RETICULOENDOTHELIAL SYSTEM (RES)
The RES is composed of special tissue macrophages, which are derived from monocytes. When released from the marrow, monocytes spend a short time in the circulation (about 24 hours) and then enter the body tissues. Within the tissues, the mono-cytes continue to differentiate into cells called macrophages, which can survive for months. Macrophages have a variety of im-portant functions. They defend the body against foreign invaders (ie, bacteria and other pathogens) via phagocytosis. They remove old or damaged cells from the circulation. They stimulate the in-flammatory process and present antigen to the immune system. Macrophages give rise to tissue histiocytes, in-cluding Kupffer cells of the liver, peritoneal macrophages, alveo-lar macrophages, and other components of the RES. Thus, the RES is a component of many other organs within the body, par-ticularly the spleen, lymph nodes, lung, and liver.
The spleen is the site of activity for most macrophages. Most of the spleen (75%) is made of red pulp; here the blood enters the ve-nous sinuses through capillaries that are surrounded by macro-phages. Within the red pulp are tiny aggregates of white pulp, consisting of B and T lymphocytes. The spleen sequesters newly re-leased reticulocytes from the marrow, removing nuclear fragments and other materials (eg, denatured hemoglobin, iron) before the now fully mature RBC returns to the circulation. Although a minority of RBCs (less than 5%) is pooled in the spleen, a signifi-cant proportion of platelets (20%–40%) is pooled here. If the spleen is enlarged, a greater proportion of RBCs and platelets can be sequestered. The spleen is a major source of hematopoiesis in fetal life. It can resume hematopoiesis later in adulthood if neces-sary (eg, in bone marrow fibrosis). The spleen has important im-munologic functions as well. It forms a substance that promotes the phagocytosis of neutrophils; it also forms the antibody IgM after exposure to antigen.