nflammation and immunity. Monocytes/macrophages and dendritic cells sh are a sentinel, receptor/effector, and presentation mode, and may eith er activate or silence specific immune reactions. In keeping with the view of monocytes/macrophages and dendritic cells as interactive senti nels, we suggest that the mononuclear phagocyte and dendritic cell sys tems be replaced by the custocyte system (custos, Lat = sentinel, guar d) as a unifying concept. Within the custocyte system, we recognize ty pe I, type II, and type III custocytes. Type I and II custocytes exhib it predominance of presentation or effector/presenter interdependency, respectively, while type III custocytes are bipolar, passing through type I- and type II-like phases during their development and in inflam matory responses. The custocyte system brings into view monocytes/macr ophages and dendritic cells as dynamic players in immunity and inflamm ation with a high degree of derivational, phenotypic, functional, and molecular plasticity.