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Overview of the Spleen

by Harry S. Jacob, MD

By structure and function, the spleen is like 2 organs. The white pulp, consisting of periarterial lymphatic sheaths and germinal centers, acts as an immune organ. The red pulp, consisting of macrophages and granulocytes lining vascular spaces (the cords and sinusoids), acts as a phagocytic organ.

The white pulp is a site of production and maturation of B and T cells. B cells in the spleen generate protective humoral antibodies; in certain autoimmune disorders (eg, immune thrombocytopenia [ITP], Coombs-positive immune hemolytic anemias), inappropriate autoantibodies to circulating blood elements also may be synthesized.

The red pulp removes antibody-coated bacteria, senescent or defective RBCs, and antibody-coated blood cells (as may occur in immune cytopenias such as ITP, Coombs-positive hemolytic anemias, and some neutropenias). The red pulp also serves as a reservoir for blood elements, especially WBCs and platelets. During its culling and pitting of RBCs, the spleen removes inclusion bodies, such as Heinz bodies (precipitates of insoluble globin), Howell-Jolly bodies (nuclear fragments), and whole nuclei; thus, after splenectomy or in the functionally hyposplenic state, RBCs with these inclusions appear in the peripheral circulation. Hematopoiesis normally occurs in the red pulp only during fetal life. Beyond fetal life, hematopoiesis may occur if injury to bone marrow (eg, by fibrosis or tumors) allows hematopoietic stem cells to circulate and repopulate the adult spleen (see Primary Myelofibrosis; see Myelodysplastic Syndrome).

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