What is unusual about the spleen is its blood supply. Normally arteries branch into smaller arterioles which branch into leaky thin-walled capillaries. From the capillaries fluid and white cells leak, but red cells remain contained and are collected into venules that coalesce into veins which return the blood to the heart. Fluid and lymphocytes that have left the capillaries are collected by lymphatics which return to the heart via the thoracic duct and the way stations situated on the lymphatics and known as lymph nodes.
In the spleen things are different. First there is a mechanism known as plasma skimming. Small arterioles known as penicillate arteries skim off the plasma and lymphocytes sideways, leaving the splenic arteries with red cells, platelets and much less plasma. The hematocrit rises from 40% to 80%. The arterioles drain not into capillaries but into sinusoids known as the Cords of Bilroth. Theodor Bilroth (1826-1894) is the father of abdominal surgery. His operations for stomach ulcers (Bilroth I and Bilroth II have only recently gone out of fashion). He was a great friend of Johannes Brahms and might have been a musician had it not been for his success in medicine.
These Bilroth Cords are lined with macrophages, and red cells delayed within them tend to get eaten. Escape from the Cords is through very narrow apertures only 1-2 microns in diameter. A red cell is 7 microns in diameter, but extremely flexible. Its shape as a biconcave disc means that it is able to squeeze through the opening. As a red cell ages it gets less flexible (like us all) and gets delayed. Red cells from some hereditary hemolytic anemias are less distensible - for example hereditary spherocytosis or pyropoikilocytosis - and tend to get trapped and eaten.
The macrophages that line the Cords have receptors for immunoglobulin. One of these receptors, FCR1 (or CD64) binds very avidly to immunoglobulin, but all the normal immunoglobulin in plasma tends to shield cells that are covered with antibody from binding to them. In the spleen the high hematocrit means that there is less plasma protecting antibody- coated red cells, and the macrophages start nipping off pieces of antibody-coated red cell membrane. Left with the same content but a smaller membrane, the red cell assumes the shape that contains the largest volume for the smallest surface area; in other words a sphere. Spheres cannot escape from the Cords of Bilroth.
Thus it is that taking out a spleen cures the hemolysis of hereditary spherocytosis and autoimmune hemolytic anemia.
When a spleen enlarges a much greater proportion of the red cells in the body are within it at any time. A normal spleen contains perhaps 100 mls of blood, a very large spleen ten times that amount. Platelets fare even worse; a very large spleen can contain 90% of the platelets in the body. There is no discernable pool of neutrophils in the spleen, however.