Phagocytic Cells and The Reticuloendothelial System

1. Phagocytic Cells and The Reticuloendothelial System Particulate matter that finds its way into the blood or tissues is rapidly removed by cells, and the property of taking up dyes, colloids, etc. was used by anatomists to define a body-wide system of phagocytic cells known as the “reticuloendothelial system” (RES), consisting of the vascular endothelium and reticular tissue cells (top right of Figure 1), and – supposedly descended from these – various types of macrophages with routine functions that included clearing up the body’s own debris and killing and digesting bacteria. However, more modern work has shown a fundamental distinction between those phagocytic cells derived from the bone marrow (blue in figure 1) and endothelial and reticular cells formed locally from the tissues themselves (yellow). Ironically, neither reticular nor endothelial cells are outstandingly phagocytic. Their function is partly structural, in maintaining the integrity of the lymphoid tissue and blood vessels, respectively. However, there is increasing awareness that both cell types have an equally important role as “signposts”, regulating the migration of hemopoietin cells from blood into the tissues and through the various sub compartments of lymphoid tissue. In contrast, the major phagocytic tissue cell is the macrophage, and it is therefore more usual today to speak of the “mononuclear phagocytic system” (MPS). The cells of the MPS are now recognized as fundamental to both the ‘recognition’ and the “mopping up” phase of the adaptive immune response. Macrophages and dendritic cells act as tissue sentinels, responding to infection and tissue damage via ‘innate’ receptors and signaling the alarm to adaptive immunity via both antigen presentation and the release of powerful cytokines. Once an adaptive immune response is established, one of the main roles of antibody is to promote and amplify phagocytosis, while T lymphocytes serve to activate macrophage microbicidal activity.