CE Grossi, National Institute for Cancer Research, University of Genoa, Genoa, Italy PM Lydyard, Department of Immunology, University College London, London, UK

The human spleen is a highly vascular organ which receives blood from the splenic artery and drains into the portal venous system. It is a discriminatory filter which removes aged erythrocytes and platelets from the circulation through phagocytic cells (macrophages) in a specialized section of the splenic venous compartment, the venous sinuses of the red pulp. This is a system of highly anastomosing vascular channels separated by cellular cords consisting of macrophages, lymphocytes and plasma cells with additional cells derived from the circulation, i.e. red cells, granulocytes and platelets. Another major component of the splenic parenchyma is the white pulp, which consists of lymphoid sheaths organized around branches of the splenic artery (periarteriolar lymphoid sheaths, PALS). As in other peripheral lymphoid tissues, the PALS are organized into T-dependent and B-dependent zones. The latter are occupied by secondary lymphoid follicles with germinal centers. The periphery of the PALS, at the boundaries between the white and the red pulp, is termed the marginal zone. This displays a unique vascular organization and is important to the function of the white pulp.

Besides providing a blood cell reservoir capable of releasing erythrocytes, granulocytes and platelets into the blood when needed, the spleen functions as the site for destruction of red cells and platelets which are phagocytosed by macrophages. The lymphoid tissue of the white pulp provides the environment for specific humoral and cellular immune responses mediated by B and T cells, predominantly against antigens circulating in the bloodstream.

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