Chapter 15 - Spleen

Abstract

The spleen has important immunologic and blood filtration functions. It is composed of two anatomically and functionally distinct regions: white pulp and red pulp. The white pulp includes lymphoid follicles composed of mostly B-cells and the periarteriolar lymphoid sheath (PALS) composed of mostly CD4-positive T-cells. Immunologically activated spleen shows lymphoid follicles with three distinct zones: germinal center, mantle zone, and marginal zone. The red pulp is composed of vascular sinuses and cords of Billroth. The sinus-lining cells, also called littoral cells, have overlapping cytoplasmic processes without tight junctions. Deformable circulating blood cells squeeze through spaces between the littoral cells and percolate through the cords of Billroth to enter the splenic sinuses and venous system and return to the systemic circulation. The red pulp contains a few lymphocytes that are predominantly CD8-positive T-cells, numerous macrophages, and a few granulocytes and plasma cells. Natural killer cells are scattered throughout the red pulp and within germinal centers. Most hematologic malignancies in the spleen represent secondary involvement. Interpretation of splenic pathology includes careful gross evaluation, ensuring optimal tissue fixation and processing, and obtaining fresh material for flow cytometric immunophenotyping and cytogenetic analysis if a hematopoietic neoplasm is suspected. Obtaining adequate clinical information is critical in diagnosing disorders involving the spleen. In this chapter, we describe the salient features of hematopoietic and nonhematopoietic disorders likely to be encountered by pathologists. These include diseases involving the white pulp and red pulp, vascular tumors, cysts and pseudotumoral lesions, circulatory abnormalities, vasculitis, and infections involving the spleen. We provide clinical features, gross and histologic findings, ancillary studies, and differential diagnoses for these entities.