red pulp

The spleen is divided histologically into red pulp and white pulp. Red pulp consists of splenic (Billroth) cords—reticular connective tissue containing macrophages, RBCs, plasma cells, and granulocytes—interspersed with vascular sinusoids lined by specialized fenestrated endothelial cells that allow a slow, tortuous blood flow ('open circulation'). This anatomic arrangement forces red blood cells to squeeze through narrow slits between endothelial cells, which serves as a functional test of RBC deformability: senescent, rigid, or abnormally shaped RBCs (e.g., spherocytes in hereditary spherocytosis, sickle cells) are trapped and phagocytosed by macrophages. Key functions of red pulp include: (1) culling — removal of old or damaged erythrocytes; (2) pitting — removal of inclusion bodies (Howell-Jolly bodies, Heinz bodies, siderocytes) without destroying the RBC; (3) iron recycling from degraded hemoglobin; and (4) storage of up to one-third of the body's platelets. Clinically, red pulp macrophage function is why post-splenectomy patients show peripheral blood findings such as Howell-Jolly bodies, target cells, and increased risk of infection by encapsulated organisms (though this risk is more related to white pulp humoral immunity). Diseases affecting red pulp function include hereditary spherocytosis, sickle cell disease, and hypersplenism (splenic sequestration). In contrast, the white pulp is the immune (lymphoid) component surrounding the central arteriole, containing T cells (periarteriolar lymphoid sheath) and B cells (follicles), responsible for antigen presentation and antibody production.