Physiology

The spleen has two major distinct functions: immune modulation and blood filtration. The spleen also participates in hematopoiesis and hematologic storage functions, although in the case of hematopoiesis, this function under normal physiologic conditions is lost shortly after birth. However, production of blood elements can be seen in certain pathologic conditions such as myeloid metaplasia. The spleen also functions as a storage compartment for blood components, namely platelets and lymphoid cells. The spleen stores approximately one-third of the body's total platelet mass; however, in the setting of splenomegaly this can increase to over 75 percent.

The spleen is an important immunologic organ and represents the largest collection of lymphoid tissue. The microcirculation of the spleen is central to its immunologic functions. The central arteries are surrounded by the periarterial lymphatic sheaths of the white pulp, which contains T lymphocytes and macrophages that sample the blood for soluble antigens. Presentation of soluble antigen by macrophages to T cells can result in the initiation of a primary or amnestic immune response. After passing the periarterial lymphatic sheath, blood reaches the surrounding lymphoid follicles, containing B cells, which if stimulated by cognate antigen can proliferate forming germinal centers. Plasma cells, mature antibody-producing cells, are formed by terminal differentiation of activated B cells. This is why many disease states mediated by autoantibodies are successfully managed with splenectomy.

The filtration function of the spleen occurs in red pulp, which is a region of open circulation (no endothelial cells) composed of reticuloendothelial cells

Lymphoid follicle

Lymphoid follicle

Figure 13-2 Schematic illustration of the splenic microvascular and lymphoid architecture.

and specialized macrophages that phagocytose antibody-coated particles. This process called opsonization is an effective method to tag harmful objects (e.g., bacteria- and viral-infected cells) for efficient removal. Additionally, injured or old cellular components of the hematopoietic system are also removed from the circulation by the reticuloendothelial cells of the red pulp.

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