White pulp is lymphoid; red pulp filters blood
Mnemonic
“Red pulp filters blood, white pulp fights infection”:
- Red pulp - cords of Billroth and venous sinusoids. Removes aged red cells and Howell-Jolly bodies, and stores platelets
- White pulp - periarteriolar lymphoid sheaths (PALS) of T cells, with B cell follicles and marginal zones
Hyposplenism therefore has two consequences, one from each pulp:
- Blood film changes: Howell-Jolly bodies, target cells, acanthocytes, and a thrombocytosis
- Infection risk from encapsulated organisms: pneumococcus, meningococcus, Haemophilus influenzae, because opsonised bacteria are cleared in the marginal zone
“Splenectomy means vaccines and penicillin”: pneumococcal, meningococcal, Hib and annual influenza vaccination, plus lifelong prophylactic penicillin, ideally given two weeks before an elective splenectomy.
Causes of hyposplenism without splenectomy: sickle cell disease (autoinfarction), coeliac disease, inflammatory bowel disease and amyloid.
Expansion
White pulp (immune function)
- Periarteriolar lymphoid sheath (PALS): T cells around the central arteriole
- Lymphoid follicles: B cells, with germinal centres when active
- Marginal zone: at the interface, containing macrophages and specialised B cells that respond to encapsulated organisms
Red pulp (filtration)
- Cords of Billroth: macrophage-rich, with an open circulation where blood leaves the vessels and must squeeze back between endothelial cells into the sinusoids
- Splenic sinusoids
The open circulation is the functional key: red cells must deform to re-enter the sinusoids, so aged, rigid or abnormal cells are trapped and destroyed. This is why spherocytes are removed and why hereditary spherocytosis improves after splenectomy.
The marginal zone explains the specific vulnerability of asplenic patients to encapsulated bacteria, since it is the site of the T-independent antibody response to polysaccharide antigens.
Culling removes whole cells; pitting removes inclusions such as Howell-Jolly bodies, whose presence on a film therefore indicates hyposplenism.