- On cut section, the spleen displays nodules of white cells termed "White Pulp" distributed within a reddish, vascular mass termed the "Red Pulp".
- Red Pulp
- The Red Pulp mostly consists of terminal branches of the splenic artery which end in specialized capillaries known as "Sheathed Capillaries". These unique capillaries simply terminate and outlet their contents into the surrounding Red Pulp. The Sheathed Capillaries take their namesake from a sheath of macrophages which surround their outlet point and filter the exiting blood. These Macrophages phagocytose old, effete erythrocytes, degrade these cells, and subsequently recycle the liberated hemoglobin as described in Heme Degradation. Healthy erythrocytes are allowed to traverse the macrophage sheath, cross into large splenic sinuses which drain into one another and eventually flow into the splenic vein.
- White Pulp
- The splenic white pulp represents small, nodular aggregations of lymphocytes containing either B-cells or T-cells. The White Pulp represents the sections of the spleen which are involved in immune responses to blood-borne microbes and are in many ways analogous to lymph nodes. Similar to the case for lymph nodes, proliferating B-cells within the White Pulp may form lymphoid follicles with germinal centers. White Pulp is often adjacent to or surrounds smaller splenic arterioles. In certain cases, small arterioles are completely encased in a sheath of T-cells and this is referred to as a "Periarteriolar Lymphoid Sheath (PALS)".