Introduction

In the 19th century, the Russian zoologist Metchnikoff discovered that certain white cells, which he termed phagocytes, were able to ingest microorganisms and other foreign materials. He recognized the biological significance of phagocytes in the defence of the organism [1]. In the early 20th century, morphologists observed that certain cells took up dyes injected intravenously (called "vital dyes", since they stained live cells). The German pathologist Aschoff identified these cells as macrophages in connective tissues, microglia in the central nervous system, endothelial cells lining vascular sinusoids, and reticular cells of lymphoid organs. He suggested that these varied cell types functioned in host defence by phagocytosing foreign invaders such as microbes, and he grouped them collectively into the reticuloendothelial system (RES) [2].

The RES obviously consists of a working complex of macrophages, dendritic reticular cells, lymphocytes, and plasma cells as arrayed in the lymph nodes, spleen, thymus, bone marrow, gut-associated lymphoid tissues, inflammatory lesions, and elsewhere [3].

It was recognized that the pinocytosis, of which the endothelial and reticular cells are capable, is fundamentally different from the active phagocytosis of macrophages. After development of the concept of mononuclear phagocytic system [4], therefore, it is now more appropriate to classify monocytes and macrophages as members of the mononuclear phagocyte system.

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