The Spleen

The spleen has the primary responsibility for mounting an immune response to blood-borne pathogens. Quantitatively, it is the most important organ in terms of lymphocyte migration and filtration. Located in the upper left quadrant of the abdomen, the spleen weighs approximately 150 g in humans. Almost 25% of the total lymphocyte population is found in the spleen. In addition, about 50% of the lymphocytes in the blood (2.5 X 1011 lymphocytes) migrate through the spleen each day (Pabst, 1988).

The spleen has a red and white pulp (Laman et al., 1992). Morphologically, the splenic artery branches until it terminates into splenic arterioles in the red pulp of the spleen (Fig. 4). The red pulp contains red cells and only small numbers of lymphocytes. A high volume of blood passes through the vascular sinuses within the red pulp. The vascular sinusoids are lined with phagocytic macrophages that function to degrade aging or defective red blood cells and ingest blood-borne pathogens. Bacterial infections cause a massive increase in the red pulp via production of immature granulocytes. Approximately 5% of the total blood flow passes through the sinusoids each minute, collecting in venules progressing in size to the splenic vein. The splenic vein carries blood from the spleen to the portal circulation.

The spleen is normally involved in hematopoiesis in select species. In rats and mice, the splenic red pulp is also involved in extramedullary hema-

Figure 4. Schematic structure of the spleen. Reprinted from Immunology, Bellanti, 1971, with permission of W. B. Saunders Co.

topoiesis of granulocytes. Normoblasts and megakaryocytes are found in the spleen.

Most of the immunocompetent cells are found in the white pulp. These lymphocyte clusters are located in the periarteriolar sheath (PALS). Roughly two-thirds of the PALS are CD4 cells while the remainder are CD8 lymphocytes. Germinal centers also exist in the white pulp. Some B cells in the zone are organized into secondary follicles. A rim of lymphocytes and macrophages in the PALS form a marginal zone. Unlike other mammals, the marginal zone predominates in the rat spleen.

The microenvironment of the marginal zone is unique. Specialized macrophages called marginal metallophilic macrophages are found in the zone. Moreover, the morphology of B cells is different relative to other areas of the spleen. There are no centrocytes or centroblasts in the germinal centers and the B-cell phenotype (IgM+, IgD-)is unique. Immunocompetent B cells, in the marginal zone, represent mixed populations. Virgin, nonactivated B cells and B memory cells are found in the zone. For reasons that are unclear, these B cells respond only to carbohydrate immunogens (e.g., pneumococcal polysaccharides) that have repeating subunits.

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