The spleen is a highly vascularized organ whose major functions are to remove senescent and damaged red blood cells and particles (such as immune complexes and opsonized microbes) from the circulation and to initiate adaptive immune responses to blood-borne antigens. The spleen weighs approximately 150 g in adults and is located in the left-upper quadrant of the abdomen. The splenic parenchyma is divided into red pulp, which is composed mainly of blood-filled vascular sinusoids, and lymphocyte-rich white pulp. Blood enters the spleen through a single splenic artery that pierces the capsule at the hilum and divides into progressively smaller branches that remain sur rounded by protective and supporting fibrous trabeculae (Fig. 1). Some of the arteriolar branches of the splenic artery end in extensive vascular sinusoids that are filled with large numbers of erythrocytes and are lined by macrophages and other cells. The sinusoids end in venules that drain into the splenic vein, which carries blood out of the spleen and into the portal circulation. The red pulp macrophages serve as an important filter for the blood by removing microbes, damaged cells, and antibody-coated (opsonized) cells and microbes. Individuals lacking spleen are susceptible to disseminated infections with encapsulated bacteria, such as pneumococci and meningococci. This is likely because such organisms are normally cleared by opsonization with antibodies followed by phagocytosis. Since the spleen is an important site of antibody production, both opsinization and phagocytic clearance of the organisms are defective in the absence of the spleen.

Fig1. Morphology of the spleen. (A) Schematic diagram of the spleen illustrating T-cell and B-cell zones, which make up the white pulp. (B) Photomicrograph of a section of human spleen showing a periarteriolar lymphoid sheath (PALS) and a lymphoid follicle with a germinal center. Surrounding these areas is the red pulp, rich in vascular sinusoids. (C) Immunofluorescence stain of white pulp in human spleen, demonstrating B cells, light blue, in a follicle and T cells, stained dark blue, in the periarteriolar lymphoid sheath. B, Courtesy Kaushik Sridhar and Robert Ohgami, MD, PhD, Department of Pathology, University of California San Francisco. C, Courtesy Andrea Radtke, PhD, laboratory of Ronald N. Germain, National Institute of Allergy and Infectious Diseases, National Institutes of Health.

The white pulp contains the cells that mediate adaptive immune responses to blood-borne antigens. In the white pulp are many collections of densely packed lymphocytes, which appear as white nodules against the background of the vascular sinusoids. The white pulp is organized around central arteries, which are branches of the splenic artery distinct from the branches that form the vascular sinusoids. Several smaller branches of each central artery pass through the lymphocyte rich area and drain into a marginal sinus. A region of specialized cells surrounding the marginal sinus, called the marginal zone, forms the boundary between the red pulp and white pulp. The architecture of the white pulp is analogous to the organization of lymph nodes, with segregated T-cell and B-cell zones. The central arteries are surrounded by cuffs of lymphocytes, most of which are T cells. Because of their anatomic location, morphologists call these T-cell zones periarteriolar lymphoid sheaths. B-cell–rich follicles occupy the space between the marginal sinus and the periarteriolar sheath. As in lymph nodes, the T-cell areas in the spleen contain a network of conduits lined by FRCs. The marginal zone just outside the marginal sinus is a distinct region populated by B cells and specialized macro phages. The architecture of the white pulp is more complex in humans than in mice, with both inner and outer marginal zones and a perifollicular zone. Antigens in the blood are delivered into the marginal sinus by circulating DCs or are transferred by the macrophages to underlying marginal zone B cells.

The anatomic arrangements of the APCs, B cells, and T cells in the splenic white pulp promote the interactions required for the efficient development of humoral immune responses, as we will discuss in Chapter 12. The segregation of T lymphocytes in the periarteriolar lymphoid sheaths and B cells in follicles and marginal zones is dependent on the production of different cytokines and chemokines by the stromal cells in these different areas, analogous to the case for lymph nodes. As in lymph nodes, the chemokine CXCL13 and its receptor CXCR5 are required for B-cell migration into the follicles, and CCL19 and CCL21 and their receptor CCR7 are required for naive T-cell migration into the periarteriolar sheath. The production of these chemokines by nonlymphoid stromal cells, such as FRCs, is stimulated by the cytokine lymphotoxin.

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مواضيع ذات صلة

Natural Killer Cells

Integrins and Integrin Ligands

Selectins and Selectin Ligands

Eosinophils

Phagocytosis

Leukocytes (White Blood Cells)

Toll-Like Receptors

Anatomy and Functions of Lymphoid Tissues : Lymph Nodes

Anatomy and Functions of Lymphoid Tissues : Bone Marrow

Populations of Lymphocytes Distinguished by History of Antigen Exposure

Development of Lymphocytes

Dendritic Cells