The spleen is divided into the white pulp, which includes the periarteriolar lymphoid sheaths (PALS) and functions as a secondary lymphoid tissue, and the macrophage‐rich red pulp, which is responsible for the removal by phagocytosis of aging erythrocytes, platelets, and some blood‐borne pathogens.
Figure 6.16 Spleen. (a) Low‐power view of human spleen showing red pulp (RP) and lymphoid white pulp (WP). Mallory’s triple stain. (Source: Image courtesy of G. Campbell.) (b) Diagrammatic representation of an area of white pulp surrounded by red pulp. (c) High‐power view of germinal center (GC) and lymphocyte mantle (M) surrounded by marginal zone (MZ) and red pulp (RP). Adjacent to the follicle, an arteriole (A) is surrounded by the periarteriolar lymphoid sheath (PALS) predominantly consisting of T‐cells. Note that the marginal zone is present only above the secondary follicle.
The white pulp constitutes circular or elongated areas (Figure 6.16a) within the erythrocyte‐containing red pulp, which possesses blood‐filled venous sinusoids (channels) lined with macrophages. As in the lymph node, the T‐ and B‐cell areas of the white pulp are segregated (Figure 6.16b). In addition to acting as a very effective blood filter removing effete (old or damaged) cells, the spleen is also important in generating immune responses against any infectious agents present in the blood. Plasmablasts and mature plasma cells are present in the area referred to as the marginal zone extending into the red pulp (Figure 6.16c).