The spleen is an odd-shaped organ that lies hidden away behind our ribs, posteriorly, on our left sides. It's functions are threefold:
- Red blood cell 'quality control' - the arterioles of the spleen mostly empty into a series of splenic cords. These lack an endothelial lining, and consist chiefly of a loose array of reticulin (a type of collagen) lined by macrophages and fibroblasts. To get back into the circulation, hopeful blood cells must squeeze themselves into the splenic sinuses that all ultimately empty into the splenic vein (and hence the general circulation). Old and damaged red blood cells aren't able to do this, and so they are doomed to remain in the cords where they are phagocytosed by the macrophages.
- Immunity - the spleen is in many ways analogous to a a lymph node, in that blood-borne pathogens are amply exposed to macrophages and dendritic cells, which in turn present them to T and B cell aggregations within the spleen. The spleen seems particularly good at preventing infections by encapsulated organisms such as Streptococcus pneumoniae, Haemophilus influenzae and Neiserria meningitidis.
- Extramedullary haemopoiesis - the spleen usually stops making our blood cells after about 7 months of gestation, but it has the ability to be reactivated if our bone marrows can't produce enough blood cells for whatever reason. (e.g. myelofibrosis, where the marrow is obliterated.)
These three functions are helpful to keep in mind when it comes to remembering the causes for splenomegaly. We'll cover that in the next post.