Fig. 1.23 Organization of the lymphoid tissues of the spleen. of lymphocytes. In each area of white pulp, blood carrying both the schematic at top left shows that the spleen consists of red pulp lymphocytes and antigen flows from a trabecular artery into a (pink areas), which is a site of red blood cell destruction, interspersed central arteriole. From this arteriole smaller blood vessels fan out, with the lymphoid white pulp. An enlargement of a small section eventually terminating in a specialized zone in the human spleen of a human spleen (top right) shows the arrangement of discrete called the perifollicular zone (PFZ), which surrounds each marginal areas of white pulp (yellow and blue) around central arterioles. zone. Cells and antigen then pass into the white pulp through open most of the white pulp is shown in transverse section, with two blood-filled spaces in the perifollicular zone. the light micrograph portions in longitudinal section. the two schematics below this at bottom left shows a transverse section of white pulp of human diagram show enlargements of a transverse section (bottom center) spleen immunostained for mature B cells. Both follicle and PAlS are and longitudinal section (bottom right) of white pulp. Surrounding surrounded by the perifollicular zone. the follicular arteriole emerges the central arteriole is the periarteriolar lymphoid sheath (PAlS), in the PAlS (arrowhead at bottom), traverses the follicle, goes made up of t cells. lymphocytes and antigen-loaded dendritic through the marginal zone, and opens into the perifollicular zone cells come together here. the follicles consist mainly of B cells; (upper arrowheads). Co, follicular B-cell corona; gC, germinal center; in secondary follicles, a germinal center is surrounded by a B-cell mZ, marginal zone; rP, red pulp; arrowheads, central arteriole. corona. the follicles are surrounded by a so-called marginal zone Photograph courtesy of n. m. milicevic.
areas of white pulp. The sheath of lymphocytes around an arteriole is called the periarteriolar lymphoid sheath (PALS) and contains mainly T cells. Lymphoid follicles occur at intervals along it, and these contain mainly B cells. An area called the marginal zone surrounds the follicle; it has few T cells, is rich in macrophages, and has a resident, noncirculating population of B cells known as marginal zone B cells. These B cells are poised to rapidly produce antibodies that have low affinity to bacterial capsular polysaccharides. These antibodies, which are discussed in Chapter 8, provide some degree of protection before the adaptive immune response is fully activated. Blood-borne microbes, soluble antigens, and antigen:antibody complexes are filtered from the blood by macrophages and immature dendritic cells within the marginal zone. Like the migration of immature dendritic cells from peripheral tissues to the T-cell areas of lymph nodes, dendritic cells in the marginal zones in the spleen migrate to the T-cell areas after taking up antigen and becoming activated; here they are able to present the antigens they carry to T cells.
villusgerminal center T cells follicle-associated epithelium subepithelial dome follicle efferent lymphatics Peyer’s patches are covered by an epithelial layer containing specialized cells called M cells, which have characteristic membrane ruf˜es M cell a dome TDA GC epithelium M cell b c