Shaped much like a loose fist and tucked under the left side of the diaphragm, the spleen is the most ignored and misunderstood of all organs. After all, we read numerous times that someone in an automobile accident or a fight has a ruptured spleen so the spleen is removed (splenectomy) and life goes on. What we don't read about is that splenectomized people later have problems with overwhelming bacterial infections of the blood (sepsis). What does this suggest as the function of the spleen?

The spleen receives blood from an artery off of the aorta. After passing through an intricate meshwork of tiny blood vessels, the blood continues to the liver. The blood vessels of the spleen are surrounded by nests of B lymphocytes - mainly of the memory type. As the blood slowly moves through the spleen, it is monitored by T-cells for any non-self invaders. If some suspicious cell or molecule is detected, it is presented to the resident B-cells for a match to an appropriate memory B-cell. Once a matching B-cell is activated, the cell divides rapidly and begins producing antibodies directed against the invading antigen. The spleen blood vessels are also lined with macrophages which swallow and digest debris in the blood such as worn out red blood cells and platelets.

In a disease such as mononucleosis, the macrophages in the spleen become overactive and trap a higher number of white blood cells. In the process, the spleen becomes swollen and may even rupture."