Mononuclear Phagocytes

The term mononuclear phagocyte covers a group of cells including the macrophages in the tissues and the ancestors of these cells, i. e. , the monocytes in the peripheral blood, and their precursors in the bone marrow, the promonocytes and monoblasts. Among the cells originating in the bone marrow, the mononuclear phagocytes constitute a separate cell line. The effector cell of this line is the macrophage, which plays an important role in the host defence against infective agents and neoplastic cells as well as in the removal of antigens, antigen-antibody complexes, effete cells, and damaged tissues. Metchnikoff ( r 892) was the first to describe the macrophage and the microphage, the latter now being called polymorphonuclear leucocyte or granulocyte, and to recognize that phagocytes not only serve as scavengers, but also play an important role in the host defence against micro-organisms. However, Haeckel ( r862) had made the first observa- tion of endocytosis 30 years before. In his book 'Die Radiolaren' published in r862, Haeckel wrote: I first observed this phenomenon in May, 1859, in a specimen of Thetis fimbria, which I had injected with an aqueous suspension of fine Indigo particles in connection with a study of the vascular system.